Evaluation of the Role of the Bvg Intermediate Phase in Bordetella pertussis during Experimental Respiratory Infection
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感染与免疫杂志 2005年第2期
Departamento de Microbiología y Parasitología, Universidad de Navarra, Pamplona, Spain
Department of Microbiology, Immunology and Molecular Genetics, School of Medicine, University of California-Los Angeles, Los Angeles
Department of Molecular, Cellular and Developmental Biology, University of California-Santa Barbara, Santa Barbara, California
ABSTRACT
The BvgAS system of Bordetella pertussis was traditionally considered to mediate a transition between two phenotypic phases (Bvg+ and Bvg–) in response to environmental signals. We characterized a third state, the intermediate (Bvgi) phase, which can be induced by introducing a 1-bp substitution into bvgS (the bvgS-I1 mutation) or by growing B. pertussis under conditions intermediate between those leading to the Bvg+ and Bvg– phases. Like B. bronchiseptica, B. pertussis displays in its Bvgi phase a characteristic colony morphology and hemolytic activity and expresses a Bvgi-phase-specific polypeptide called BipA, whose synthesis is regulated by bvgAS at the transcriptional level. Based on our results, we hypothesize that the Bvgi phase of B. pertussis may be involved in facilitating transmission between hosts. Thus, a B. pertussis mutant carrying the bvgS-I1 mutation (GMT1i) persisted at wild-type levels only in the upper murine respiratory tract. Interestingly, a bipA deletion derivative of GMT1i displayed a reduced ability to colonize the nasal cavity of mice compared with GMT1i. However, in experimental mixed infections GMT1i expressing the Bvgi phase could establish an initial colonization in the nose and trachea of mice as efficiently as GMT1, but the wild-type strain outcompeted GMT1i at a later time point at all sites of the respiratory tract, suggesting that the Bvgi phase does not serve as a phenotypic phase specialized in colonization. Finally, even though B. pertussis expresses in vitro the Bvgi phase at the human nasal temperature, anti-BipA antibodies were undetectable in a large collection of sera from pertussis patients.
INTRODUCTION
In his 1960 landmark article, B. W. Lacey reported that the etiological agent of whooping cough, Bordetella pertussis, was able to undergo profound phenotypic and antigenic alterations in response to changes in growth temperature or as a result of the addition of certain ions to the culture medium. After a methodical and thorough analysis, he identified three antigenically distinct phases, which he designated X mode, I mode and C mode, and coined the term antigenic modulation to define the transition (entirely reversible) leading from X mode through I mode to C mode (21). In the same report, he accurately anticipated that "modulation in B. pertussis is not a saltative change but a process of continuous change leading at equilibrium to one of an infinite number of antigenic states."
Some 25 years later, the locus governing this sophisticated transition was identified, cloned (34, 37), and found to encode a two-component regulatory system which was designated BvgAS (formerly vir) (3, 35). The BvgAS signaling pathway uses a complex four-step His-Asp-His-Asp phosphotransfer mechanism involving three domains located in the sensor component of the system (BvgS) and a fourth domain, a second receiver, located in the response regulator of the system (BvgA; for a review, see reference 7). Once phosphorylated, BvgA has increased affinity for Bvg-activated promoters and can also function as a repressor depending on the location of the BvgA-P binding sites within the promoter. In addition, each Bvg-activated promoter requires a specific level of BvgA-P to achieve its maximum transcriptional activity.
Initially, BvgAS was considered to mediate a biphasic transition between a virulent phase, the Bvg+ phase (equivalent to X mode in Lacey's terminology), and an avirulent phase, the Bvg– phase (corresponding to Lacey's C mode). At the molecular level, the Bvg+ phase is characterized by the expression of the vast majority of virulence factors including all the known adhesins and proteinaceous toxins which are encoded by the so-called vir activated genes (vags). Conversely, the Bvg– phase is defined by the lack of activation of the vags, as well as by the cessation of repression (mediated by the product of the bvgR locus) (25, 26) of the so-called vir-repressed genes (vrgs) encoding outer membrane proteins of unknown function (20, 32).
All these molecular events translate into macroscopically detectable differences between B. pertussis cultures growing on solid medium supplemented with blood and thus, the Bvg+ phase colonies are small, domed (hemispheric), and hemolytic and the Bvg–-phase colonies are large, flat, and nonhemolytic. While B. pertussis expresses the Bvg+ phase when growing at 37°C in the absence of certain ions, including magnesium sulfate and nicotinic acid (modulators), incubation below 30°C or addition of an appropriate amount of modulators to the culture medium brings about the transition to the Bvg– phase. In marked contrast with these two phenotypic phases, the third phenotypic state reported by Lacey in Bordetella remained virtually forgotten for almost 40 years.
Instrumental in reviving the interest on this intermediate phase was the discovery of a spontaneous mutant of a closely related member of the Bordetella genus (B. bronchiseptica RB53i) displaying in vivo and in vitro phenotypes (pathogenic potential and ability to survive under nutrient-limiting conditions, respectively) intermediate between those characteristic of the Bvg+ and Bvg– phases (11). Subsequent genetic analyses revealed that, besides the constitutive mutation (bvgS-C3) which was already present in the chromosome of RB53 and makes this strain insensitive to modulation, RB53i carried another point mutation in its bvgS gene. This second mutation was found to result in a single-amino-acid substitution of four residues C-terminal to the BvgS primary site of autophosphorylation. When this point mutation (called bvgS-I1) was introduced into wild-type B. bronchiseptica, the resultant strain (RB50i) displayed an antigenic profile as well as a colony morphology and hemolytic activity under nonmodulating conditions indistinguishable from that of a semimodulated Bordetella culture (11). Therefore, the phenotypic state stably expressed by RB53i or displayed by an unmodulated culture of RB50i was seemingly equivalent to that termed I mode by Lacey and was subsequently designated the Bvgi phase.
At the molecular level, the Bvgi phase was characterized by the absence of Bvg-repressed phenotypes and by the expression of a subset of vags corresponding to those not requiring full (Bvg+ phase) levels of BvgA-P for their transcription (known as class 2 genes; see reference 7). However, the most prominent feature of the Bvgi-phase antigenic profile was the expression of Bvgi-phase-specific polypeptides (Bips), whose existence was also deduced by Lacey. Further studies led to the identification of bipA, the first Bvgi-phase-specific gene, the characterization of the bipA gene product, BipA (36), and the study of the mechanism by which BvgAS controls the expression of bipA (12).
Whereas the open reading frame of B. bronchiseptica bipA is predicted to encode a 1,578-amino-acid protein, the equivalent B. pertussis open reading frame (29) code for a 1,308-amino-acid protein. Both gene products exhibit homology to intimin and invasin at their N termini, and B. bronchiseptica BipA has been shown to be exposed on the cell surface (36). In addition, polypeptides cross-reacting with B. bronchiseptica BipA have been detected in lysates obtained from several B. pertussis strains (14, 36) but not from human strains of B. parapertussis (14). However, the role of the Bvgi phase in the life cycle of B. bronchiseptica and the potential contribution of BipA to its pathogenicity are still unclear. Thus, a B. bronchiseptica bipA deletion mutant did not show any detectable colonization defect in the context of a natural model of pathogen-host interaction (36), although additional unpublished evidence (8) supports the hypothesis that the Bvgi phase of B. bronchiseptica may be necessary for transmission between hosts. In contrast, there is an almost complete void of information regarding the potential role of the Bvgi phase of B. pertussis during the experimental respiratory infection. This fact prompted the investigation presented in this work.
MATERIALS AND METHODS
Bacterial strains and growth conditions. The bacterial strains used in this study are listed in Table 1. All the Bordetella strains were grown on Bordet-Gengou (BBL) agar plates supplemented with either 15% or 7.5% defibrinated sheep blood (Oxoid, Basingstoke, United Kingdom) for B. pertussis and B. bronchiseptica, respectively. Plates were incubated at 37°C in loosely fitted jars for 96 h (for B. pertussis) or 48 h (for B. bronchiseptica). To ensure absence of Bvg–-phase mutants in cultures, an aliquot of each culture was plated on BG-blood agar, incubated for the appropriate time, and colonies were visually inspected for hemolytic activity.
When it was necessary to grow Bordetella under Bvg–-phase (modulating) conditions, nicotinic acid was added to the medium at a final concentration of 16 mM. Whole-cell lysates for sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (PAGE) analysis were prepared from cells grown on BG-blood agar.
Escherichia coli strains were grown in LB medium. When necessary, culture media were supplemented with the appropriate combinations of the following antibiotics: nalidixic acid (50 μg/ml), kanamycin (50 μg/ml), ampicillin (100 μg/ml), gentamicin (20 μg/ml), streptomycin (20 μg/ml), and cephalexin (10 μg/ml).
Construction of mutant strains by allelic exchange. The plasmids used in this study are listed in Table 1. GMT1i and BPC1i were constructed via allelic exchange by introducing the bvgS-I1-mutation carried by the plasmid pGMT65 in a 988-bp EcoRI-BamHI fragment into the chromosomes of GMT1 and BPC1, respectively. The mutated fragment was generated by overlapping PCR (4, 38) with genomic DNA from B. pertussis GMT1 as the DNA template. The two DNA segments necessary for this reaction were generated in two independent PCRs with the following oligonucleotides: bvgi-1 (5'-GCT GGA ATT CAT GCG CGT GCT CA-3') and bvgi-2 (5'-GCG TTC ATC GGC ATG CGG ATC TCG TG-3') for the first PCR, and bvgi-3 (5'-CAC GAG ATC CGC ATG CCG ATG AAC GC-3') and bvgi-4 (5'-CGG GAT CCT GCC ACC CAC TTG CGA GA-3') for the second. These oligonucleotides (purchased from Life Technologies, Gibco BRL, Grand Island, N.Y.) were designed to anneal at positions 2557, 3003, 3523, and 3502, respectively, of the published bvgAS sequence of B. pertussis 165 (GenBank accession number M25401). Note both that primers bvgi-1 and bvgi-4 were engineered to include an EcoRI and a BamHI site, respectively, at their 5' end and that primers bvgi-2 and bvgi-3 incorporate the bvgS-I1 point mutation in their sequence.
The 988-bp fragment containing the bvgS-I1 mutation was generated in a new PCR with oligonucleotides bvgi-1 and bvgi-4 in the presence of a 1:1 molar ratio of the two previously amplified fragments. The resultant PCR product was cut with EcoRI and BamHI and ligated to EcoRI- and BamHI-digested pEGBR to generate pGMT65. DNA sequencing of the PCR-generated segment confirmed that no other alterations besides the bvgS-I1 mutation were present in this plasmid. pGMT65 was mobilized from E. coli SM10 into B. pertussis GMT1 and BPC1 by conjugation. Exconjugants were selected on BG-blood agar supplemented with Gm and then grown on BG-blood agar without antibiotic selection to allow for the loss of the plasmid. Plasmid-cured clones were selected on BG-blood agar supplemented with 5% sucrose. Clones in which the allelic replacement had occurred as intended were readily detected by their phenotypic characteristics (see below).
Attempts to construct derivatives of B. pertussis carrying in-frame deletions in bipA by allelic exchange with the sucrose counterselection method were repeatedly unsuccessful since, for unknown reasons, all the selected exconjugants unexpectedly displayed a total insensitivity to sucrose. Mutant strains GMT1-DBA, GMT1i-DBA, and GG2-DBA were finally constructed by allelic exchange with pSS1129-based plasmids and an alternative counterselection method already described (33). As parental strains for these constructions, spontaneous nalidixic acid-resistant mutants of GMT1, GMT1i, and GG2 (GMT1-Nal, GMT1i-Nal, and GG2-Nal, respectively) were used.
The 0.6-kb DNA fragment carrying the bipA deletion allele was generated by overlapping PCR (4, 38) with genomic DNA from B. pertussis GMT1 as the DNA template. In this fragment, the deletion junction is flanked by two 0.3-kb segments; the one 5' to the deletion encompasses the first 37 codons of the bipA ORF as well as 162 bp upstream the BipA start codon, whereas the portion 3' to the deletion includes the last 55 codons of the bipA open reading frame and 210-bp downstream of the BipA stop codon. This mutation results in the loss of the 93% of the bipA open reading frame. The fragments upstream and downstream of the deletion junction were generated in two independent PCRs with oligonucleotides bipA-1 (5'-CCT GTG GCC GGA GAG ATG TA-3') and bipA-2 (5'-GGT TCT GGC GAC GCG GGC T-3') to generate the bipA 5' end-containing fragment, and bipA-3 (5'-AGC CCG CGT CGC CAG AAC CCG CCG GGC CGT GAT TGC-3') and bipA-4 (5'-CGC CGC ATC CAT CGC CGT-3') to generate the corresponding 3' portion. These oligonucleotides (purchased from Genset Oligos, Paris, France) were designed to anneal at positions 1164465, 1164738, 1168389, and 1168764, respectively, of the published B. pertussis Tohama I chromosome sequence (29) (GenBank accession number NC_002929). Note the presence of the deletion allele in the sequence of primer bipA-3 as well as the complementarity exhibited by primers bipA-2 and bipA-3 over a long stretch of nucleotides.
The 0.6-kb fragment containing the deletion allele was generated in a new PCR with oligonucleotides bvgi-1 and bvgi-4 in the presence of a 1:1 molar ratio of the two previously amplified fragments. The Taq-amplified PCR product was directly ligated into 3'-T overhangs of pCR2.1 to construct pNur5 and then the 0.6-kb BamHI-XbaI fragment of this plasmid containing the bipA deletion allele was ligated to BamHI- and NheI-digested pSS1129 to generate pNur8. The 0.6-kb PCR-generated DNA fragment was sequenced to confirm that the bipA open reading frame had been maintained. pNur8 was mobilized from E. coli SM10 into B. pertussis GMT1, GMT1i, and GG2 by conjugation. Exconjugants were selected on BG-blood agar supplemented with nalidixic acid and gentamicin (streptomycin cannot be used as exconjugants become streptomycin sensitive) and then grown on BG-blood agar without antibiotic selection to allow for the loss of the plasmid. Plasmid-cured clones were selected on BG-blood agar supplemented with streptomycin, and clones in which the genetic rearrangement had occurred as intended were identified by PCR with primers bipA-1 and bipA-4.
BPC1 was constructed by allelic exchange with plasmid pJM503 (28) and the method described for the construction of GMT1-DBA. Likewise, the Bvg–- phase-locked mutant GG2 was constructed with the same allelic exchange procedure, resulting in an in-frame deletion of a 233-bp XcmI fragment of bvgA in B. pertussis strain GMT1 (Gordon and Cotter, unpublished results).
To use as controls throughout the study, strains essentially identical to B. bronchiseptica RB50i and RB53i were constructed via allelic exchange by introducing the bvgS-I1 mutation carried by the plasmid pEG129 into the chromosomes of RB50 and RB53, respectively, as described by Cotter and Miller (11).
Construction of strains carrying lacZ fusions to B. pertussis bipA and quantitation of -galactosidase activity. A 595-bp DNA fragment internal to bipA was generated by PCR with genomic DNA from B. pertussis GMT1 as template and oligonucleotides (Genset Oligos) bipA-5 (5'-GCG AAT TCT GGT GGG CGG CAG CAT CC-3') and bipA-6 (5'-GCG GAT CCA GCG GCG CAT ACA CAT T-3'), engineered to incorporate EcoRI and BamHI sites, respectively, at their 5' ends. These oligonucleotides were designed to anneal at positions 1164768 and 1165363, respectively, of the published B. pertussis Tohama I chromosome sequence (29) (GenBank accession number NC_002929). The PCR product was digested with EcoRI and BamHI and cloned into EcoRI- and BamHI-digested pEGZ (i.e., upstream of the promoterless lacZ gene) to generate pNur2. This plasmid was transformed into competent E. coli DH5 cells and mobilized into B. pertussis GMT1, GMT1i, and GG2 by triparental conjugation with the tra functions provided by plasmid pBRK2013. Exconjugants were selected on BG-blood agar supplemented with gentamicin. -Galactosidase activity was measured as already described (24) in cells grown in BG-blood agar for 72 h. To detect the occurrence of spontaneous bvgAS mutants, aliquots of each test suspension were plated onto BG-blood agar. The statistical significance of differences in expression was determined by two-tailed Student's t test comparisons.
SDS-PAGE and Western immunoblotting. SDS-PAGE and Western immunoblotting were performed as previously described (22, 24). Briefly, bacterial cells suspended in phosphate-buffered saline (PBS) were lysed by addition of 2x SDS-PAGE sample buffer (30). Bacterial lysates were boiled for 5 min, stacked in an SDS-4% polyacrylamide gel and separated in an SDS-7.5% polyacrylamide gel. Proteins were transferred to either polyvinylidene difluoride or nitrocellulose membranes for immunoblotting. Membranes were incubated with a 1:1,000 dilution of one of the following primary antibodies: serum from a rat infected with B. bronchiseptica RB53i (antiserum S3), serum from a rat hyperimmunized with killed whole B. pertussis GMT1i cells (antiserum S219), or sera from patients convalescing from pertussis. Antigen-antibody complexes were detected with a 1:5,000 dilution of horseradish peroxidase-conjugated antibody of the appropriate specificity (Amersham International, Little Chalfont, United Kingdom) and visualized by an enhanced chemiluminescence technique (Amersham) according to the manufacturer's instructions.
Experimental animals. Experimental intranasal inoculations of mice with B. pertussis were performed as previously described (23). Briefly, female BALB/c mice were obtained from Harlan Spain (Harlan Interfauna Iberica S.A., Barcelona, Spain) at 3 weeks of age. To confirm that they were Bordetella-free, two animals of each lot were euthanized, and samples of nasal, tracheal, and lung tissue were removed and cultured on BG-blood agar. Inocula were prepared from B. pertussis cells grown on BG-blood agar for 3 days and consisted of 106 CFU administered intranasally in 50 μl of PBS while the animals were slightly anesthetized by halothane inhalation. Each experimental group consisted of at least three mice (for time point zero) or at least five mice (for time point 8 days).
At each experimental time point, mice were anesthetized by halothane inhalation, a sample of blood was obtained, and then the animals were sacrificed by cervical dislocation. The chest cavity was opened, and approximately 0.5 cm of trachea and the right lung lobe were removed and placed in PBS for homogenization. The nose was dissected, and the entire nasal septum and adjacent tissues were removed and placed in PBS. Tissues were homogenized with tissue grinders, and aliquots of the suspensions were plated on BG-blood agar for viable count. The lower limits of detection for these counts were 4 CFU for both the nasal and tracheal samples and 50 CFU (5,000 in some experiments) for the lung samples. Viable counts below the lower limit of detection were arbitrarily assigned to half the lower limit of detection for each assay.
In all the animal experiments except for the respiratory mixed infections, statistical significance was determined by two-way analysis of variance, and when interaction was found to be significant, the means were compared with contrasts. For the respiratory mixed infection of mice with GMT1 and GMT1i, an identical experimental protocol was followed, although, in this case, the inoculum consisted of 106 CFU of each bacterial strain suspended in a final volume of 50 μl of PBS. To ensure that equal numbers of CFU for each strain had been delivered to the animals, appropriate dilutions of the inocula were plated on BG-blood agar for viable count. The two colonial types recovered from the animals were easily identified as formed by GMT1 or GMT1i by visually inspecting their distinct morphology and hemolytic activity. For these mixed infections, the differences in colonization both in the trachea and in the lungs were analyzed by the Wilcoxon signed rank test (due to the existence of a significant number of values falling below the lower limit of detection), whereas for the nasal septum a mixed factorial analysis of variance, with a between-subjects factor (time) and a within-subjects factor (strains), was used. All the statistical analyses were performed with SPSS software (SPSS 11.0 for Windows; SPSS Inc., Chicago, Ill.).
Intranasal inoculation of a Bordetella-free 3-week-old female Wistar rat (Harlan Interfauna Iberica S.A., Barcelona, Spain) with B. bronchiseptica RB53i was performed by placing a 10-μl drop of PBS containing 200 CFU of RB53i into one nostril as already described (24). The rat immune serum (antiserum S3) was obtained 3 weeks after the inoculation.
To obtain an antiserum against the Bvgi phase of B. pertussis GMT1i (antiserum S219), a 4-week-old Bordetella-free Wistar rat was inoculated intraperitoneally with 109 killed whole cells of B. pertussis GMT1i suspended in 200 μl of PBS, essentially as previously described (32). Briefly, GMT1i cells grown on BG-blood agar for 3 days were washed once with saline, and the optical density at 540 nm of the suspension was then adjusted with saline to 0.12 (109 CFU/ml, approximately). Bacterial cells were killed by adding merthiolate and erythromycin to the suspensions, both at a final concentration of 0.01% (wt/vol), and by incubating the resultant mixture at 37°C for 3 h. This treatment killed 100% of the sampled cells as assessed by viable counting of the treated suspension on BG plates. Prior to each inoculation, whole cells were washed twice with saline. Three weeks after the primary immunization, the animal received by the same route a second dose of freshly prepared inoculum containing this time 107 killed cells of GMT1i. The immune antiserum was collected 2 weeks after the secondary immunization.
All the animal protocols used in this study were approved by the University of Navarra Animal Research Committee (protocol number 039/00).
PCR, cloning, and sequencing. PCRs were performed with the following conditions: 3 mM MgCl2, 5% dimethyl sulfoxide, 1 U of Taq polymerase (PerkinElmer, Wellesley, Mass.), 250 μM each of the four deoxynucleoside triphosphates, and 20 pmol of each primer were combined and brought to a total volume of 25 μl. As the source of template DNA, a small portion of a colony was resuspended in the solution. A Perkin-Elmer GeneAmp 2400 thermal cycler was used for the reactions. The cycling parameters were incubation at 95°C for 5 min, followed by 30 cycles of 95°C for 1 min, 55°C for 1 min, and 72°C for 1 min, and a final incubation at 72°C for 5 min. PCR products were cloned into the pCR2.1 vector with the TA cloning kit (Invitrogen, Carlsbad, Calif.) according to the manufacturer's instructions. Plasmids containing the cloned PCR products were submitted for sequencing to Sistemas Genomicos, S.L. (Paterna, Spain).
RESULTS
Construction and phenotypic characterization of B. pertussis Bvgi-phase mutants. In B. bronchiseptica it was previously shown that the presence of the bvgS-I1 mutation leads to the expression of the Bvgi phase under growth conditions that would otherwise promote growth in the Bvg+ phase (11). However, the bvgS-I1-carrying derivative of strain RB50, designated RB50i, was not totally locked in the Bvgi phase but still retained its capacity to modulate to the Bvg– phase when grown under fully modulating (Bvg–-phase) conditions. To determine whether B. pertussis can stably display an equivalent phenotype, we introduced by allelic exchange the bvgS-I1 point mutation into the chromosome of wild-type B. pertussis GMT1, generating strain GMT1i.
Unlike the wild-type strain, though similar to the behavior described for RB50i, when GMT1i was grown under nonmodulating (Bvg+-phase) conditions, it gave rise to colonies whose morphology and hemolysis were intermediate between those of the Bvg+ and Bvg– phases (i.e., Bvgi-phase-like colonies). However, GMT1i behaved like the wild-type strain when grown under other conditions, rendering Bvgi-phase-like colonies and Bvg–-phase-like colonies when grown under semimodulating (3 mM nicotinic acid) or fully modulating (16 mM nicotinic acid) conditions, respectively (data not shown). This result suggests that, as shown in the equivalent B. bronchiseptica mutant, the single-amino-acid substitution carried by the BvgAS system of GMT1i alters the phosphorylation cascade in a way such that only intermediate amounts of BvgA-P are produced even under nonmodulating (Bvg+-phase) conditions.
To construct a B. pertussis mutant expressing the Bvgi phase under any growth conditions, we introduced the bvgS-I1 mutation into strain BPC1, a B. pertussis GMT1 derivative carrying the bvgS-C3 mutation. The resultant mutant strain was designated BPC1i and, as expected, gave rise to Bvgi-phase-like colonies when grown under Bvg+ and Bvg–-phase conditions (data not shown). GMT1i and BPC1i retained their phenotype after repeated passes on BG-blood agar, and neither of them were prone to revert to the wild-type phenotype or showed an increase in the rate of phase variation (irreversible transition to the Bvg– phase) compared to the corresponding isogenic parental strain.
Comparative analysis of bipA transcriptional activity in GMT1, GMT1i, and a Bvg–-phase-locked isogenic mutant. To study the role of the Bvgi phase of B. pertussis, we first sought to characterize in detail the pattern of Bvg-dependent regulation of bipA, the only gene hitherto reported in a wild-type strain of a closely related member of the Bordetella genus, B. bronchiseptica, to be upregulated under semimodulating conditions (12). For this purpose, we constructed derivatives of GMT1, GMT1i, and GG2 (an isogenic Bvg–-phase-locked mutant of GMT1; see Table 1) carrying chromosomal bipA-lacZ transcriptional fusions and measured the production of -galactosidase in the resultant strains, termed GMT1-BAL, GMT1i-BAL, and GG2-BAL, respectively.
As shown in Fig. 1, growth of GMT1-BAL under semimodulating conditions (2.5 to 5 mM nicotinic acid) resulted in a four- to fivefold increase in bipA transcription with respect to levels measured in cells from the same strain grown under Bvg+ or Bvg–-phase conditions. These differences were found to be highly significant (P = 0.001). However, this physiological upregulation of bipA was dwarfed by that detected in the bvgS-I1-carrying derivative of GMT1-BAL, GMT1i-BAL, grown under nonmodulating conditions (25-fold with respect to the level measured in unmodulated cells of GMT1-BAL). The difference in bipA transcriptional activity (6-fold) between GMT1i and GMT1 grown under semimodulating conditions (P < 0.001) suggests that addition of submodulating amounts of nicotinic acid does not suffice to recreate a full Bvgi-phase status in the wild-type strain, thus resembling the behavior reported to occur with a Bvg–-phase-specific phenotype (frl expression) in a B. bronchiseptica Bvg–-phase-locked mutant (11). In sharp contrast with the previous observation, transcriptional activity of bipA was almost undetectable in the isogenic Bvg–-phase-locked mutant of GMT1-BAL (GG2-BAL). These results complement and confirm those obtained by Fuchslocher and collaborators (14) with a similar experimental design and the same B. pertussis Bvgi mutant whose construction we report in the present study (GMT1i).
Characterization of the expression profile of B. pertussis Bvgi-phase-specific polypeptides cross-reacting with B. bronchiseptica Bvgi-phase-specific polypeptides. With an antibody directed against the C terminus of B. bronchiseptica BipA, Stockbauer and collaborators identified in cell lysates of B. pertussis GMT1i two major cross-reacting bands of approximately 140 and 115 kDa that were absent in cells from Bvg+ and Bvg–-phase-locked derivatives (36). To expand this analysis, we obtained whole-cell extracts of GMT1 and of its isogenic Bvgi mutants (GMT1i and BPC1i) grown with various concentrations of nicotinic acid and analyzed them side by side by Western blot with serum from a rat infected with B. bronchiseptica RB53i (antiserum S3). As shown in Fig. 2A, antiserum S3 detected the presence of a type of polypeptides that was present in all the lysates irrespective of their modulation status and therefore can be classified as Bvg-independent polypeptides (stars in Fig. 2A). In contrast, a second set of bands were undetectable under both Bvg+-phase and Bvg–-phase conditions (0 and 16 mM nicotinic acid, respectively), thus being exclusively expressed by B. pertussis GMT1 when growing under semimodulating conditions (i.e., 3.2 and 5 mM nicotinic acid; arrows in Fig. 2A).
These polypeptides consisted of a major band of 140 kDa and two minor bands of approximately 100 and 115 kDa. In view of the molecular mass of the deduced B. pertussis BipA gene product (137 kDa; GenBank accession number NC_002929-BP1112) (29) and of the aforementioned results by Stockbauer et al., it is likely that the slowest-running polypeptide could correspond to the B. pertussis BipA homolog. Consistent with this hypothesis, the range of expression of the 140-kDa polypeptide as a function of the concentration of nicotinic acid (Fig. 2A) nearly correlated with the range of bipA transcriptional activity shown in Fig. 1.
These results demonstrate that the expression of Bvgi-phase polypeptides in B. pertussis (and presumably the rest of Bvgi-phase-specific phenotypes) can be induced by growing the wild-type strain in the presence of levels of a chemical modulator somewhat intermediate (e.g., 3.2 mM and 5 mM nicotinic acid in Fig. 2A) between those leading to growth in the Bvg+ phase (0 mM) or in the Bvg– phase (16 mM). Interestingly, Fig. 2 also shows that this conclusion holds strictly true when, instead of a chemical modulator, a physical modulator potentially having a more relevant physiological role (temperature) is used. Thus, production of Bvgi-phase polypeptides could only be detected in GMT1 when this strain was incubated at temperatures intermediate (i.e., 30°C in Fig. 2B) between those leading to growth in the Bvg+ and Bvg– phases (37°C and 26°C, respectively). In addition and regardless the type of modulator used, the profile of Bvgi-phase polypeptides expressed by GMT1 matched that of its isogenic Bvgi-phase mutant (BPC1i), as shown both in Fig. 2A and 2B.
Finally, results shown in Fig. 2A confirm that, although GMT1i displays the most characteristic marker of the Bvgi-phase (expression of Bvgi-phase polypeptides) when grown under Bvg+ or Bvgi-phase conditions, it still retains its capacity to gradually modulate to the Bvg– phase when increasing amounts of nicotinic acid are added to the culture medium. Thus, expression of Bvgi-phase polypeptides dropped to undetectable levels when GMT1i was grown in the presence of 16 mM nicotinic acid. On the contrary, these modulating conditions not only failed to downregulate Bvgi-phase polypeptide expression in BPC1i (as expected of a mutant stably locked in the Bvgi phase) but, for unknown reasons, induced the expression of levels of Bvgi-phase polypeptides noticeably higher than those measured in cells of the same strain grown under Bvg+-phase conditions.
Construction and phenotypic characterization of bipA strains of B. pertussis. Stockbauer and collaborators showed that all the Bvgi-phase-specific polypeptides detected in B. bronchiseptica RB50i cell lysates by an antiserum of the same specificity as antiserum S3 are most probably BipA breakdown products since inactivation of bipA in RB50i brings about the loss of the entire Bvgi-phase-specific reactivity (36). To study the contribution of BipA to the Bvgi-phase-specific antigenic profile of B. pertussis, we analyzed BipA expression in a B. pertussis bipA mutant. For this purpose, we first made use of the bipA-lacZ fusion-carrying derivative of GMT1i (GMT1i-BAL) since the insertion of the DNA segment internal to bipA and transcriptionally fused to the lacZ gene should result in the inactivation of bipA in this mutant.
Lysates of GMT1i and GMT1i-BAL were separated by SDS-PAGE, transferred to polyvinylidene difluoride and probed with antiserum S3. As shown in Fig. 3A, none of the Bvgi-phase-specific polypeptides expressed by GMT1i was detected in the lysate of the B. pertussis bipA mutant. As a control, the same bipA-lacZ fusion was introduced by homologous recombination into the chromosome of B. bronchiseptica RB50i to construct RB50i-BAL. In agreement with published observations (36) and similar to the results obtained with the equivalent B. pertussis strains (Fig. 3A), disruption of bipA in RB50i-BAL abolished all the Bvgi-phase-specific reactivity detected by antiserum S3 in its parental strain (RB50i; data not shown). However, since this phenomenon could also be due to unintended polar effects caused by the insertion of the plasmid on genes downstream of bipA and transcriptionally linked to it, we constructed by allelic exchange a GMT1i derivative carrying an in-frame deletion in bipA and designated it GMT1i-DBA.
As shown in Fig. 3B, Western blot analysis of GMT1i-DBA with S3 antiserum revealed that this mutant did not express any of the three major Bvgi-phase-specific polypeptides and that its antigenic pattern was indistinguishable from that of the wild-type strain grown under Bvg+-phase conditions. These results indicate that, similar to B. bronchiseptica, the fast-running (100-kDa and 115-kDa in B. pertussis) Bvgi-phase-specific polypeptides are in all likelihood BipA breakdown products and therefore BipA seems to be the only B. pertussis Bvgi-phase-specific antigen detectable by serum from a rat infected with a B. bronchiseptica Bvgi-phase-locked mutant. Nevertheless, it cannot be excluded that the appropriate expression of the 100-kDa and 115-kDa polypeptides may depend on the presence of a wild-type bipA allele and/or a functional BipA protein.
Search for B. pertussis Bvgi-phase-specific antigens other than BipA. To determine whether B. pertussis possesses an exclusive set of Bvgi-phase-specific antigens not shared with B. bronchiseptica, we first compared the antigenic profile of a B. pertussis Bvgi-phase-locked mutant (BPC1i) with that of its equivalent B. bronchiseptica mutant (RB53i) by Western immunoblot with sera from mice infected with B. pertussis GMT1i for 20 days. Not surprisingly, due to the poor antibody response elicited by B. pertussis when used to infect mice by the intranasal route (15), none of the tested sera showed a significant reactivity against any of the cell extracts (data not shown).
To circumvent this problem, we instead raised an antiserum against the Bvgi phase of B. pertussis by immunizing a rat by the intraperitoneal route with suspensions containing killed whole cells of GMT1i. This immune serum was designated antiserum S219. To be able to differentiate the anti-BipA-specific antibody response from that potentially directed against other Bips, we analyzed side by side the antigenic profile of GMT1i and that of its isogenic BipA deletion mutant (GMT1i-DBA) by Western blot with antiserum S219. Intriguingly, although we expected to detect an antibody response directed against a rather wide variety of B. pertussis cell epitopes, a significant part of the detectable reactivity was found to be directed against Bvgi-phase-specific polypeptides (Fig. 4) and strongly resembled that exhibited by antiserum S3 (see for example Fig. 3B). Similar to the results obtained with antiserum S3, inactivation of bipA in GMT1i-DBA sufficed to abolish all the Bvgi-phase-specific reactivity detected by antiserum S219 in GMT1i. Our results demonstrate that, similar to B. bronchiseptica (11, 36), BipA behaves as the immunodominant Bvgi-phase-specific antigen of B. pertussis, at least in rats immunized with killed whole cells by the intraperitoneal route.
Ability of GMT1i to colonize and persist in the respiratory tract of mice. To study whether expression of phases other than the Bvg+ phase enables B. pertussis to colonize and persist in vivo, we performed a comparative analysis of the pathogenic potential of GMT1 and GMT1i in a mouse model of respiratory infection. To ensure delivery of large numbers of bacteria to the entire respiratory tract, we inoculated groups of eight mice intranasally with a high-volume, high-dose inoculum (50 μl of PBS containing 106 CFU of the corresponding strain). Approximately 1 h later we sacrificed three mice per group to determine the initial level of colonization reached by GMT1 and GMT1i in the upper (nasal septum) and lower (trachea and lungs) respiratory tract. These levels of colonization were compared again in the remaining animals at day 8 postinoculation, approximately coinciding with the peak of infection in our experimental model. Since the virulence of GMT1 in a mouse model of respiratory infection had never been subjected to a side-to-side comparison with that of a well-characterized B. pertussis strain, we also included in this analysis the prototype B. pertussis strain, Tohama I.
As shown in Fig. 5, while mice inoculated with GMT1i had numbers of CFU at the beginning of the experiment (day 0) comparable to those of mice inoculated with GMT1 at all the sites of the respiratory tract, GMT1i showed a marked defect in colonization in the lower respiratory tract of mice compared to the wild-type strain at day 8 postinoculation. Specifically, the numbers of CFU recovered from the trachea and the lungs of animals infected with the wild-type strain were 100 and 10,000 times higher, respectively, than those obtained from mice infected with its isogenic mutant lacking the capacity to express the Bvg+ phase. These results, which resemble those obtained with RB50i in a natural host of B. bronchiseptica, the rat (11), demonstrate that Bvg phases of B. pertussis other than the Bvg+ phase are not sufficient for the colonization of the lower respiratory tract of mice.
Interestingly, the ability of GMT1i to colonize the nasal septum of mice was indistinguishable from that of its parental strain at day 8 postinoculation (Fig. 5). It is important to note that, as we have previously shown with an identical animal model (23), a Bvg–-phase-locked mutant of B. pertussis is fully impaired for the colonization of the entire respiratory tract of mice and cannot even be recovered from the animal as early as day 6 postinoculation. Therefore, these results indicate that the Bvg+ phase of B. pertussis is not strictly necessary for the colonization of the upper respiratory tract of mice, since a strain devoid of this phase can colonize the nasal cavity at wild-type levels. Finally, our results demonstrate that GMT1 exhibits an ability to colonize and persist in the mouse respiratory tract similar to that of the B. pertussis prototype strain Tohama I (Fig. 5).
Experimental respiratory mixed infection with GMT1 and GMT1i. If the colonization defect exhibited by GMT1i in the lower murine respiratory tract was due to an inability to express one or several Bvg+-phase-specific factors at wild-type levels, coinoculation of the mutant along with the wild-type strain in a mixed respiratory infection could potentially restore that defect. To test this hypothesis, we followed the experimental design outlined in the previous section, but this time the animals were inoculated with a mixed suspension of GMT1 and GMT1i (106 and 106 CFU) suspended in 50 μl of PBS. As shown in Fig. 6, the ability of GMT1 to colonize the murine respiratory tract was not affected by concomitant infection with GMT1i at any experimental time point since the wild-type strain colonized all sites of the coinfected animals at levels indistinguishable from those reached when administered unmixed (see Fig. 5 and two independent experiments [not shown]).
In contrast, the presence of the wild-type strain not only proved to be insufficient to restore the colonization defect displayed by GMT1i but contributed to make it more pronounced. Thus, unlike the behavior described in single-infection experiments, GMT1i was recovered in lower numbers than GMT1 (P < 0.05) from the lungs of the animals at day 0. These differences became much more prominent at day 8 postinoculation, when GMT1 outcompeted GMT1i not only, as expected, in the lower respiratory tract (P < 0.05), but also in the upper respiratory tract (P < 0.01). These results demonstrate that the Bvg+ phase is better endowed than the Bvgi phase for colonization of the upper murine respiratory tract, although the latter phase can make up for the former in the absence of competition between the two. In addition, our results confirm that the expression of the Bvg+ phase is absolutely required, at least temporarily, for the colonization of the lower respiratory tract. Finally, our observations indicate that the virulence factors that enable GMT1 to reach wild-type levels of colonization in the murine respiratory tract cannot be exploited by its isogenic B. pertussis Bvgi mutant to restore its colonization defect.
Assessment of the role of BipA in the pathogenesis of B. pertussis. To determine whether BipA plays a role in the pathogenicity of B. pertussis, we first constructed a mutant equivalent to GMT1i-DBA (i.e., carrying an in-frame deletion in bipA) in the wild-type strain (GMT1) background and designated it GMT1-DBA. We then compared the ability of GMT1-DBA to colonize and persist in the respiratory tract of mice with that of GMT1 with the same experimental design of respiratory infection described above. As shown in Fig. 7A, GMT1-DBA was found to be as capable as the wild-type strain to colonize all the sites of the murine respiratory tract during the entire course of the experiment. Our results are consistent with previous observations reporting that a B. bronchiseptica mutant carrying an in-frame deletion in bipA was not defective for the colonization of any site of the respiratory tract of the rabbit, one of its natural hosts (36).
Nevertheless, since the experiment shown in Fig. 7A was performed with bipA mutants constructed in the wild-type strain background, it cannot be ruled out that the bacteria never underwent a transition to the Bvgi phase in vivo in their experimental host. To determine whether actual expression of BipA in vivo provides B. pertussis with an adaptive advantage when this pathogen is genetically forced to express its Bvgi phase inside its host, we compared the ability of GMT1i to colonize the murine respiratory tract with that of its isogenic mutant carrying an in-frame deletion in bipA (GMT1i-DBA). At the same time, this experiment potentially allowed us to investigate whether BipA facilitates the initial interaction between B. pertussis and the respiratory epithelium by comparing the level of colonization reached by the unmodulated GMT1i and GMT1i-DBA cells (i.e., expressing and lacking BipA, respectively) shortly after the inoculation.
As shown in Fig. 7B, although GMT1i-DBA showed a tendency to colonize the murine respiratory tract at levels slightly lower than GMT1i, this difference was not found to be significant at the beginning of the experiment. Interestingly, this deficiency became more pronounced after 8 days of infection, and thus the level of colonization reached by the bipA mutant in the nasal septum at the end of the experiment was markedly lower (P = 0.002; i.e., P < 0.01) than that of its parental strain. It is important to point out that the actual inocula used in these assays (as in all the animal experiments presented in this work) were plated on BG-blood agar for viable counts to ensure that equal numbers of CFU of the corresponding strain had been delivered to the animals (n = 8 in this particular experiment).
Search for serological markers indicating that B. pertussis may modulate to the Bvgi phase in vivo. To indirectly assess whether wild-type B. pertussis undergoes a transition to the Bvgi phase in vivo, we screened sera from 100 patients (80 children and 20 adults) convalescent from pertussis for anti-Bvgi-phase-specific antibodies. For that purpose, whole-cell extracts of B. pertussis GMT1 and GMT1i were separated by SDS-PAGE in adjacent gel lanes, transferred to polyvinylidene difluoride, and probed with the corresponding serum sample. As a positive control, duplicate lysates were probed with antiserum S3. Whereas the presence of an antibody response of varying intensity against Bvg+-phase-specific polypeptides and Bvg-independent polypeptides (stars and arrows, respectively, in Fig. 8) was often detectable in the sera, antibodies against Bvgi-phase-specific polypeptides were not detected in any sample. In light of these results, it was not surprising to likewise be unable to detect this type of antibodies in sera from a nonnatural host of B. pertussis (the mouse) infected with GMT1 for at least 20 days (data not shown). In contrast, positive detection of BipA by antiserum S3 in all the assays demonstrated that the protein was present in the GMT1i extract. These results suggest that if B. pertussis undergoes a transition to the Bvgi phase in vivo, this phenomenon (temporally and/or spatially) passes unnoticed by the immune system of its host.
DISCUSSION
Despite intense investigation since Lacey's pioneering observations in 1960, the role of phenotypic modulation in the B. pertussis life cycle remains an enigma. Results from two independent reports performed with different experimental models of respiratory infection showed that a Bvg+-phase-locked mutant of B. pertussis colonized the respiratory tract as efficiently as the wild type (23, 27) whereas a Bvg–-phase-locked mutant was cleared soon after the beginning of the experiment, even from the upper respiratory tract (23). In addition, both studies showed that B. pertussis mutants manipulated to ectopically express vrgs displayed a significant colonization defect in the lower respiratory tract of mice. These observations suggest that the Bvg+ phase of B. pertussis is necessary and sufficient for the colonization of the murine respiratory tract. In contrast, the observation that vrg6 was not required for full B. pertussis virulence in the mouse, as was initially reported, and that the transcriptional activity from the vrg6 promoter was very low throughout the respiratory infection (23) argue against a role for the Bvg– phase during respiratory infection.
Paradoxically, the host seems to be the only niche where the BvgAS-mediated transition to the Bvg– phase in B. pertussis can be expected to occur, since this Bordetella species, unlike its close relative B. bronchiseptica (9), is not able to survive under the nutrient-limiting conditions prevailing in the environment. Alternatively, as other authors have proposed, the Bvg– phase could be a nonfunctional remnant that B. pertussis retained from its progenitor, B. bronchiseptica (9). However, even if the latter hypothesis is correct, the BvgAS-mediated phenotypic modulation may still be necessary in the life cycle of B. pertussis to facilitate its alteration at least between the Bvg+ and the Bvgi phases, as the present study indicates.
First, our results show that B. pertussis expresses the Bvgi phase in vitro when growing at 30°C, thereby confirming early observations reported by Lacey, though in a different strain background (21). Although a number of environmental signals that Bordetella can sense may be important in vivo, temperature appears, in all likelihood, as a relevant modulator for B. pertussis inside its host. In fact, the temperature in the human nasal cavity ranges from 30.3 to 32.3°C when measured at an ambient temperature of 23°C (6, 18) and is notably similar to that of rabbits and small rodents (6, 17, 18). Therefore, it seems likely that B. pertussis may express its Bvgi phase in that niche. Consistent with this hypothesis, the only site of the respiratory tract where GMT1i was able to persist at wild-type levels was the upper respiratory tract. These results demonstrate that the Bvg+ phase of B. pertussis is not strictly necessary for the colonization of the upper respiratory tract of mice.
In conflict with a potential role for the Bvgi phase in vivo, we failed to detect anti-BipA-specific antibodies in a large collection of sera from patients convalescent from pertussis. To reconcile these observations, we propose that the Bvgi phase of B. pertussis may be necessary for transmission between hosts, as may be the case for B. bronchiseptica (8). According to this hypothesis, which has been previously postulated for the members of the B. bronchiseptica cluster (9), transition to the Bvgi phase in response to low temperature in the distal upper respiratory tract might facilitate the release of B. pertussis from the nasal epithelium and/or the initial interaction with the new host. In line with our results, this hypothetical transition should be short-lasting enough to pass unnoticed to the immune system of the host, thereby precluding the elicitation of an anti-BipA-specific antibody response. Likewise, in its new host B. pertussis may need to undergo a transition back to the Bvg+ phase to prevent immune recognition of BipA and subsequent production of anti-BipA antibodies.
Alternatively, although we have demonstrated by immunizing rats with GMT1i that B. pertussis BipA is highly immunogenic, the possibility remains that this protein may not be immunogenic in humans naturally infected with B. pertussis. If so, this feature of B. pertussis BipA would be exclusive of the human pathogen since B. bronchiseptica BipA has also been shown to be highly immunogenic in both naturally (Martínez de Tejada and Lorenzo-Pajuelo, unpublished results) and experimentally infected animals (36).
Based on our results, several lines of evidence support a hypothetical role of the Bvgi phase in the transmission of B. pertussis. First, we have shown that a B. pertussis strain expressing the Bvgi phase (GMT1i grown under nonmodulating conditions) can establish an initial colonization in the nasal septum and the trachea of mice at wild-type levels even when coinoculated with an isogenic strain expressing the Bvg+ phase (GMT1 grown under nonmodulating conditions). More importantly, we demonstrated that in the absence of wild-type cells GMT1i can persist in the upper respiratory tract as efficiently as the wild-type strain. Taken together, these observations suggest that a wild-type B. pertussis strain expressing the Bvgi phase would be sufficiently well endowed to initiate the colonization of the respiratory tract immediately after a transmission event.
Interestingly, our results show that the expression of BipA conferred on GMT1i a noticeably higher (although non-statistically significant) ability to establish colonization in both the upper and lower respiratory tracts of mice compared with a BipA– isogenic strain. In this context, it is tempting to speculate that the poor interaction of the bipA derivative of GMT1i with the murine respiratory tract at the initial stages of infection may have resulted at a later experimental time point in the marked defect in colonization displayed by this strain in the upper respiratory tract. On the other hand, results from the coinoculation experiments show that the Bvg+ phase is also better endowed than the Bvgi phase for the colonization of the upper murine respiratory tract. This fact indicates that the Bvgi phase does not appear to be a phenotypic phase specialized in the colonization of any site of the respiratory tract, not even of the nasal cavity. On the contrary, our observations are consistent with the rapid transition from the Bvgi phase to the Bvg+ phase postulated by our hypothesis. Finally, the fact that loss of bipA by deletion in the wild-type strain background did not cause any detectable colonization defect supports our hypothesis that BipA expression is not strictly required for the colonization of the respiratory tract but rather necessary only immediately before and/or after a transmission event.
Nevertheless, a definitive conclusion cannot be formulated since the specific conditions encountered by B. pertussis in the mouse respiratory tract may not be adequate to promote a transition to the Bvgi phase. Obviously, in this hypothetical scenario, our animal model would not allow us to detect a colonization defect caused by the absence of BipA. To address this specific point, we plan to monitor bipA transcription during the experimental respiratory infection with an in vivo reporter system. Furthermore, to more stringently test our hypothesis, we plan to carry out experiments with B. bronchiseptica (which possesses Bvgi phenotypes very similar to those of the human pathogen [11 and results shown in the present work]) and a model mimicking natural transmission to investigate whether immunization with BipA confers on the animals a certain degree of protection from being naturally infected. Likewise, we will test whether BipA-immunized animals are less infectious than those not immunized but similarly infected with wild-type B. bronchiseptica.
The biochemical characterization of GMT1 and GMT1i revealed that, as shown in B. bronchiseptica (12, 36), BvgAS also controls the expression of BipA at the level of transcription in B. pertussis. This conclusion is in good agreement with recent results published by Fuschlocher and collaborators with the same mutant whose construction is reported in the present work (14). In addition, our results show that BipA expression is undetectable under fully modulating (Bvg– phase) conditions (16 mM nicotinic acid in our experiments). Consistent with this, the bipA-lacZ fusion was found to be fully inactive when introduced in the chromosome of a Bvg–-phase-locked mutant (GG2) and this strain did not produce detectable amounts of BipA. Therefore, taken together, our results demonstrate that bipA is a Bvgi-phase-specific gene. This conclusion confirms previous studies on B. bronchiseptica bipA (12, 36) and B. pertussis bipA (14) but contradicts conclusions from other authors reporting either upregulation of bipA under Bvg–-phase conditions, thus regarding bipA as a vrg (2) or irregular transcriptional behaviors (16).
At the protein level, comparison of the Bvgi-phase-specific antigenic profile of GMT1i with that of its isogenic bipA mutant (GMT1i-DBA) indicates that BipA is the only B. pertussis Bip detectable with serum raised against the Bvgi phase of either B. bronchiseptica or B. pertussis. Interestingly, DNA microarray analyses have revealed the existence of additional Bvgi-phase-specific genes in Bordetella (5). To circumvent a potential interference caused by BipA (i.e., immunodominance) and to simultaneously complement at the protein level this search for Bvgi-phase polypeptide-encoding genes, we plan to use an antiserum raised against GMT1i-DBA to identify Bvgi-phase polypeptides other than BipA.
Finally, our results also show that the Bvgi phases of B. pertussis and B. bronchiseptica differ markedly in the range of modulating conditions at which they are expressed. Thus, whereas the Bvgi phase of B. bronchiseptica is detectable over a narrow range of concentrations of nicotinic acid (approximately 0 to 1.6 mM) (11), B. pertussis needs a larger amount of modulating agent (up to 2 mM nicotinic acid) to switch to the Bvgi phase. In this regard, it is worth noting that, unlike strain Tohama I, GMT1 belongs to a group of B. pertussis strains characterized by their high sensitivity to modulation (24). Interestingly, with a B. bronchiseptica chimeric strain carrying the bvgAS locus of B. pertussis Tohama I, it has been shown that both upregulation and downregulation of bipA transcription require levels of nicotinic acid twice as high as those shown here for GMT1 (12). Whether these differences between the Bvgi phases of B. pertussis and B. bronchiseptica are related to peculiarities in their life cycle and their pathogenicity remains to be elucidated.
ACKNOWLEDGMENTS
We are indebted to Wirsing von Knig (German Reference Laboratory for Pertussis, Krefeld, Germany) and his collaborators for the generous gift of sera from patients convalescing from pertussis. We are also thankful to Trevor Stenson for providing us with scientific reports unavailable at our institution.
This work was supported by grants from the Ministerio de Sanidad y Consumo (FIS-01/0729) and from Proyectos de Investigacion Universidad de Navarra (PIUNA-9922) to G.M.T. and by doctoral fellowships from Gobierno de Navarra and from Friends of the University of Navarra Incorporated to N.V.-I. and A.C.-M., respectively.
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Department of Microbiology, Immunology and Molecular Genetics, School of Medicine, University of California-Los Angeles, Los Angeles
Department of Molecular, Cellular and Developmental Biology, University of California-Santa Barbara, Santa Barbara, California
ABSTRACT
The BvgAS system of Bordetella pertussis was traditionally considered to mediate a transition between two phenotypic phases (Bvg+ and Bvg–) in response to environmental signals. We characterized a third state, the intermediate (Bvgi) phase, which can be induced by introducing a 1-bp substitution into bvgS (the bvgS-I1 mutation) or by growing B. pertussis under conditions intermediate between those leading to the Bvg+ and Bvg– phases. Like B. bronchiseptica, B. pertussis displays in its Bvgi phase a characteristic colony morphology and hemolytic activity and expresses a Bvgi-phase-specific polypeptide called BipA, whose synthesis is regulated by bvgAS at the transcriptional level. Based on our results, we hypothesize that the Bvgi phase of B. pertussis may be involved in facilitating transmission between hosts. Thus, a B. pertussis mutant carrying the bvgS-I1 mutation (GMT1i) persisted at wild-type levels only in the upper murine respiratory tract. Interestingly, a bipA deletion derivative of GMT1i displayed a reduced ability to colonize the nasal cavity of mice compared with GMT1i. However, in experimental mixed infections GMT1i expressing the Bvgi phase could establish an initial colonization in the nose and trachea of mice as efficiently as GMT1, but the wild-type strain outcompeted GMT1i at a later time point at all sites of the respiratory tract, suggesting that the Bvgi phase does not serve as a phenotypic phase specialized in colonization. Finally, even though B. pertussis expresses in vitro the Bvgi phase at the human nasal temperature, anti-BipA antibodies were undetectable in a large collection of sera from pertussis patients.
INTRODUCTION
In his 1960 landmark article, B. W. Lacey reported that the etiological agent of whooping cough, Bordetella pertussis, was able to undergo profound phenotypic and antigenic alterations in response to changes in growth temperature or as a result of the addition of certain ions to the culture medium. After a methodical and thorough analysis, he identified three antigenically distinct phases, which he designated X mode, I mode and C mode, and coined the term antigenic modulation to define the transition (entirely reversible) leading from X mode through I mode to C mode (21). In the same report, he accurately anticipated that "modulation in B. pertussis is not a saltative change but a process of continuous change leading at equilibrium to one of an infinite number of antigenic states."
Some 25 years later, the locus governing this sophisticated transition was identified, cloned (34, 37), and found to encode a two-component regulatory system which was designated BvgAS (formerly vir) (3, 35). The BvgAS signaling pathway uses a complex four-step His-Asp-His-Asp phosphotransfer mechanism involving three domains located in the sensor component of the system (BvgS) and a fourth domain, a second receiver, located in the response regulator of the system (BvgA; for a review, see reference 7). Once phosphorylated, BvgA has increased affinity for Bvg-activated promoters and can also function as a repressor depending on the location of the BvgA-P binding sites within the promoter. In addition, each Bvg-activated promoter requires a specific level of BvgA-P to achieve its maximum transcriptional activity.
Initially, BvgAS was considered to mediate a biphasic transition between a virulent phase, the Bvg+ phase (equivalent to X mode in Lacey's terminology), and an avirulent phase, the Bvg– phase (corresponding to Lacey's C mode). At the molecular level, the Bvg+ phase is characterized by the expression of the vast majority of virulence factors including all the known adhesins and proteinaceous toxins which are encoded by the so-called vir activated genes (vags). Conversely, the Bvg– phase is defined by the lack of activation of the vags, as well as by the cessation of repression (mediated by the product of the bvgR locus) (25, 26) of the so-called vir-repressed genes (vrgs) encoding outer membrane proteins of unknown function (20, 32).
All these molecular events translate into macroscopically detectable differences between B. pertussis cultures growing on solid medium supplemented with blood and thus, the Bvg+ phase colonies are small, domed (hemispheric), and hemolytic and the Bvg–-phase colonies are large, flat, and nonhemolytic. While B. pertussis expresses the Bvg+ phase when growing at 37°C in the absence of certain ions, including magnesium sulfate and nicotinic acid (modulators), incubation below 30°C or addition of an appropriate amount of modulators to the culture medium brings about the transition to the Bvg– phase. In marked contrast with these two phenotypic phases, the third phenotypic state reported by Lacey in Bordetella remained virtually forgotten for almost 40 years.
Instrumental in reviving the interest on this intermediate phase was the discovery of a spontaneous mutant of a closely related member of the Bordetella genus (B. bronchiseptica RB53i) displaying in vivo and in vitro phenotypes (pathogenic potential and ability to survive under nutrient-limiting conditions, respectively) intermediate between those characteristic of the Bvg+ and Bvg– phases (11). Subsequent genetic analyses revealed that, besides the constitutive mutation (bvgS-C3) which was already present in the chromosome of RB53 and makes this strain insensitive to modulation, RB53i carried another point mutation in its bvgS gene. This second mutation was found to result in a single-amino-acid substitution of four residues C-terminal to the BvgS primary site of autophosphorylation. When this point mutation (called bvgS-I1) was introduced into wild-type B. bronchiseptica, the resultant strain (RB50i) displayed an antigenic profile as well as a colony morphology and hemolytic activity under nonmodulating conditions indistinguishable from that of a semimodulated Bordetella culture (11). Therefore, the phenotypic state stably expressed by RB53i or displayed by an unmodulated culture of RB50i was seemingly equivalent to that termed I mode by Lacey and was subsequently designated the Bvgi phase.
At the molecular level, the Bvgi phase was characterized by the absence of Bvg-repressed phenotypes and by the expression of a subset of vags corresponding to those not requiring full (Bvg+ phase) levels of BvgA-P for their transcription (known as class 2 genes; see reference 7). However, the most prominent feature of the Bvgi-phase antigenic profile was the expression of Bvgi-phase-specific polypeptides (Bips), whose existence was also deduced by Lacey. Further studies led to the identification of bipA, the first Bvgi-phase-specific gene, the characterization of the bipA gene product, BipA (36), and the study of the mechanism by which BvgAS controls the expression of bipA (12).
Whereas the open reading frame of B. bronchiseptica bipA is predicted to encode a 1,578-amino-acid protein, the equivalent B. pertussis open reading frame (29) code for a 1,308-amino-acid protein. Both gene products exhibit homology to intimin and invasin at their N termini, and B. bronchiseptica BipA has been shown to be exposed on the cell surface (36). In addition, polypeptides cross-reacting with B. bronchiseptica BipA have been detected in lysates obtained from several B. pertussis strains (14, 36) but not from human strains of B. parapertussis (14). However, the role of the Bvgi phase in the life cycle of B. bronchiseptica and the potential contribution of BipA to its pathogenicity are still unclear. Thus, a B. bronchiseptica bipA deletion mutant did not show any detectable colonization defect in the context of a natural model of pathogen-host interaction (36), although additional unpublished evidence (8) supports the hypothesis that the Bvgi phase of B. bronchiseptica may be necessary for transmission between hosts. In contrast, there is an almost complete void of information regarding the potential role of the Bvgi phase of B. pertussis during the experimental respiratory infection. This fact prompted the investigation presented in this work.
MATERIALS AND METHODS
Bacterial strains and growth conditions. The bacterial strains used in this study are listed in Table 1. All the Bordetella strains were grown on Bordet-Gengou (BBL) agar plates supplemented with either 15% or 7.5% defibrinated sheep blood (Oxoid, Basingstoke, United Kingdom) for B. pertussis and B. bronchiseptica, respectively. Plates were incubated at 37°C in loosely fitted jars for 96 h (for B. pertussis) or 48 h (for B. bronchiseptica). To ensure absence of Bvg–-phase mutants in cultures, an aliquot of each culture was plated on BG-blood agar, incubated for the appropriate time, and colonies were visually inspected for hemolytic activity.
When it was necessary to grow Bordetella under Bvg–-phase (modulating) conditions, nicotinic acid was added to the medium at a final concentration of 16 mM. Whole-cell lysates for sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (PAGE) analysis were prepared from cells grown on BG-blood agar.
Escherichia coli strains were grown in LB medium. When necessary, culture media were supplemented with the appropriate combinations of the following antibiotics: nalidixic acid (50 μg/ml), kanamycin (50 μg/ml), ampicillin (100 μg/ml), gentamicin (20 μg/ml), streptomycin (20 μg/ml), and cephalexin (10 μg/ml).
Construction of mutant strains by allelic exchange. The plasmids used in this study are listed in Table 1. GMT1i and BPC1i were constructed via allelic exchange by introducing the bvgS-I1-mutation carried by the plasmid pGMT65 in a 988-bp EcoRI-BamHI fragment into the chromosomes of GMT1 and BPC1, respectively. The mutated fragment was generated by overlapping PCR (4, 38) with genomic DNA from B. pertussis GMT1 as the DNA template. The two DNA segments necessary for this reaction were generated in two independent PCRs with the following oligonucleotides: bvgi-1 (5'-GCT GGA ATT CAT GCG CGT GCT CA-3') and bvgi-2 (5'-GCG TTC ATC GGC ATG CGG ATC TCG TG-3') for the first PCR, and bvgi-3 (5'-CAC GAG ATC CGC ATG CCG ATG AAC GC-3') and bvgi-4 (5'-CGG GAT CCT GCC ACC CAC TTG CGA GA-3') for the second. These oligonucleotides (purchased from Life Technologies, Gibco BRL, Grand Island, N.Y.) were designed to anneal at positions 2557, 3003, 3523, and 3502, respectively, of the published bvgAS sequence of B. pertussis 165 (GenBank accession number M25401). Note both that primers bvgi-1 and bvgi-4 were engineered to include an EcoRI and a BamHI site, respectively, at their 5' end and that primers bvgi-2 and bvgi-3 incorporate the bvgS-I1 point mutation in their sequence.
The 988-bp fragment containing the bvgS-I1 mutation was generated in a new PCR with oligonucleotides bvgi-1 and bvgi-4 in the presence of a 1:1 molar ratio of the two previously amplified fragments. The resultant PCR product was cut with EcoRI and BamHI and ligated to EcoRI- and BamHI-digested pEGBR to generate pGMT65. DNA sequencing of the PCR-generated segment confirmed that no other alterations besides the bvgS-I1 mutation were present in this plasmid. pGMT65 was mobilized from E. coli SM10 into B. pertussis GMT1 and BPC1 by conjugation. Exconjugants were selected on BG-blood agar supplemented with Gm and then grown on BG-blood agar without antibiotic selection to allow for the loss of the plasmid. Plasmid-cured clones were selected on BG-blood agar supplemented with 5% sucrose. Clones in which the allelic replacement had occurred as intended were readily detected by their phenotypic characteristics (see below).
Attempts to construct derivatives of B. pertussis carrying in-frame deletions in bipA by allelic exchange with the sucrose counterselection method were repeatedly unsuccessful since, for unknown reasons, all the selected exconjugants unexpectedly displayed a total insensitivity to sucrose. Mutant strains GMT1-DBA, GMT1i-DBA, and GG2-DBA were finally constructed by allelic exchange with pSS1129-based plasmids and an alternative counterselection method already described (33). As parental strains for these constructions, spontaneous nalidixic acid-resistant mutants of GMT1, GMT1i, and GG2 (GMT1-Nal, GMT1i-Nal, and GG2-Nal, respectively) were used.
The 0.6-kb DNA fragment carrying the bipA deletion allele was generated by overlapping PCR (4, 38) with genomic DNA from B. pertussis GMT1 as the DNA template. In this fragment, the deletion junction is flanked by two 0.3-kb segments; the one 5' to the deletion encompasses the first 37 codons of the bipA ORF as well as 162 bp upstream the BipA start codon, whereas the portion 3' to the deletion includes the last 55 codons of the bipA open reading frame and 210-bp downstream of the BipA stop codon. This mutation results in the loss of the 93% of the bipA open reading frame. The fragments upstream and downstream of the deletion junction were generated in two independent PCRs with oligonucleotides bipA-1 (5'-CCT GTG GCC GGA GAG ATG TA-3') and bipA-2 (5'-GGT TCT GGC GAC GCG GGC T-3') to generate the bipA 5' end-containing fragment, and bipA-3 (5'-AGC CCG CGT CGC CAG AAC CCG CCG GGC CGT GAT TGC-3') and bipA-4 (5'-CGC CGC ATC CAT CGC CGT-3') to generate the corresponding 3' portion. These oligonucleotides (purchased from Genset Oligos, Paris, France) were designed to anneal at positions 1164465, 1164738, 1168389, and 1168764, respectively, of the published B. pertussis Tohama I chromosome sequence (29) (GenBank accession number NC_002929). Note the presence of the deletion allele in the sequence of primer bipA-3 as well as the complementarity exhibited by primers bipA-2 and bipA-3 over a long stretch of nucleotides.
The 0.6-kb fragment containing the deletion allele was generated in a new PCR with oligonucleotides bvgi-1 and bvgi-4 in the presence of a 1:1 molar ratio of the two previously amplified fragments. The Taq-amplified PCR product was directly ligated into 3'-T overhangs of pCR2.1 to construct pNur5 and then the 0.6-kb BamHI-XbaI fragment of this plasmid containing the bipA deletion allele was ligated to BamHI- and NheI-digested pSS1129 to generate pNur8. The 0.6-kb PCR-generated DNA fragment was sequenced to confirm that the bipA open reading frame had been maintained. pNur8 was mobilized from E. coli SM10 into B. pertussis GMT1, GMT1i, and GG2 by conjugation. Exconjugants were selected on BG-blood agar supplemented with nalidixic acid and gentamicin (streptomycin cannot be used as exconjugants become streptomycin sensitive) and then grown on BG-blood agar without antibiotic selection to allow for the loss of the plasmid. Plasmid-cured clones were selected on BG-blood agar supplemented with streptomycin, and clones in which the genetic rearrangement had occurred as intended were identified by PCR with primers bipA-1 and bipA-4.
BPC1 was constructed by allelic exchange with plasmid pJM503 (28) and the method described for the construction of GMT1-DBA. Likewise, the Bvg–- phase-locked mutant GG2 was constructed with the same allelic exchange procedure, resulting in an in-frame deletion of a 233-bp XcmI fragment of bvgA in B. pertussis strain GMT1 (Gordon and Cotter, unpublished results).
To use as controls throughout the study, strains essentially identical to B. bronchiseptica RB50i and RB53i were constructed via allelic exchange by introducing the bvgS-I1 mutation carried by the plasmid pEG129 into the chromosomes of RB50 and RB53, respectively, as described by Cotter and Miller (11).
Construction of strains carrying lacZ fusions to B. pertussis bipA and quantitation of -galactosidase activity. A 595-bp DNA fragment internal to bipA was generated by PCR with genomic DNA from B. pertussis GMT1 as template and oligonucleotides (Genset Oligos) bipA-5 (5'-GCG AAT TCT GGT GGG CGG CAG CAT CC-3') and bipA-6 (5'-GCG GAT CCA GCG GCG CAT ACA CAT T-3'), engineered to incorporate EcoRI and BamHI sites, respectively, at their 5' ends. These oligonucleotides were designed to anneal at positions 1164768 and 1165363, respectively, of the published B. pertussis Tohama I chromosome sequence (29) (GenBank accession number NC_002929). The PCR product was digested with EcoRI and BamHI and cloned into EcoRI- and BamHI-digested pEGZ (i.e., upstream of the promoterless lacZ gene) to generate pNur2. This plasmid was transformed into competent E. coli DH5 cells and mobilized into B. pertussis GMT1, GMT1i, and GG2 by triparental conjugation with the tra functions provided by plasmid pBRK2013. Exconjugants were selected on BG-blood agar supplemented with gentamicin. -Galactosidase activity was measured as already described (24) in cells grown in BG-blood agar for 72 h. To detect the occurrence of spontaneous bvgAS mutants, aliquots of each test suspension were plated onto BG-blood agar. The statistical significance of differences in expression was determined by two-tailed Student's t test comparisons.
SDS-PAGE and Western immunoblotting. SDS-PAGE and Western immunoblotting were performed as previously described (22, 24). Briefly, bacterial cells suspended in phosphate-buffered saline (PBS) were lysed by addition of 2x SDS-PAGE sample buffer (30). Bacterial lysates were boiled for 5 min, stacked in an SDS-4% polyacrylamide gel and separated in an SDS-7.5% polyacrylamide gel. Proteins were transferred to either polyvinylidene difluoride or nitrocellulose membranes for immunoblotting. Membranes were incubated with a 1:1,000 dilution of one of the following primary antibodies: serum from a rat infected with B. bronchiseptica RB53i (antiserum S3), serum from a rat hyperimmunized with killed whole B. pertussis GMT1i cells (antiserum S219), or sera from patients convalescing from pertussis. Antigen-antibody complexes were detected with a 1:5,000 dilution of horseradish peroxidase-conjugated antibody of the appropriate specificity (Amersham International, Little Chalfont, United Kingdom) and visualized by an enhanced chemiluminescence technique (Amersham) according to the manufacturer's instructions.
Experimental animals. Experimental intranasal inoculations of mice with B. pertussis were performed as previously described (23). Briefly, female BALB/c mice were obtained from Harlan Spain (Harlan Interfauna Iberica S.A., Barcelona, Spain) at 3 weeks of age. To confirm that they were Bordetella-free, two animals of each lot were euthanized, and samples of nasal, tracheal, and lung tissue were removed and cultured on BG-blood agar. Inocula were prepared from B. pertussis cells grown on BG-blood agar for 3 days and consisted of 106 CFU administered intranasally in 50 μl of PBS while the animals were slightly anesthetized by halothane inhalation. Each experimental group consisted of at least three mice (for time point zero) or at least five mice (for time point 8 days).
At each experimental time point, mice were anesthetized by halothane inhalation, a sample of blood was obtained, and then the animals were sacrificed by cervical dislocation. The chest cavity was opened, and approximately 0.5 cm of trachea and the right lung lobe were removed and placed in PBS for homogenization. The nose was dissected, and the entire nasal septum and adjacent tissues were removed and placed in PBS. Tissues were homogenized with tissue grinders, and aliquots of the suspensions were plated on BG-blood agar for viable count. The lower limits of detection for these counts were 4 CFU for both the nasal and tracheal samples and 50 CFU (5,000 in some experiments) for the lung samples. Viable counts below the lower limit of detection were arbitrarily assigned to half the lower limit of detection for each assay.
In all the animal experiments except for the respiratory mixed infections, statistical significance was determined by two-way analysis of variance, and when interaction was found to be significant, the means were compared with contrasts. For the respiratory mixed infection of mice with GMT1 and GMT1i, an identical experimental protocol was followed, although, in this case, the inoculum consisted of 106 CFU of each bacterial strain suspended in a final volume of 50 μl of PBS. To ensure that equal numbers of CFU for each strain had been delivered to the animals, appropriate dilutions of the inocula were plated on BG-blood agar for viable count. The two colonial types recovered from the animals were easily identified as formed by GMT1 or GMT1i by visually inspecting their distinct morphology and hemolytic activity. For these mixed infections, the differences in colonization both in the trachea and in the lungs were analyzed by the Wilcoxon signed rank test (due to the existence of a significant number of values falling below the lower limit of detection), whereas for the nasal septum a mixed factorial analysis of variance, with a between-subjects factor (time) and a within-subjects factor (strains), was used. All the statistical analyses were performed with SPSS software (SPSS 11.0 for Windows; SPSS Inc., Chicago, Ill.).
Intranasal inoculation of a Bordetella-free 3-week-old female Wistar rat (Harlan Interfauna Iberica S.A., Barcelona, Spain) with B. bronchiseptica RB53i was performed by placing a 10-μl drop of PBS containing 200 CFU of RB53i into one nostril as already described (24). The rat immune serum (antiserum S3) was obtained 3 weeks after the inoculation.
To obtain an antiserum against the Bvgi phase of B. pertussis GMT1i (antiserum S219), a 4-week-old Bordetella-free Wistar rat was inoculated intraperitoneally with 109 killed whole cells of B. pertussis GMT1i suspended in 200 μl of PBS, essentially as previously described (32). Briefly, GMT1i cells grown on BG-blood agar for 3 days were washed once with saline, and the optical density at 540 nm of the suspension was then adjusted with saline to 0.12 (109 CFU/ml, approximately). Bacterial cells were killed by adding merthiolate and erythromycin to the suspensions, both at a final concentration of 0.01% (wt/vol), and by incubating the resultant mixture at 37°C for 3 h. This treatment killed 100% of the sampled cells as assessed by viable counting of the treated suspension on BG plates. Prior to each inoculation, whole cells were washed twice with saline. Three weeks after the primary immunization, the animal received by the same route a second dose of freshly prepared inoculum containing this time 107 killed cells of GMT1i. The immune antiserum was collected 2 weeks after the secondary immunization.
All the animal protocols used in this study were approved by the University of Navarra Animal Research Committee (protocol number 039/00).
PCR, cloning, and sequencing. PCRs were performed with the following conditions: 3 mM MgCl2, 5% dimethyl sulfoxide, 1 U of Taq polymerase (PerkinElmer, Wellesley, Mass.), 250 μM each of the four deoxynucleoside triphosphates, and 20 pmol of each primer were combined and brought to a total volume of 25 μl. As the source of template DNA, a small portion of a colony was resuspended in the solution. A Perkin-Elmer GeneAmp 2400 thermal cycler was used for the reactions. The cycling parameters were incubation at 95°C for 5 min, followed by 30 cycles of 95°C for 1 min, 55°C for 1 min, and 72°C for 1 min, and a final incubation at 72°C for 5 min. PCR products were cloned into the pCR2.1 vector with the TA cloning kit (Invitrogen, Carlsbad, Calif.) according to the manufacturer's instructions. Plasmids containing the cloned PCR products were submitted for sequencing to Sistemas Genomicos, S.L. (Paterna, Spain).
RESULTS
Construction and phenotypic characterization of B. pertussis Bvgi-phase mutants. In B. bronchiseptica it was previously shown that the presence of the bvgS-I1 mutation leads to the expression of the Bvgi phase under growth conditions that would otherwise promote growth in the Bvg+ phase (11). However, the bvgS-I1-carrying derivative of strain RB50, designated RB50i, was not totally locked in the Bvgi phase but still retained its capacity to modulate to the Bvg– phase when grown under fully modulating (Bvg–-phase) conditions. To determine whether B. pertussis can stably display an equivalent phenotype, we introduced by allelic exchange the bvgS-I1 point mutation into the chromosome of wild-type B. pertussis GMT1, generating strain GMT1i.
Unlike the wild-type strain, though similar to the behavior described for RB50i, when GMT1i was grown under nonmodulating (Bvg+-phase) conditions, it gave rise to colonies whose morphology and hemolysis were intermediate between those of the Bvg+ and Bvg– phases (i.e., Bvgi-phase-like colonies). However, GMT1i behaved like the wild-type strain when grown under other conditions, rendering Bvgi-phase-like colonies and Bvg–-phase-like colonies when grown under semimodulating (3 mM nicotinic acid) or fully modulating (16 mM nicotinic acid) conditions, respectively (data not shown). This result suggests that, as shown in the equivalent B. bronchiseptica mutant, the single-amino-acid substitution carried by the BvgAS system of GMT1i alters the phosphorylation cascade in a way such that only intermediate amounts of BvgA-P are produced even under nonmodulating (Bvg+-phase) conditions.
To construct a B. pertussis mutant expressing the Bvgi phase under any growth conditions, we introduced the bvgS-I1 mutation into strain BPC1, a B. pertussis GMT1 derivative carrying the bvgS-C3 mutation. The resultant mutant strain was designated BPC1i and, as expected, gave rise to Bvgi-phase-like colonies when grown under Bvg+ and Bvg–-phase conditions (data not shown). GMT1i and BPC1i retained their phenotype after repeated passes on BG-blood agar, and neither of them were prone to revert to the wild-type phenotype or showed an increase in the rate of phase variation (irreversible transition to the Bvg– phase) compared to the corresponding isogenic parental strain.
Comparative analysis of bipA transcriptional activity in GMT1, GMT1i, and a Bvg–-phase-locked isogenic mutant. To study the role of the Bvgi phase of B. pertussis, we first sought to characterize in detail the pattern of Bvg-dependent regulation of bipA, the only gene hitherto reported in a wild-type strain of a closely related member of the Bordetella genus, B. bronchiseptica, to be upregulated under semimodulating conditions (12). For this purpose, we constructed derivatives of GMT1, GMT1i, and GG2 (an isogenic Bvg–-phase-locked mutant of GMT1; see Table 1) carrying chromosomal bipA-lacZ transcriptional fusions and measured the production of -galactosidase in the resultant strains, termed GMT1-BAL, GMT1i-BAL, and GG2-BAL, respectively.
As shown in Fig. 1, growth of GMT1-BAL under semimodulating conditions (2.5 to 5 mM nicotinic acid) resulted in a four- to fivefold increase in bipA transcription with respect to levels measured in cells from the same strain grown under Bvg+ or Bvg–-phase conditions. These differences were found to be highly significant (P = 0.001). However, this physiological upregulation of bipA was dwarfed by that detected in the bvgS-I1-carrying derivative of GMT1-BAL, GMT1i-BAL, grown under nonmodulating conditions (25-fold with respect to the level measured in unmodulated cells of GMT1-BAL). The difference in bipA transcriptional activity (6-fold) between GMT1i and GMT1 grown under semimodulating conditions (P < 0.001) suggests that addition of submodulating amounts of nicotinic acid does not suffice to recreate a full Bvgi-phase status in the wild-type strain, thus resembling the behavior reported to occur with a Bvg–-phase-specific phenotype (frl expression) in a B. bronchiseptica Bvg–-phase-locked mutant (11). In sharp contrast with the previous observation, transcriptional activity of bipA was almost undetectable in the isogenic Bvg–-phase-locked mutant of GMT1-BAL (GG2-BAL). These results complement and confirm those obtained by Fuchslocher and collaborators (14) with a similar experimental design and the same B. pertussis Bvgi mutant whose construction we report in the present study (GMT1i).
Characterization of the expression profile of B. pertussis Bvgi-phase-specific polypeptides cross-reacting with B. bronchiseptica Bvgi-phase-specific polypeptides. With an antibody directed against the C terminus of B. bronchiseptica BipA, Stockbauer and collaborators identified in cell lysates of B. pertussis GMT1i two major cross-reacting bands of approximately 140 and 115 kDa that were absent in cells from Bvg+ and Bvg–-phase-locked derivatives (36). To expand this analysis, we obtained whole-cell extracts of GMT1 and of its isogenic Bvgi mutants (GMT1i and BPC1i) grown with various concentrations of nicotinic acid and analyzed them side by side by Western blot with serum from a rat infected with B. bronchiseptica RB53i (antiserum S3). As shown in Fig. 2A, antiserum S3 detected the presence of a type of polypeptides that was present in all the lysates irrespective of their modulation status and therefore can be classified as Bvg-independent polypeptides (stars in Fig. 2A). In contrast, a second set of bands were undetectable under both Bvg+-phase and Bvg–-phase conditions (0 and 16 mM nicotinic acid, respectively), thus being exclusively expressed by B. pertussis GMT1 when growing under semimodulating conditions (i.e., 3.2 and 5 mM nicotinic acid; arrows in Fig. 2A).
These polypeptides consisted of a major band of 140 kDa and two minor bands of approximately 100 and 115 kDa. In view of the molecular mass of the deduced B. pertussis BipA gene product (137 kDa; GenBank accession number NC_002929-BP1112) (29) and of the aforementioned results by Stockbauer et al., it is likely that the slowest-running polypeptide could correspond to the B. pertussis BipA homolog. Consistent with this hypothesis, the range of expression of the 140-kDa polypeptide as a function of the concentration of nicotinic acid (Fig. 2A) nearly correlated with the range of bipA transcriptional activity shown in Fig. 1.
These results demonstrate that the expression of Bvgi-phase polypeptides in B. pertussis (and presumably the rest of Bvgi-phase-specific phenotypes) can be induced by growing the wild-type strain in the presence of levels of a chemical modulator somewhat intermediate (e.g., 3.2 mM and 5 mM nicotinic acid in Fig. 2A) between those leading to growth in the Bvg+ phase (0 mM) or in the Bvg– phase (16 mM). Interestingly, Fig. 2 also shows that this conclusion holds strictly true when, instead of a chemical modulator, a physical modulator potentially having a more relevant physiological role (temperature) is used. Thus, production of Bvgi-phase polypeptides could only be detected in GMT1 when this strain was incubated at temperatures intermediate (i.e., 30°C in Fig. 2B) between those leading to growth in the Bvg+ and Bvg– phases (37°C and 26°C, respectively). In addition and regardless the type of modulator used, the profile of Bvgi-phase polypeptides expressed by GMT1 matched that of its isogenic Bvgi-phase mutant (BPC1i), as shown both in Fig. 2A and 2B.
Finally, results shown in Fig. 2A confirm that, although GMT1i displays the most characteristic marker of the Bvgi-phase (expression of Bvgi-phase polypeptides) when grown under Bvg+ or Bvgi-phase conditions, it still retains its capacity to gradually modulate to the Bvg– phase when increasing amounts of nicotinic acid are added to the culture medium. Thus, expression of Bvgi-phase polypeptides dropped to undetectable levels when GMT1i was grown in the presence of 16 mM nicotinic acid. On the contrary, these modulating conditions not only failed to downregulate Bvgi-phase polypeptide expression in BPC1i (as expected of a mutant stably locked in the Bvgi phase) but, for unknown reasons, induced the expression of levels of Bvgi-phase polypeptides noticeably higher than those measured in cells of the same strain grown under Bvg+-phase conditions.
Construction and phenotypic characterization of bipA strains of B. pertussis. Stockbauer and collaborators showed that all the Bvgi-phase-specific polypeptides detected in B. bronchiseptica RB50i cell lysates by an antiserum of the same specificity as antiserum S3 are most probably BipA breakdown products since inactivation of bipA in RB50i brings about the loss of the entire Bvgi-phase-specific reactivity (36). To study the contribution of BipA to the Bvgi-phase-specific antigenic profile of B. pertussis, we analyzed BipA expression in a B. pertussis bipA mutant. For this purpose, we first made use of the bipA-lacZ fusion-carrying derivative of GMT1i (GMT1i-BAL) since the insertion of the DNA segment internal to bipA and transcriptionally fused to the lacZ gene should result in the inactivation of bipA in this mutant.
Lysates of GMT1i and GMT1i-BAL were separated by SDS-PAGE, transferred to polyvinylidene difluoride and probed with antiserum S3. As shown in Fig. 3A, none of the Bvgi-phase-specific polypeptides expressed by GMT1i was detected in the lysate of the B. pertussis bipA mutant. As a control, the same bipA-lacZ fusion was introduced by homologous recombination into the chromosome of B. bronchiseptica RB50i to construct RB50i-BAL. In agreement with published observations (36) and similar to the results obtained with the equivalent B. pertussis strains (Fig. 3A), disruption of bipA in RB50i-BAL abolished all the Bvgi-phase-specific reactivity detected by antiserum S3 in its parental strain (RB50i; data not shown). However, since this phenomenon could also be due to unintended polar effects caused by the insertion of the plasmid on genes downstream of bipA and transcriptionally linked to it, we constructed by allelic exchange a GMT1i derivative carrying an in-frame deletion in bipA and designated it GMT1i-DBA.
As shown in Fig. 3B, Western blot analysis of GMT1i-DBA with S3 antiserum revealed that this mutant did not express any of the three major Bvgi-phase-specific polypeptides and that its antigenic pattern was indistinguishable from that of the wild-type strain grown under Bvg+-phase conditions. These results indicate that, similar to B. bronchiseptica, the fast-running (100-kDa and 115-kDa in B. pertussis) Bvgi-phase-specific polypeptides are in all likelihood BipA breakdown products and therefore BipA seems to be the only B. pertussis Bvgi-phase-specific antigen detectable by serum from a rat infected with a B. bronchiseptica Bvgi-phase-locked mutant. Nevertheless, it cannot be excluded that the appropriate expression of the 100-kDa and 115-kDa polypeptides may depend on the presence of a wild-type bipA allele and/or a functional BipA protein.
Search for B. pertussis Bvgi-phase-specific antigens other than BipA. To determine whether B. pertussis possesses an exclusive set of Bvgi-phase-specific antigens not shared with B. bronchiseptica, we first compared the antigenic profile of a B. pertussis Bvgi-phase-locked mutant (BPC1i) with that of its equivalent B. bronchiseptica mutant (RB53i) by Western immunoblot with sera from mice infected with B. pertussis GMT1i for 20 days. Not surprisingly, due to the poor antibody response elicited by B. pertussis when used to infect mice by the intranasal route (15), none of the tested sera showed a significant reactivity against any of the cell extracts (data not shown).
To circumvent this problem, we instead raised an antiserum against the Bvgi phase of B. pertussis by immunizing a rat by the intraperitoneal route with suspensions containing killed whole cells of GMT1i. This immune serum was designated antiserum S219. To be able to differentiate the anti-BipA-specific antibody response from that potentially directed against other Bips, we analyzed side by side the antigenic profile of GMT1i and that of its isogenic BipA deletion mutant (GMT1i-DBA) by Western blot with antiserum S219. Intriguingly, although we expected to detect an antibody response directed against a rather wide variety of B. pertussis cell epitopes, a significant part of the detectable reactivity was found to be directed against Bvgi-phase-specific polypeptides (Fig. 4) and strongly resembled that exhibited by antiserum S3 (see for example Fig. 3B). Similar to the results obtained with antiserum S3, inactivation of bipA in GMT1i-DBA sufficed to abolish all the Bvgi-phase-specific reactivity detected by antiserum S219 in GMT1i. Our results demonstrate that, similar to B. bronchiseptica (11, 36), BipA behaves as the immunodominant Bvgi-phase-specific antigen of B. pertussis, at least in rats immunized with killed whole cells by the intraperitoneal route.
Ability of GMT1i to colonize and persist in the respiratory tract of mice. To study whether expression of phases other than the Bvg+ phase enables B. pertussis to colonize and persist in vivo, we performed a comparative analysis of the pathogenic potential of GMT1 and GMT1i in a mouse model of respiratory infection. To ensure delivery of large numbers of bacteria to the entire respiratory tract, we inoculated groups of eight mice intranasally with a high-volume, high-dose inoculum (50 μl of PBS containing 106 CFU of the corresponding strain). Approximately 1 h later we sacrificed three mice per group to determine the initial level of colonization reached by GMT1 and GMT1i in the upper (nasal septum) and lower (trachea and lungs) respiratory tract. These levels of colonization were compared again in the remaining animals at day 8 postinoculation, approximately coinciding with the peak of infection in our experimental model. Since the virulence of GMT1 in a mouse model of respiratory infection had never been subjected to a side-to-side comparison with that of a well-characterized B. pertussis strain, we also included in this analysis the prototype B. pertussis strain, Tohama I.
As shown in Fig. 5, while mice inoculated with GMT1i had numbers of CFU at the beginning of the experiment (day 0) comparable to those of mice inoculated with GMT1 at all the sites of the respiratory tract, GMT1i showed a marked defect in colonization in the lower respiratory tract of mice compared to the wild-type strain at day 8 postinoculation. Specifically, the numbers of CFU recovered from the trachea and the lungs of animals infected with the wild-type strain were 100 and 10,000 times higher, respectively, than those obtained from mice infected with its isogenic mutant lacking the capacity to express the Bvg+ phase. These results, which resemble those obtained with RB50i in a natural host of B. bronchiseptica, the rat (11), demonstrate that Bvg phases of B. pertussis other than the Bvg+ phase are not sufficient for the colonization of the lower respiratory tract of mice.
Interestingly, the ability of GMT1i to colonize the nasal septum of mice was indistinguishable from that of its parental strain at day 8 postinoculation (Fig. 5). It is important to note that, as we have previously shown with an identical animal model (23), a Bvg–-phase-locked mutant of B. pertussis is fully impaired for the colonization of the entire respiratory tract of mice and cannot even be recovered from the animal as early as day 6 postinoculation. Therefore, these results indicate that the Bvg+ phase of B. pertussis is not strictly necessary for the colonization of the upper respiratory tract of mice, since a strain devoid of this phase can colonize the nasal cavity at wild-type levels. Finally, our results demonstrate that GMT1 exhibits an ability to colonize and persist in the mouse respiratory tract similar to that of the B. pertussis prototype strain Tohama I (Fig. 5).
Experimental respiratory mixed infection with GMT1 and GMT1i. If the colonization defect exhibited by GMT1i in the lower murine respiratory tract was due to an inability to express one or several Bvg+-phase-specific factors at wild-type levels, coinoculation of the mutant along with the wild-type strain in a mixed respiratory infection could potentially restore that defect. To test this hypothesis, we followed the experimental design outlined in the previous section, but this time the animals were inoculated with a mixed suspension of GMT1 and GMT1i (106 and 106 CFU) suspended in 50 μl of PBS. As shown in Fig. 6, the ability of GMT1 to colonize the murine respiratory tract was not affected by concomitant infection with GMT1i at any experimental time point since the wild-type strain colonized all sites of the coinfected animals at levels indistinguishable from those reached when administered unmixed (see Fig. 5 and two independent experiments [not shown]).
In contrast, the presence of the wild-type strain not only proved to be insufficient to restore the colonization defect displayed by GMT1i but contributed to make it more pronounced. Thus, unlike the behavior described in single-infection experiments, GMT1i was recovered in lower numbers than GMT1 (P < 0.05) from the lungs of the animals at day 0. These differences became much more prominent at day 8 postinoculation, when GMT1 outcompeted GMT1i not only, as expected, in the lower respiratory tract (P < 0.05), but also in the upper respiratory tract (P < 0.01). These results demonstrate that the Bvg+ phase is better endowed than the Bvgi phase for colonization of the upper murine respiratory tract, although the latter phase can make up for the former in the absence of competition between the two. In addition, our results confirm that the expression of the Bvg+ phase is absolutely required, at least temporarily, for the colonization of the lower respiratory tract. Finally, our observations indicate that the virulence factors that enable GMT1 to reach wild-type levels of colonization in the murine respiratory tract cannot be exploited by its isogenic B. pertussis Bvgi mutant to restore its colonization defect.
Assessment of the role of BipA in the pathogenesis of B. pertussis. To determine whether BipA plays a role in the pathogenicity of B. pertussis, we first constructed a mutant equivalent to GMT1i-DBA (i.e., carrying an in-frame deletion in bipA) in the wild-type strain (GMT1) background and designated it GMT1-DBA. We then compared the ability of GMT1-DBA to colonize and persist in the respiratory tract of mice with that of GMT1 with the same experimental design of respiratory infection described above. As shown in Fig. 7A, GMT1-DBA was found to be as capable as the wild-type strain to colonize all the sites of the murine respiratory tract during the entire course of the experiment. Our results are consistent with previous observations reporting that a B. bronchiseptica mutant carrying an in-frame deletion in bipA was not defective for the colonization of any site of the respiratory tract of the rabbit, one of its natural hosts (36).
Nevertheless, since the experiment shown in Fig. 7A was performed with bipA mutants constructed in the wild-type strain background, it cannot be ruled out that the bacteria never underwent a transition to the Bvgi phase in vivo in their experimental host. To determine whether actual expression of BipA in vivo provides B. pertussis with an adaptive advantage when this pathogen is genetically forced to express its Bvgi phase inside its host, we compared the ability of GMT1i to colonize the murine respiratory tract with that of its isogenic mutant carrying an in-frame deletion in bipA (GMT1i-DBA). At the same time, this experiment potentially allowed us to investigate whether BipA facilitates the initial interaction between B. pertussis and the respiratory epithelium by comparing the level of colonization reached by the unmodulated GMT1i and GMT1i-DBA cells (i.e., expressing and lacking BipA, respectively) shortly after the inoculation.
As shown in Fig. 7B, although GMT1i-DBA showed a tendency to colonize the murine respiratory tract at levels slightly lower than GMT1i, this difference was not found to be significant at the beginning of the experiment. Interestingly, this deficiency became more pronounced after 8 days of infection, and thus the level of colonization reached by the bipA mutant in the nasal septum at the end of the experiment was markedly lower (P = 0.002; i.e., P < 0.01) than that of its parental strain. It is important to point out that the actual inocula used in these assays (as in all the animal experiments presented in this work) were plated on BG-blood agar for viable counts to ensure that equal numbers of CFU of the corresponding strain had been delivered to the animals (n = 8 in this particular experiment).
Search for serological markers indicating that B. pertussis may modulate to the Bvgi phase in vivo. To indirectly assess whether wild-type B. pertussis undergoes a transition to the Bvgi phase in vivo, we screened sera from 100 patients (80 children and 20 adults) convalescent from pertussis for anti-Bvgi-phase-specific antibodies. For that purpose, whole-cell extracts of B. pertussis GMT1 and GMT1i were separated by SDS-PAGE in adjacent gel lanes, transferred to polyvinylidene difluoride, and probed with the corresponding serum sample. As a positive control, duplicate lysates were probed with antiserum S3. Whereas the presence of an antibody response of varying intensity against Bvg+-phase-specific polypeptides and Bvg-independent polypeptides (stars and arrows, respectively, in Fig. 8) was often detectable in the sera, antibodies against Bvgi-phase-specific polypeptides were not detected in any sample. In light of these results, it was not surprising to likewise be unable to detect this type of antibodies in sera from a nonnatural host of B. pertussis (the mouse) infected with GMT1 for at least 20 days (data not shown). In contrast, positive detection of BipA by antiserum S3 in all the assays demonstrated that the protein was present in the GMT1i extract. These results suggest that if B. pertussis undergoes a transition to the Bvgi phase in vivo, this phenomenon (temporally and/or spatially) passes unnoticed by the immune system of its host.
DISCUSSION
Despite intense investigation since Lacey's pioneering observations in 1960, the role of phenotypic modulation in the B. pertussis life cycle remains an enigma. Results from two independent reports performed with different experimental models of respiratory infection showed that a Bvg+-phase-locked mutant of B. pertussis colonized the respiratory tract as efficiently as the wild type (23, 27) whereas a Bvg–-phase-locked mutant was cleared soon after the beginning of the experiment, even from the upper respiratory tract (23). In addition, both studies showed that B. pertussis mutants manipulated to ectopically express vrgs displayed a significant colonization defect in the lower respiratory tract of mice. These observations suggest that the Bvg+ phase of B. pertussis is necessary and sufficient for the colonization of the murine respiratory tract. In contrast, the observation that vrg6 was not required for full B. pertussis virulence in the mouse, as was initially reported, and that the transcriptional activity from the vrg6 promoter was very low throughout the respiratory infection (23) argue against a role for the Bvg– phase during respiratory infection.
Paradoxically, the host seems to be the only niche where the BvgAS-mediated transition to the Bvg– phase in B. pertussis can be expected to occur, since this Bordetella species, unlike its close relative B. bronchiseptica (9), is not able to survive under the nutrient-limiting conditions prevailing in the environment. Alternatively, as other authors have proposed, the Bvg– phase could be a nonfunctional remnant that B. pertussis retained from its progenitor, B. bronchiseptica (9). However, even if the latter hypothesis is correct, the BvgAS-mediated phenotypic modulation may still be necessary in the life cycle of B. pertussis to facilitate its alteration at least between the Bvg+ and the Bvgi phases, as the present study indicates.
First, our results show that B. pertussis expresses the Bvgi phase in vitro when growing at 30°C, thereby confirming early observations reported by Lacey, though in a different strain background (21). Although a number of environmental signals that Bordetella can sense may be important in vivo, temperature appears, in all likelihood, as a relevant modulator for B. pertussis inside its host. In fact, the temperature in the human nasal cavity ranges from 30.3 to 32.3°C when measured at an ambient temperature of 23°C (6, 18) and is notably similar to that of rabbits and small rodents (6, 17, 18). Therefore, it seems likely that B. pertussis may express its Bvgi phase in that niche. Consistent with this hypothesis, the only site of the respiratory tract where GMT1i was able to persist at wild-type levels was the upper respiratory tract. These results demonstrate that the Bvg+ phase of B. pertussis is not strictly necessary for the colonization of the upper respiratory tract of mice.
In conflict with a potential role for the Bvgi phase in vivo, we failed to detect anti-BipA-specific antibodies in a large collection of sera from patients convalescent from pertussis. To reconcile these observations, we propose that the Bvgi phase of B. pertussis may be necessary for transmission between hosts, as may be the case for B. bronchiseptica (8). According to this hypothesis, which has been previously postulated for the members of the B. bronchiseptica cluster (9), transition to the Bvgi phase in response to low temperature in the distal upper respiratory tract might facilitate the release of B. pertussis from the nasal epithelium and/or the initial interaction with the new host. In line with our results, this hypothetical transition should be short-lasting enough to pass unnoticed to the immune system of the host, thereby precluding the elicitation of an anti-BipA-specific antibody response. Likewise, in its new host B. pertussis may need to undergo a transition back to the Bvg+ phase to prevent immune recognition of BipA and subsequent production of anti-BipA antibodies.
Alternatively, although we have demonstrated by immunizing rats with GMT1i that B. pertussis BipA is highly immunogenic, the possibility remains that this protein may not be immunogenic in humans naturally infected with B. pertussis. If so, this feature of B. pertussis BipA would be exclusive of the human pathogen since B. bronchiseptica BipA has also been shown to be highly immunogenic in both naturally (Martínez de Tejada and Lorenzo-Pajuelo, unpublished results) and experimentally infected animals (36).
Based on our results, several lines of evidence support a hypothetical role of the Bvgi phase in the transmission of B. pertussis. First, we have shown that a B. pertussis strain expressing the Bvgi phase (GMT1i grown under nonmodulating conditions) can establish an initial colonization in the nasal septum and the trachea of mice at wild-type levels even when coinoculated with an isogenic strain expressing the Bvg+ phase (GMT1 grown under nonmodulating conditions). More importantly, we demonstrated that in the absence of wild-type cells GMT1i can persist in the upper respiratory tract as efficiently as the wild-type strain. Taken together, these observations suggest that a wild-type B. pertussis strain expressing the Bvgi phase would be sufficiently well endowed to initiate the colonization of the respiratory tract immediately after a transmission event.
Interestingly, our results show that the expression of BipA conferred on GMT1i a noticeably higher (although non-statistically significant) ability to establish colonization in both the upper and lower respiratory tracts of mice compared with a BipA– isogenic strain. In this context, it is tempting to speculate that the poor interaction of the bipA derivative of GMT1i with the murine respiratory tract at the initial stages of infection may have resulted at a later experimental time point in the marked defect in colonization displayed by this strain in the upper respiratory tract. On the other hand, results from the coinoculation experiments show that the Bvg+ phase is also better endowed than the Bvgi phase for the colonization of the upper murine respiratory tract. This fact indicates that the Bvgi phase does not appear to be a phenotypic phase specialized in the colonization of any site of the respiratory tract, not even of the nasal cavity. On the contrary, our observations are consistent with the rapid transition from the Bvgi phase to the Bvg+ phase postulated by our hypothesis. Finally, the fact that loss of bipA by deletion in the wild-type strain background did not cause any detectable colonization defect supports our hypothesis that BipA expression is not strictly required for the colonization of the respiratory tract but rather necessary only immediately before and/or after a transmission event.
Nevertheless, a definitive conclusion cannot be formulated since the specific conditions encountered by B. pertussis in the mouse respiratory tract may not be adequate to promote a transition to the Bvgi phase. Obviously, in this hypothetical scenario, our animal model would not allow us to detect a colonization defect caused by the absence of BipA. To address this specific point, we plan to monitor bipA transcription during the experimental respiratory infection with an in vivo reporter system. Furthermore, to more stringently test our hypothesis, we plan to carry out experiments with B. bronchiseptica (which possesses Bvgi phenotypes very similar to those of the human pathogen [11 and results shown in the present work]) and a model mimicking natural transmission to investigate whether immunization with BipA confers on the animals a certain degree of protection from being naturally infected. Likewise, we will test whether BipA-immunized animals are less infectious than those not immunized but similarly infected with wild-type B. bronchiseptica.
The biochemical characterization of GMT1 and GMT1i revealed that, as shown in B. bronchiseptica (12, 36), BvgAS also controls the expression of BipA at the level of transcription in B. pertussis. This conclusion is in good agreement with recent results published by Fuschlocher and collaborators with the same mutant whose construction is reported in the present work (14). In addition, our results show that BipA expression is undetectable under fully modulating (Bvg– phase) conditions (16 mM nicotinic acid in our experiments). Consistent with this, the bipA-lacZ fusion was found to be fully inactive when introduced in the chromosome of a Bvg–-phase-locked mutant (GG2) and this strain did not produce detectable amounts of BipA. Therefore, taken together, our results demonstrate that bipA is a Bvgi-phase-specific gene. This conclusion confirms previous studies on B. bronchiseptica bipA (12, 36) and B. pertussis bipA (14) but contradicts conclusions from other authors reporting either upregulation of bipA under Bvg–-phase conditions, thus regarding bipA as a vrg (2) or irregular transcriptional behaviors (16).
At the protein level, comparison of the Bvgi-phase-specific antigenic profile of GMT1i with that of its isogenic bipA mutant (GMT1i-DBA) indicates that BipA is the only B. pertussis Bip detectable with serum raised against the Bvgi phase of either B. bronchiseptica or B. pertussis. Interestingly, DNA microarray analyses have revealed the existence of additional Bvgi-phase-specific genes in Bordetella (5). To circumvent a potential interference caused by BipA (i.e., immunodominance) and to simultaneously complement at the protein level this search for Bvgi-phase polypeptide-encoding genes, we plan to use an antiserum raised against GMT1i-DBA to identify Bvgi-phase polypeptides other than BipA.
Finally, our results also show that the Bvgi phases of B. pertussis and B. bronchiseptica differ markedly in the range of modulating conditions at which they are expressed. Thus, whereas the Bvgi phase of B. bronchiseptica is detectable over a narrow range of concentrations of nicotinic acid (approximately 0 to 1.6 mM) (11), B. pertussis needs a larger amount of modulating agent (up to 2 mM nicotinic acid) to switch to the Bvgi phase. In this regard, it is worth noting that, unlike strain Tohama I, GMT1 belongs to a group of B. pertussis strains characterized by their high sensitivity to modulation (24). Interestingly, with a B. bronchiseptica chimeric strain carrying the bvgAS locus of B. pertussis Tohama I, it has been shown that both upregulation and downregulation of bipA transcription require levels of nicotinic acid twice as high as those shown here for GMT1 (12). Whether these differences between the Bvgi phases of B. pertussis and B. bronchiseptica are related to peculiarities in their life cycle and their pathogenicity remains to be elucidated.
ACKNOWLEDGMENTS
We are indebted to Wirsing von Knig (German Reference Laboratory for Pertussis, Krefeld, Germany) and his collaborators for the generous gift of sera from patients convalescing from pertussis. We are also thankful to Trevor Stenson for providing us with scientific reports unavailable at our institution.
This work was supported by grants from the Ministerio de Sanidad y Consumo (FIS-01/0729) and from Proyectos de Investigacion Universidad de Navarra (PIUNA-9922) to G.M.T. and by doctoral fellowships from Gobierno de Navarra and from Friends of the University of Navarra Incorporated to N.V.-I. and A.C.-M., respectively.
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