Utility of the VITEK 2 Advanced Expert System for Identification of Extended-Spectrum -Lactamase Production in Enterobacter spp.
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微生物临床杂志 2006年第1期
Division of Epidemiology
Clinical Microbiology Laboratory, Tel Aviv Sourasky Medical Center, 6 Weizmann St., Tel Aviv, Israel
ABSTRACT
Forty clinical isolates of Enterobacter spp. were identified as extended-spectrum -lactamase (ESBL) producers by disk diffusion. The VITEK 2 Advanced Expert System (AES) identified the ESBL phenotype in only 25 isolates (62.5%), and erroneously reported cephalosporin susceptibility in 11 isolates (28%). Refinements in the AES are required in order to improve ESBL detection in Enterobacter.
TEXT
Accurate identification of extended-spectrum -lactamase (ESBL) production is essential for the appropriate reporting of antimicrobial susceptibility results, since ESBLs render penicillins, cephalosporins, and aztreonam inadequate for treatment of serious infections (2). While guidelines exist for ESBL detection in Escherichia coli, Klebsiella spp., and Proteus mirabilis isolates only (2), these enzymes are produced by a wide variety of other gram-negative organisms as well, including Enterobacter spp. (3, 8, 12, 13).
The VITEK 2 Advanced Expert System (AES; bioMerieux, Durham, NC) is an automated system that uses the antimicrobial susceptibility data generated to suggest the phenotype of the tested isolate and thereby determine susceptibility or resistance to antibiotics not tested (10). It has been used successfully to determine ESBL presence in E. coli and Klebsiella spp. (14, 16). Although ESBL detection in Enterobacter spp. by earlier automated systems, including VITEK, was hampered by the production of AmpC -lactamase (17), the more recently developed AES has not been evaluated for Enterobacter specifically. Given the high proportion of ESBL production among Enterobacter isolates at our institution and elsewhere (8, 12) and the fact that the AES includes ESBL production among the resistance mechanisms it suggests for Enterobacter isolates, we sought to determine the accuracy of the AES in ESBL detection among clinical isolates of Enterobacter.
Forty unique-patient isolates of Enterobacter were included (30 E. cloacae isolates and 10 E. aerogenes isolates). These epidemiologically unrelated isolates were identified phenotypically as ESBL producers via the disk diffusion method established for E. coli, Klebsiella spp., and P. mirabilis (2, 3), using 30-μg cefotaxime-, ceftazidime-, and cefepime-impregnated disks with and without 10 μg clavulanic acid (cefotaxime- and ceftazidime-containing disks made by Oxoid [Basingstoke, Hampshire, England]; cefepime-containing disks were prepared in-house). Although CLSI does not recommend the disk diffusion method for Enterobacter, we and others have previously noted a strong correlation between demonstration of a clavulanic acid effect and the presence of an ESBL gene (3, 12, 15). E. coli ATCC 25922 and Klebsiella pneumoniae ATCC 700603 were used as negative and positive controls for ESBL production, respectively.
Susceptibility profiles and suggested resistance mechanisms were recorded by the AES for each isolate. All isolates not identified as ESBL producers by VITEK were subjected to further study in order to demonstrate ESBL presence. Screening for the common ESBL gene families (TEM, SHV, OXA, and CTX-M) was performed by PCR (primers recorded in Table 1). Isolates for which PCR failed to amplify a known ESBL gene were subjected to further phenotypic testing to confirm the presence of a -lactamase able to hydrolyze ceftriaxone and inhibited by clavulanate, i.e., an ESBL. -Lactamase activity of sonicated cells was measured spectrophotometrically using 25 μM ceftriaxone in 50 mM phosphate buffer (pH 7.0) as the substrate. Inhibition by 2.5 μM clavulanate was measured after 5-min preincubation. In addition, these isolates were subjected to isoelectric focusing to locate the isoelectric point (pI) of the -lactamases produced, according to methods we have previously described (12).
The VITEK 2 AES identified 25 of 40 isolates (62.5%) as ESBL producers (Table 2). For 17, ESBL production was the only resistance mechanism suggested, and for 8, production of a high-level cephalosporinase was also proposed. Of the 15 isolates not identified by AES as ESBL producers (Tables 3 and 4), 11 (73%) were reported to be susceptible to at least one cephalosporin, contrary to published guidelines for susceptibility reporting in ESBL producers (2). The following alternative resistance mechanisms were reported for these isolates by AES: high-level cephalosporinase (13 isolates), impermeability (2 isolates), and other -lactamases (3 isolates).
In 6 of the 15 discrepant isolates, PCR amplified an ESBL gene (Table 3). Three PCR products belonging to the SHV family were confirmed as the ESBL gene blaSHV-12 by sequencing. The remaining three PCR products belonged to the CTX-M family of ESBL genes. ESBL activity was phenotypically confirmed in cell lysates of the nine discrepant isolates for which an ESBL gene was not amplified by demonstrating ceftriaxone hydrolysis that was inhibited in the presence of clavulanate (Table 4). Results of isoelectric focusing were consistent with the production of at least one -lactamase by each of these isolates. Their pI values are reported in Table 4.
A number of studies have sought to determine the reliability of the VITEK system for ESBL detection in Enterobacteriaceae isolates, most with satisfactory results (4-7, 9, 14, 16). These studies, however, have involved primarily isolates of E. coli and Klebsiella spp. ESBL detection in Enterobacter by automated systems is more complicated because of the production of chromosomally encoded AmpC-type enzymes, which, unlike ESBLs, are not inhibited by clavulanate and may even be induced by it and therefore may nullify the ability of the VITEK system to identify ESBL production based on the clavulanic acid effect. Indeed, in one study looking specifically at ESBL detection in Enterobacter spp. (17), of 31 ESBL-producing isolates, the VITEK detection test, using cefotaxime and ceftazidime alone and in combination with clavulanic acid, was positive for only 2 (6.5%).
The AES enhances the ability of the VITEK system to identify ESBLs by basing its phenotype determination on the distribution of MICs for various -lactam antibiotics rather than simply on neutralization by clavulanic acid (1, 6, 11). Sanders et al. found a high degree of accuracy of the AES in resistance mechanism detection in Enterobacter (92%), but insufficient data preclude a determination of the accuracy of the AES in ESBL detection specifically (11). Two groups have reported 90% agreement between the VITEK 2 AES and reference genotype data in ESBL detection overall in Enterobacteriaceae, including in AmpC-inducible species, though in each study, only a few Enterobacter isolates were tested (1, 6).
It seems apparent that the VITEK 2 AES, while an appropriate diagnostic tool for ESBL detection in E. coli and Klebsiella spp., is less reliable for their detection in Enterobacter spp. In our study, AES had a sensitivity of 62.5% in identifying ESBL-producing Enterobacter spp., compared with disk diffusion. In 15 discrepant cases in which molecular testing was performed, PCR confirmed the presence of an ESBL gene in 40%, and ceftriaxone hydrolysis inhibited by clavulanate in cell lysate confirmed ESBL production in the remainder. Since the definition of ESBLs is phenotypically based (2), these isolates should be considered ESBL producers despite the absence of amplification of the main ESBL gene families by PCR. Whether this absence is indicative of ESBL genes not belonging to the common gene families remains to be determined. An unknown percentage of ESBL-producing Enterobacter species will not be detected by either VITEK or disk diffusion. Thus, the true sensitivity of the AES in ESBL detection is likely even lower than we report.
Misidentification of ESBL producers by the AES led to erroneous reporting of cephalosporin susceptibility in nearly three-quarters of discrepant cases. Refinements in the AES are therefore required to improve the accuracy of ESBL detection in Enterobacter spp. Inclusion of cefepime or cefpirome alone and with a -lactamase inhibitor in susceptibility testing may improve performance, as these agents are less efficiently hydrohlyzed by AmpC enzymes than are earlier-generation cephalosporins (17). Theoretically, tazobactam may be a more appropriate -lactamase inhibitor than clavulanic acid, as it is a weaker inducer of AmpC enzymes. Additional studies are required before the VITEK 2 AES can be used as a sole method of detection of ESBL production in Enterobacter spp.
ACKNOWLEDGMENTS
This work was supported in part by a grant from the Sapir Fund, Mifal Hapayis, Israel.
REFERENCES
Canton, R., M. Perez-Vazquez, A. Oliver, T. M. Coque, E. Loza, F. Ponz, and F. Baquero. 2001. Validation of the VITEK2 and the Advance Expert System with a collection of Enterobacteriaceae harboring extended spectrum or inhibitor resistant -lactamases. Diagn. Microbiol. Infect. Dis. 41:65-70.
Clinical and Laboratory Standards Institute. 2005. Performance standards for antimicrobial susceptibility testing, 15th informational supplement. M100-S15. Clinical and Laboratory Standards Institute, Wayne, Pa.
Ho, P. L., R. H. Shek, K. H. Chow, R. S. Duan, G. C. Mak, E. L. Lai, W. C. Yam, K. W. Tsang, and W. M. Lai. 2005. Detection and characterization of extended-spectrum -lactamases among bloodstream isolates of Enterobacter spp. in Hong Kong, 2000-2002. J. Antimicrob. Chemother. 55:326-332.
Leverstein-van Hall, M. A., A. C. Fluit, A. Paauw, A. T. Box, S. Brisse, and J. Verhoef. 2002. Evaluation of the Etest ESBL and the BD Phoenix, VITEK 1, and VITEK 2 automated instruments for detection of extended-spectrum -lactamases in multiresistant Escherichia coli and Klebsiella spp. J. Clin. Microbiol. 40:3703-3711.
Linscott, A. J., and W. J. Brown. 2005. Evaluation of four commercially available extended-spectrum -lactamase phenotypic confirmation tests. J. Clin. Microbiol. 43:1081-1085.
Livermore, D. M., M. Struelens, J. Amorim, F. Baquero, J. Bille, R. Canton, S. Henning, S. Gatermann, A. Marchese, H. Mittermayer, C. Nonhoff, K. J. Oakton, F. Praplan, H. Ramos, G. C. Schito, J. Van Eldere, J. Verhaegen, J. Verhoef, and M. R. Visser. 2002. Multicentre evaluation of the VITEK 2 Advanced Expert System for interpretive reading of antimicrobial resistance tests. J. Antimicrob. Chemother. 49:289-300.
Midolo, P. D., D. Matthews, C. D. Fernandez, and T. G. Kerr. 2002. Detection of extended spectrum -lactamases in the routine clinical microbiology laboratory. Pathology 34:362-364.
Paterson, D. L., F. Rossi, F. Baquero, P. R. Hsueh, G. L. Woods, V. Satishchandran, T. A. Snyder, C. M. Harvey, H. Teppler, M. J. Dinubile, and J. W. Chow. 2005. In vitro susceptibilities of aerobic and facultative gram-negative bacilli isolated from patients with intra-abdominal infections worldwide: the 2003 Study for Monitoring Antimicrobial Resistance Trends (SMART). J. Antimicrob. Chemother. 55:965-973.
Sanders, C. C., A. L. Barry, J. A. Washington, C. Shubert, E. S. Moland, M. M. Traczewski, C. Knapp, and R. Mulder. 1996. Detection of extended-spectrum--lactamase-producing members of the family Enterobacteriaceae with Vitek ESBL test. J. Clin. Microbiol. 34:2997-3001.
Sanders, C. C., M. Peyret, E. S. Moland, S. J. Cavalieri, C. Shubert, K. S. Thomson, J. M. Boeufgras, and W. E. Sanders, Jr. 2001. Potential impact of the VITEK 2 system and the Advanced Expert System on the clinical laboratory of a university-based hospital. J. Clin. Microbiol. 39:2379-2385.
Sanders, C. C., M. Peyret, E. S. Moland, C. Shubert, K. S. Thomson, J. M. Boeufgras, and W. E. Sanders, Jr. 2000. Ability of the VITEK 2 advanced expert system to identify -lactam phenotypes in isolates of Enterobacteriaceae and Pseudomonas aeruginosa. J. Clin. Microbiol. 38:570-574.
Schlesinger, J., S. Navon-Venezia, I. Chmelnitsky, O. Hammer-Munz, A. Leavitt, H. S. Gold, M. J. Schwaber, and Y. Carmeli. 2005. Extended-spectrum -lactamases among Enterobacter isolates obtained in Tel Aviv, Israel. Antimicrob. Agents Chemother. 49:1150-1156.
Schwaber, M. J., P. M. Raney, J. K. Rasheed, J. W. Biddle, P. Williams, J. E. McGowan, Jr., and F. C. Tenover. 2004. Utility of NCCLS guidelines for identifying extended-spectrum -lactamases in non-Escherichia coli and non-Klebsiella spp. of Enterobacteriaceae. J. Clin. Microbiol. 42:294-298.
Sorlozano, A., J. Gutierrez, G. Piedrola, and M. J. Soto. 2005. Acceptable performance of VITEK 2 system to detect extended-spectrum -lactamases in clinical isolates of Escherichia coli: a comparative study of phenotypic commercial methods and NCCLS guidelines. Diagn. Microbiol. Infect. Dis. 51:191-193.
Sturenburg, E., I. Sobottka, D. Noor, R. Laufs, and D. Mack. 2004. Evaluation of a new cefepime-clavulanate ESBL Etest to detect extended-spectrum -lactamases in an Enterobacteriaceae strain collection. J. Antimicrob. Chemother. 54:134-138.
Sturenburg, E., I. Sobottka, H. H. Feucht, D. Mack, and R. Laufs. 2003. Comparison of BDPhoenix and VITEK2 automated antimicrobial susceptibility test systems for extended-spectrum -lactamase detection in Escherichia coli and Klebsiella species clinical isolates. Diagn. Microbiol. Infect. Dis. 45:29-34.
Tzelepi, E., P. Giakkoupi, D. Sofianou, V. Loukova, A. Kemeroglou, and A. Tsakris. 2000. Detection of extended-spectrum -lactamases in clinical isolates of Enterobacter cloacae and Enterobacter aerogenes. J. Clin. Microbiol. 38:542-546.(Mitchell J. Schwaber, Shi)
Clinical Microbiology Laboratory, Tel Aviv Sourasky Medical Center, 6 Weizmann St., Tel Aviv, Israel
ABSTRACT
Forty clinical isolates of Enterobacter spp. were identified as extended-spectrum -lactamase (ESBL) producers by disk diffusion. The VITEK 2 Advanced Expert System (AES) identified the ESBL phenotype in only 25 isolates (62.5%), and erroneously reported cephalosporin susceptibility in 11 isolates (28%). Refinements in the AES are required in order to improve ESBL detection in Enterobacter.
TEXT
Accurate identification of extended-spectrum -lactamase (ESBL) production is essential for the appropriate reporting of antimicrobial susceptibility results, since ESBLs render penicillins, cephalosporins, and aztreonam inadequate for treatment of serious infections (2). While guidelines exist for ESBL detection in Escherichia coli, Klebsiella spp., and Proteus mirabilis isolates only (2), these enzymes are produced by a wide variety of other gram-negative organisms as well, including Enterobacter spp. (3, 8, 12, 13).
The VITEK 2 Advanced Expert System (AES; bioMerieux, Durham, NC) is an automated system that uses the antimicrobial susceptibility data generated to suggest the phenotype of the tested isolate and thereby determine susceptibility or resistance to antibiotics not tested (10). It has been used successfully to determine ESBL presence in E. coli and Klebsiella spp. (14, 16). Although ESBL detection in Enterobacter spp. by earlier automated systems, including VITEK, was hampered by the production of AmpC -lactamase (17), the more recently developed AES has not been evaluated for Enterobacter specifically. Given the high proportion of ESBL production among Enterobacter isolates at our institution and elsewhere (8, 12) and the fact that the AES includes ESBL production among the resistance mechanisms it suggests for Enterobacter isolates, we sought to determine the accuracy of the AES in ESBL detection among clinical isolates of Enterobacter.
Forty unique-patient isolates of Enterobacter were included (30 E. cloacae isolates and 10 E. aerogenes isolates). These epidemiologically unrelated isolates were identified phenotypically as ESBL producers via the disk diffusion method established for E. coli, Klebsiella spp., and P. mirabilis (2, 3), using 30-μg cefotaxime-, ceftazidime-, and cefepime-impregnated disks with and without 10 μg clavulanic acid (cefotaxime- and ceftazidime-containing disks made by Oxoid [Basingstoke, Hampshire, England]; cefepime-containing disks were prepared in-house). Although CLSI does not recommend the disk diffusion method for Enterobacter, we and others have previously noted a strong correlation between demonstration of a clavulanic acid effect and the presence of an ESBL gene (3, 12, 15). E. coli ATCC 25922 and Klebsiella pneumoniae ATCC 700603 were used as negative and positive controls for ESBL production, respectively.
Susceptibility profiles and suggested resistance mechanisms were recorded by the AES for each isolate. All isolates not identified as ESBL producers by VITEK were subjected to further study in order to demonstrate ESBL presence. Screening for the common ESBL gene families (TEM, SHV, OXA, and CTX-M) was performed by PCR (primers recorded in Table 1). Isolates for which PCR failed to amplify a known ESBL gene were subjected to further phenotypic testing to confirm the presence of a -lactamase able to hydrolyze ceftriaxone and inhibited by clavulanate, i.e., an ESBL. -Lactamase activity of sonicated cells was measured spectrophotometrically using 25 μM ceftriaxone in 50 mM phosphate buffer (pH 7.0) as the substrate. Inhibition by 2.5 μM clavulanate was measured after 5-min preincubation. In addition, these isolates were subjected to isoelectric focusing to locate the isoelectric point (pI) of the -lactamases produced, according to methods we have previously described (12).
The VITEK 2 AES identified 25 of 40 isolates (62.5%) as ESBL producers (Table 2). For 17, ESBL production was the only resistance mechanism suggested, and for 8, production of a high-level cephalosporinase was also proposed. Of the 15 isolates not identified by AES as ESBL producers (Tables 3 and 4), 11 (73%) were reported to be susceptible to at least one cephalosporin, contrary to published guidelines for susceptibility reporting in ESBL producers (2). The following alternative resistance mechanisms were reported for these isolates by AES: high-level cephalosporinase (13 isolates), impermeability (2 isolates), and other -lactamases (3 isolates).
In 6 of the 15 discrepant isolates, PCR amplified an ESBL gene (Table 3). Three PCR products belonging to the SHV family were confirmed as the ESBL gene blaSHV-12 by sequencing. The remaining three PCR products belonged to the CTX-M family of ESBL genes. ESBL activity was phenotypically confirmed in cell lysates of the nine discrepant isolates for which an ESBL gene was not amplified by demonstrating ceftriaxone hydrolysis that was inhibited in the presence of clavulanate (Table 4). Results of isoelectric focusing were consistent with the production of at least one -lactamase by each of these isolates. Their pI values are reported in Table 4.
A number of studies have sought to determine the reliability of the VITEK system for ESBL detection in Enterobacteriaceae isolates, most with satisfactory results (4-7, 9, 14, 16). These studies, however, have involved primarily isolates of E. coli and Klebsiella spp. ESBL detection in Enterobacter by automated systems is more complicated because of the production of chromosomally encoded AmpC-type enzymes, which, unlike ESBLs, are not inhibited by clavulanate and may even be induced by it and therefore may nullify the ability of the VITEK system to identify ESBL production based on the clavulanic acid effect. Indeed, in one study looking specifically at ESBL detection in Enterobacter spp. (17), of 31 ESBL-producing isolates, the VITEK detection test, using cefotaxime and ceftazidime alone and in combination with clavulanic acid, was positive for only 2 (6.5%).
The AES enhances the ability of the VITEK system to identify ESBLs by basing its phenotype determination on the distribution of MICs for various -lactam antibiotics rather than simply on neutralization by clavulanic acid (1, 6, 11). Sanders et al. found a high degree of accuracy of the AES in resistance mechanism detection in Enterobacter (92%), but insufficient data preclude a determination of the accuracy of the AES in ESBL detection specifically (11). Two groups have reported 90% agreement between the VITEK 2 AES and reference genotype data in ESBL detection overall in Enterobacteriaceae, including in AmpC-inducible species, though in each study, only a few Enterobacter isolates were tested (1, 6).
It seems apparent that the VITEK 2 AES, while an appropriate diagnostic tool for ESBL detection in E. coli and Klebsiella spp., is less reliable for their detection in Enterobacter spp. In our study, AES had a sensitivity of 62.5% in identifying ESBL-producing Enterobacter spp., compared with disk diffusion. In 15 discrepant cases in which molecular testing was performed, PCR confirmed the presence of an ESBL gene in 40%, and ceftriaxone hydrolysis inhibited by clavulanate in cell lysate confirmed ESBL production in the remainder. Since the definition of ESBLs is phenotypically based (2), these isolates should be considered ESBL producers despite the absence of amplification of the main ESBL gene families by PCR. Whether this absence is indicative of ESBL genes not belonging to the common gene families remains to be determined. An unknown percentage of ESBL-producing Enterobacter species will not be detected by either VITEK or disk diffusion. Thus, the true sensitivity of the AES in ESBL detection is likely even lower than we report.
Misidentification of ESBL producers by the AES led to erroneous reporting of cephalosporin susceptibility in nearly three-quarters of discrepant cases. Refinements in the AES are therefore required to improve the accuracy of ESBL detection in Enterobacter spp. Inclusion of cefepime or cefpirome alone and with a -lactamase inhibitor in susceptibility testing may improve performance, as these agents are less efficiently hydrohlyzed by AmpC enzymes than are earlier-generation cephalosporins (17). Theoretically, tazobactam may be a more appropriate -lactamase inhibitor than clavulanic acid, as it is a weaker inducer of AmpC enzymes. Additional studies are required before the VITEK 2 AES can be used as a sole method of detection of ESBL production in Enterobacter spp.
ACKNOWLEDGMENTS
This work was supported in part by a grant from the Sapir Fund, Mifal Hapayis, Israel.
REFERENCES
Canton, R., M. Perez-Vazquez, A. Oliver, T. M. Coque, E. Loza, F. Ponz, and F. Baquero. 2001. Validation of the VITEK2 and the Advance Expert System with a collection of Enterobacteriaceae harboring extended spectrum or inhibitor resistant -lactamases. Diagn. Microbiol. Infect. Dis. 41:65-70.
Clinical and Laboratory Standards Institute. 2005. Performance standards for antimicrobial susceptibility testing, 15th informational supplement. M100-S15. Clinical and Laboratory Standards Institute, Wayne, Pa.
Ho, P. L., R. H. Shek, K. H. Chow, R. S. Duan, G. C. Mak, E. L. Lai, W. C. Yam, K. W. Tsang, and W. M. Lai. 2005. Detection and characterization of extended-spectrum -lactamases among bloodstream isolates of Enterobacter spp. in Hong Kong, 2000-2002. J. Antimicrob. Chemother. 55:326-332.
Leverstein-van Hall, M. A., A. C. Fluit, A. Paauw, A. T. Box, S. Brisse, and J. Verhoef. 2002. Evaluation of the Etest ESBL and the BD Phoenix, VITEK 1, and VITEK 2 automated instruments for detection of extended-spectrum -lactamases in multiresistant Escherichia coli and Klebsiella spp. J. Clin. Microbiol. 40:3703-3711.
Linscott, A. J., and W. J. Brown. 2005. Evaluation of four commercially available extended-spectrum -lactamase phenotypic confirmation tests. J. Clin. Microbiol. 43:1081-1085.
Livermore, D. M., M. Struelens, J. Amorim, F. Baquero, J. Bille, R. Canton, S. Henning, S. Gatermann, A. Marchese, H. Mittermayer, C. Nonhoff, K. J. Oakton, F. Praplan, H. Ramos, G. C. Schito, J. Van Eldere, J. Verhaegen, J. Verhoef, and M. R. Visser. 2002. Multicentre evaluation of the VITEK 2 Advanced Expert System for interpretive reading of antimicrobial resistance tests. J. Antimicrob. Chemother. 49:289-300.
Midolo, P. D., D. Matthews, C. D. Fernandez, and T. G. Kerr. 2002. Detection of extended spectrum -lactamases in the routine clinical microbiology laboratory. Pathology 34:362-364.
Paterson, D. L., F. Rossi, F. Baquero, P. R. Hsueh, G. L. Woods, V. Satishchandran, T. A. Snyder, C. M. Harvey, H. Teppler, M. J. Dinubile, and J. W. Chow. 2005. In vitro susceptibilities of aerobic and facultative gram-negative bacilli isolated from patients with intra-abdominal infections worldwide: the 2003 Study for Monitoring Antimicrobial Resistance Trends (SMART). J. Antimicrob. Chemother. 55:965-973.
Sanders, C. C., A. L. Barry, J. A. Washington, C. Shubert, E. S. Moland, M. M. Traczewski, C. Knapp, and R. Mulder. 1996. Detection of extended-spectrum--lactamase-producing members of the family Enterobacteriaceae with Vitek ESBL test. J. Clin. Microbiol. 34:2997-3001.
Sanders, C. C., M. Peyret, E. S. Moland, S. J. Cavalieri, C. Shubert, K. S. Thomson, J. M. Boeufgras, and W. E. Sanders, Jr. 2001. Potential impact of the VITEK 2 system and the Advanced Expert System on the clinical laboratory of a university-based hospital. J. Clin. Microbiol. 39:2379-2385.
Sanders, C. C., M. Peyret, E. S. Moland, C. Shubert, K. S. Thomson, J. M. Boeufgras, and W. E. Sanders, Jr. 2000. Ability of the VITEK 2 advanced expert system to identify -lactam phenotypes in isolates of Enterobacteriaceae and Pseudomonas aeruginosa. J. Clin. Microbiol. 38:570-574.
Schlesinger, J., S. Navon-Venezia, I. Chmelnitsky, O. Hammer-Munz, A. Leavitt, H. S. Gold, M. J. Schwaber, and Y. Carmeli. 2005. Extended-spectrum -lactamases among Enterobacter isolates obtained in Tel Aviv, Israel. Antimicrob. Agents Chemother. 49:1150-1156.
Schwaber, M. J., P. M. Raney, J. K. Rasheed, J. W. Biddle, P. Williams, J. E. McGowan, Jr., and F. C. Tenover. 2004. Utility of NCCLS guidelines for identifying extended-spectrum -lactamases in non-Escherichia coli and non-Klebsiella spp. of Enterobacteriaceae. J. Clin. Microbiol. 42:294-298.
Sorlozano, A., J. Gutierrez, G. Piedrola, and M. J. Soto. 2005. Acceptable performance of VITEK 2 system to detect extended-spectrum -lactamases in clinical isolates of Escherichia coli: a comparative study of phenotypic commercial methods and NCCLS guidelines. Diagn. Microbiol. Infect. Dis. 51:191-193.
Sturenburg, E., I. Sobottka, D. Noor, R. Laufs, and D. Mack. 2004. Evaluation of a new cefepime-clavulanate ESBL Etest to detect extended-spectrum -lactamases in an Enterobacteriaceae strain collection. J. Antimicrob. Chemother. 54:134-138.
Sturenburg, E., I. Sobottka, H. H. Feucht, D. Mack, and R. Laufs. 2003. Comparison of BDPhoenix and VITEK2 automated antimicrobial susceptibility test systems for extended-spectrum -lactamase detection in Escherichia coli and Klebsiella species clinical isolates. Diagn. Microbiol. Infect. Dis. 45:29-34.
Tzelepi, E., P. Giakkoupi, D. Sofianou, V. Loukova, A. Kemeroglou, and A. Tsakris. 2000. Detection of extended-spectrum -lactamases in clinical isolates of Enterobacter cloacae and Enterobacter aerogenes. J. Clin. Microbiol. 38:542-546.(Mitchell J. Schwaber, Shi)