Clinical Significance of Coagulase-Negative Staphylococci Recovered fr
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《微生物临床杂志》
Laboratory Medicine Services, Changi General Hospital, Singapore
ABSTRACT
Laboratory criteria were used to select coagulase-negative staphylococci isolated from nonsterile body sites for further investigation. Fifty-seven percent of the study isolates were clinically significant, predominantly causing community-acquired soft tissue infections. There were species-related differences in virulence, with 91% of Staphylococcus lugdunensis isolates clinically significant compared with 11% of S. haemolyticus isolates.
TEXT
Staphylococci are normal skin commensals (1) and are frequently isolated from clinical specimens. The isolation of coagulase-negative staphylococci (CoNS) from nonsterile sites is usually ignored or reported without further species level identification. Routine species level identification may better define the clinical spectrum of disease caused by CoNS. However, the reference method (7) is labor-intensive, while the use of commercial identification kits is not cost-effective.
This study used a simple laboratory algorithm (Table 1) to select CoNS isolated from nonsterile sites for further investigation using an abbreviated identification scheme (4, 6). Quantitative culture using a calibrated loop system (8) was performed for urine specimens, while other specimens were inoculated using semiquantitative methods (14).
CoNS were identified by Gram stain, the presence of catalase, bacitracin resistance, and the absence of free coagulase (1). Isolates positive for ornithine decarboxylase and pyrrolidonyl arylamidase (OBIS PYR; Oxoid, United Kingdom) were identified as Staphylococcus lugdunensis (9). Urinary isolates that were resistant to novobiocin, exhibited urease activity, and failed to reduce nitrates were identified as S. saprophyticus (subspecies saprophyticus) (1). Following these initial screening tests, CoNS isolates were identified to the species level by a simplified two-step identification method (6). CoNS that remained unidentified were tested using ID32 Staph (bioMerieux, France) and ID-GP cards (bioMerieux, France). Clinical details were collected retrospectively from patient case records. Statistical data were generated using StatCalc (EpiInfo; CDC). Isolates were classified as nosocomial if the sample was collected more than 48 h following admission to the hospital. All other isolates were classified as community acquired. Standardized criteria for the diagnosis of nosocomial infections were used to determine the clinical significance of test isolates (3, 5).
One hundred seventy-nine CoNS isolates were cultured from 168 unique samples, which were predominantly swabs (n = 126; 70%) and urine (n = 25; 14%). Eighty-seven (49%) of the isolates were cultured from operative samples, and 145 (81%) of the isolates were classified as community acquired. A single colonial morphotype of CoNS was present in primary culture from 156 samples, while two morphotypes of CoNS were present in the remaining 12 samples. Nineteen (11%) CoNS isolates were isolated in mixed cultures, with 1+ growth of other organisms, predominantly diptheroids. Assessment of semiquantitative growth (14) showed that 77 isolates (51%) were recovered in 1+ growth, 62 (41%) in 2+ growth, and 12 (8%) in 3+ growth. Four isolates were not identified by the simplified testing method, three of which remained unidentifiable by the commercial identification systems. The most common species was S. epidermidis, followed by S. lugdunensis. The latter was overrepresented in samples taken from the pelvic girdle, accounting for 55% of isolates from this region of the body (relative risk [RR], 2.6; 95% confidence interval, 1.7 to 4.0). Other CoNS were equally distributed across other body sites.
One hundred two (57%) isolates were found to be clinically significant. No laboratory features were sufficiently discriminative to differentiate clinically significant isolates from commensal skin flora. Community-acquired CoNS isolates were more likely to be clinically significant than nosocomial isolates (RR, 3.7; 95% confidence interval, 1.8 to 7.8). S. lugdunensis was the predominant CoNS causing clinically significant infections (n = 48; 45% of clinically significant isolates), followed by S. epidermidis (n = 34; 33%). S. lugdunensis was predominantly associated with superficial abscesses and infections of sebaceous glands (Table 2) . The pathogenicity of CoNS varied across different species. The most virulent species was S. lugdunensis, with 91% of all isolates associated with clinically significant infections, followed by S. saprophyticus (88%). In contrast, 52% and 11% of S. epidermidis and S. haemolyticus isolates, respectively, were clinically significant.
CoNS are common isolates from nonsterile body sites and have traditionally been described as skin commensals, with substantially less virulence than S. aureus. There are few data on differences in virulence and pathogenicity among species of CoNS. In this study, CoNS isolates from nonsterile sites were selected using simple laboratory criteria. More than half of the study isolates were associated with clinically significant infections, and three species (S. lugdunensis, S. epidermidis, and S. saprophyticus) accounted for 86% of these infections. An earlier study reported that only 21% of CoNS isolates were considered to be clinically significant (10). However, the earlier study was conducted without preselection criteria and included cultures from sterile body sites.
The data from this study reinforce the propensity of S. lugdunensis to be associated with acute cutaneous infections. Previous investigators have reported the involvement of S. lugdunensis in breast and soft tissue abscesses (2, 11, 13), and other reports have also noted the frequent isolation of this organism from the pelvic region (2, 12).
Most of the clinically significant infections were community acquired. The hospital population we studied did not include significant numbers of immunocompromised patients. Also, nosocomial isolates of CoNS are commonly present in polymicrobial cultures, which would have been excluded by the screening criteria. None of the other laboratory criteria analyzed for the study isolates were able to differentiate clinically significant isolates from colonizers. Density of growth, measured by semiquantitative methods, or the presence of a single CoNS species on culture did not prove sufficiently discriminative.
Some factors may have influenced the results of this study. First, the standardized clinical criteria used to prevent bias were designed for the surveillance of nosocomial infections and may not be entirely suitable for the diagnosis of community-acquired infections. Secondly, prior antibiotic therapy may have influenced the culture results. However, no documented evidence of prior antibiotic therapy was noted in most samples received from community sources.
The results of this study suggest that when the study screening algorithm is applied to CoNS isolated from community-acquired infections, over two-thirds of isolates will be clinically significant. There was evidence of species-related variation in virulence, with S. lugdunensis and S. saprophyticus identified as the most pathogenic isolates. A combination of a simplified identification system for CoNS, such as that used in this study, and the application of the screening algorithmn would allow clinical laboratories to correctly identify and report the CoNS isolates that are most likely to be clinically relevant.
ACKNOWLEDGMENTS
This work was supported by a grant from the National Medical Research Council, Singapore.
FOOTNOTES
Corresponding author. Mailing address: Laboratory Medicine Services, Changi General Hospital, 2 Simei Street 3, Singapore, 529889. Phone: 065-68504935. Fax: 065-64269507. E-mail: siew_yong_ng@cgh.com.sg.
REFERENCES
Bannerman, T. L. 2003. Staphylococcus, Micrococcus, and other catalase-positive cocci that grow aerobically, p. 384-404. In P. R. Murray, E. J. Baron, J. H. Jorgensen, M. A. Pfaller, and R. H. Yolken (ed.), Manual of Clinical Microbiology, 8th ed., vol. 1. American Society for Microbiology, Washington, D.C.
Bellamy, R., and T. Barkham. 2002. Staphylococcus lugdunensis infection sites: predominance of abscesses in the pelvic girdle region. Clin. Infect. Dis. 35:E32-E34.
Crowe, M. J., and E. M. Cooke. 1998. Review of case definitions for nosocomial infection—towards a consensus. Presentation by the Nosocomial Infection Surveillance Unit (NISU) to the Hospital Infection Liaison Group, subcommittee of the Federation of Infection Societies (FIS). J. Hosp. Infect. 39:3-11.
De Paulis, A. N., S. C. Predari, C. D. Chazarreta, and J. E. Santoianni. 2003. Five-test simple scheme for species-level identification of clinically significant coagulase-negative staphylococci. J. Clin. Microbiol. 41:1219-1224.
Emori, T. G., D. H. Culver, T. C. Horan, W. R. Jarvis, J. W. White, D. R. Olson, S. Banerjee, J. R. Edwards, W. J. Martone, and R. P. Gaynes. 1991. National nosocomial infections surveillance system (NNIS): description of surveillance methods. Am. J. Infect. Control 19:19-35.
Ieven, M., J. Verhoeven, S. R. Pattyn, and H. Goossens. 1995. Rapid and economical method for species identification of clinically significant coagulase-negative staphylococci. J. Clin. Microbiol. 33:1060-1063.
Kloos, W. E., and K. H. Schleifer. 1975. Simplified scheme for routine identification of human Staphylococcus species. J. Clin. Microbiol. 1:82-88.
Pezzlo, M., and M. K. York. 2004. Urine cultures, p. 3.12.1-3.12.31. In H. D. Isenberg (ed.), Clinical microbiology procedures handbook, 2nd ed., vol. 1. American Society for Microbiology, Washington, D.C.
Schnitzler, N., R. Meilicke, G. Conrads, D. Frank, and G. Haase. 1998. Staphylococcus lugdunensis: report of a case of peritonitis and an easy-to-perform screening strategy. J. Clin. Microbiol. 36:812-813.
Sewell, C. M., J. E. Clarridge, E. J. Young, and R. K. Guthrie. 1982. Clinical significance of coagulase-negative staphylococci. J. Clin. Microbiol. 16:236-239.
Vandenesch, F., S. J. Eykyn, J. Etienne, and J. Lemozy. 1995. Skin and post-surgical wound infections due to Staphylococcus lugdunensis. Clin. Microbiol. Infect. 1:73-74.
van der Mee-Marquet, N., A. Achard, L. Mereghetti, A. Danton, M. Minier, and R. Quentin. 2003. Staphylococcus lugdunensis infections: high frequency of inguinal area carriage. J. Clin. Microbiol. 41:1404-1409.
Waghorn, D. J. 1994. Staphylococcus lugdunensis as a cause of breast abscess. Clin. Infect. Dis. 19:814-815.
York, M. K. 2004. Processing, isolation, detection, and interpretation of aerobic bacteriology cultures, p. 3.3.2.1-3.3.2.14. In H. D. Isenberg (ed.), Clinical microbiology procedures handbook, 2nd ed., vol. 1. American Society for Microbiology, Washington, D.C.(Thean Yen Tan, Siew Yong Ng, and Wan Xin)
ABSTRACT
Laboratory criteria were used to select coagulase-negative staphylococci isolated from nonsterile body sites for further investigation. Fifty-seven percent of the study isolates were clinically significant, predominantly causing community-acquired soft tissue infections. There were species-related differences in virulence, with 91% of Staphylococcus lugdunensis isolates clinically significant compared with 11% of S. haemolyticus isolates.
TEXT
Staphylococci are normal skin commensals (1) and are frequently isolated from clinical specimens. The isolation of coagulase-negative staphylococci (CoNS) from nonsterile sites is usually ignored or reported without further species level identification. Routine species level identification may better define the clinical spectrum of disease caused by CoNS. However, the reference method (7) is labor-intensive, while the use of commercial identification kits is not cost-effective.
This study used a simple laboratory algorithm (Table 1) to select CoNS isolated from nonsterile sites for further investigation using an abbreviated identification scheme (4, 6). Quantitative culture using a calibrated loop system (8) was performed for urine specimens, while other specimens were inoculated using semiquantitative methods (14).
CoNS were identified by Gram stain, the presence of catalase, bacitracin resistance, and the absence of free coagulase (1). Isolates positive for ornithine decarboxylase and pyrrolidonyl arylamidase (OBIS PYR; Oxoid, United Kingdom) were identified as Staphylococcus lugdunensis (9). Urinary isolates that were resistant to novobiocin, exhibited urease activity, and failed to reduce nitrates were identified as S. saprophyticus (subspecies saprophyticus) (1). Following these initial screening tests, CoNS isolates were identified to the species level by a simplified two-step identification method (6). CoNS that remained unidentified were tested using ID32 Staph (bioMerieux, France) and ID-GP cards (bioMerieux, France). Clinical details were collected retrospectively from patient case records. Statistical data were generated using StatCalc (EpiInfo; CDC). Isolates were classified as nosocomial if the sample was collected more than 48 h following admission to the hospital. All other isolates were classified as community acquired. Standardized criteria for the diagnosis of nosocomial infections were used to determine the clinical significance of test isolates (3, 5).
One hundred seventy-nine CoNS isolates were cultured from 168 unique samples, which were predominantly swabs (n = 126; 70%) and urine (n = 25; 14%). Eighty-seven (49%) of the isolates were cultured from operative samples, and 145 (81%) of the isolates were classified as community acquired. A single colonial morphotype of CoNS was present in primary culture from 156 samples, while two morphotypes of CoNS were present in the remaining 12 samples. Nineteen (11%) CoNS isolates were isolated in mixed cultures, with 1+ growth of other organisms, predominantly diptheroids. Assessment of semiquantitative growth (14) showed that 77 isolates (51%) were recovered in 1+ growth, 62 (41%) in 2+ growth, and 12 (8%) in 3+ growth. Four isolates were not identified by the simplified testing method, three of which remained unidentifiable by the commercial identification systems. The most common species was S. epidermidis, followed by S. lugdunensis. The latter was overrepresented in samples taken from the pelvic girdle, accounting for 55% of isolates from this region of the body (relative risk [RR], 2.6; 95% confidence interval, 1.7 to 4.0). Other CoNS were equally distributed across other body sites.
One hundred two (57%) isolates were found to be clinically significant. No laboratory features were sufficiently discriminative to differentiate clinically significant isolates from commensal skin flora. Community-acquired CoNS isolates were more likely to be clinically significant than nosocomial isolates (RR, 3.7; 95% confidence interval, 1.8 to 7.8). S. lugdunensis was the predominant CoNS causing clinically significant infections (n = 48; 45% of clinically significant isolates), followed by S. epidermidis (n = 34; 33%). S. lugdunensis was predominantly associated with superficial abscesses and infections of sebaceous glands (Table 2) . The pathogenicity of CoNS varied across different species. The most virulent species was S. lugdunensis, with 91% of all isolates associated with clinically significant infections, followed by S. saprophyticus (88%). In contrast, 52% and 11% of S. epidermidis and S. haemolyticus isolates, respectively, were clinically significant.
CoNS are common isolates from nonsterile body sites and have traditionally been described as skin commensals, with substantially less virulence than S. aureus. There are few data on differences in virulence and pathogenicity among species of CoNS. In this study, CoNS isolates from nonsterile sites were selected using simple laboratory criteria. More than half of the study isolates were associated with clinically significant infections, and three species (S. lugdunensis, S. epidermidis, and S. saprophyticus) accounted for 86% of these infections. An earlier study reported that only 21% of CoNS isolates were considered to be clinically significant (10). However, the earlier study was conducted without preselection criteria and included cultures from sterile body sites.
The data from this study reinforce the propensity of S. lugdunensis to be associated with acute cutaneous infections. Previous investigators have reported the involvement of S. lugdunensis in breast and soft tissue abscesses (2, 11, 13), and other reports have also noted the frequent isolation of this organism from the pelvic region (2, 12).
Most of the clinically significant infections were community acquired. The hospital population we studied did not include significant numbers of immunocompromised patients. Also, nosocomial isolates of CoNS are commonly present in polymicrobial cultures, which would have been excluded by the screening criteria. None of the other laboratory criteria analyzed for the study isolates were able to differentiate clinically significant isolates from colonizers. Density of growth, measured by semiquantitative methods, or the presence of a single CoNS species on culture did not prove sufficiently discriminative.
Some factors may have influenced the results of this study. First, the standardized clinical criteria used to prevent bias were designed for the surveillance of nosocomial infections and may not be entirely suitable for the diagnosis of community-acquired infections. Secondly, prior antibiotic therapy may have influenced the culture results. However, no documented evidence of prior antibiotic therapy was noted in most samples received from community sources.
The results of this study suggest that when the study screening algorithm is applied to CoNS isolated from community-acquired infections, over two-thirds of isolates will be clinically significant. There was evidence of species-related variation in virulence, with S. lugdunensis and S. saprophyticus identified as the most pathogenic isolates. A combination of a simplified identification system for CoNS, such as that used in this study, and the application of the screening algorithmn would allow clinical laboratories to correctly identify and report the CoNS isolates that are most likely to be clinically relevant.
ACKNOWLEDGMENTS
This work was supported by a grant from the National Medical Research Council, Singapore.
FOOTNOTES
Corresponding author. Mailing address: Laboratory Medicine Services, Changi General Hospital, 2 Simei Street 3, Singapore, 529889. Phone: 065-68504935. Fax: 065-64269507. E-mail: siew_yong_ng@cgh.com.sg.
REFERENCES
Bannerman, T. L. 2003. Staphylococcus, Micrococcus, and other catalase-positive cocci that grow aerobically, p. 384-404. In P. R. Murray, E. J. Baron, J. H. Jorgensen, M. A. Pfaller, and R. H. Yolken (ed.), Manual of Clinical Microbiology, 8th ed., vol. 1. American Society for Microbiology, Washington, D.C.
Bellamy, R., and T. Barkham. 2002. Staphylococcus lugdunensis infection sites: predominance of abscesses in the pelvic girdle region. Clin. Infect. Dis. 35:E32-E34.
Crowe, M. J., and E. M. Cooke. 1998. Review of case definitions for nosocomial infection—towards a consensus. Presentation by the Nosocomial Infection Surveillance Unit (NISU) to the Hospital Infection Liaison Group, subcommittee of the Federation of Infection Societies (FIS). J. Hosp. Infect. 39:3-11.
De Paulis, A. N., S. C. Predari, C. D. Chazarreta, and J. E. Santoianni. 2003. Five-test simple scheme for species-level identification of clinically significant coagulase-negative staphylococci. J. Clin. Microbiol. 41:1219-1224.
Emori, T. G., D. H. Culver, T. C. Horan, W. R. Jarvis, J. W. White, D. R. Olson, S. Banerjee, J. R. Edwards, W. J. Martone, and R. P. Gaynes. 1991. National nosocomial infections surveillance system (NNIS): description of surveillance methods. Am. J. Infect. Control 19:19-35.
Ieven, M., J. Verhoeven, S. R. Pattyn, and H. Goossens. 1995. Rapid and economical method for species identification of clinically significant coagulase-negative staphylococci. J. Clin. Microbiol. 33:1060-1063.
Kloos, W. E., and K. H. Schleifer. 1975. Simplified scheme for routine identification of human Staphylococcus species. J. Clin. Microbiol. 1:82-88.
Pezzlo, M., and M. K. York. 2004. Urine cultures, p. 3.12.1-3.12.31. In H. D. Isenberg (ed.), Clinical microbiology procedures handbook, 2nd ed., vol. 1. American Society for Microbiology, Washington, D.C.
Schnitzler, N., R. Meilicke, G. Conrads, D. Frank, and G. Haase. 1998. Staphylococcus lugdunensis: report of a case of peritonitis and an easy-to-perform screening strategy. J. Clin. Microbiol. 36:812-813.
Sewell, C. M., J. E. Clarridge, E. J. Young, and R. K. Guthrie. 1982. Clinical significance of coagulase-negative staphylococci. J. Clin. Microbiol. 16:236-239.
Vandenesch, F., S. J. Eykyn, J. Etienne, and J. Lemozy. 1995. Skin and post-surgical wound infections due to Staphylococcus lugdunensis. Clin. Microbiol. Infect. 1:73-74.
van der Mee-Marquet, N., A. Achard, L. Mereghetti, A. Danton, M. Minier, and R. Quentin. 2003. Staphylococcus lugdunensis infections: high frequency of inguinal area carriage. J. Clin. Microbiol. 41:1404-1409.
Waghorn, D. J. 1994. Staphylococcus lugdunensis as a cause of breast abscess. Clin. Infect. Dis. 19:814-815.
York, M. K. 2004. Processing, isolation, detection, and interpretation of aerobic bacteriology cultures, p. 3.3.2.1-3.3.2.14. In H. D. Isenberg (ed.), Clinical microbiology procedures handbook, 2nd ed., vol. 1. American Society for Microbiology, Washington, D.C.(Thean Yen Tan, Siew Yong Ng, and Wan Xin)