Prevalence of the ST239 Clone of Methicillin-Resistant Staphylococcus aureus and Differences in Antimicrobial Susceptibilities of ST239 and
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微生物临床杂志 2005年第8期
Department of Microbiology
Institute of Infectious Diseases
Department of Internal Medicine, Kyungpook National University School of Medicine, Daegu, Korea
ABSTRACT
A total of 188 nonduplicate methicillin-resistant Staphylococcus aureus (MRSA) isolates obtained between 2001 and 2004 in a university hospital in Daegu, Korea, were analyzed for their clonal types by molecular typing techniques, including multilocus sequence typing, spaA typing, staphylococcal chromosomal cassette mec (SCCmec) typing, and pulsed-field gel electrophoresis (PFGE). They were examined for their antimicrobial susceptibilities. The majority (87%) of MRSA isolates belonged to sequence type 239 (ST239; n = 100; 53%) and ST5 (n = 63, 34%) on the basis of sequence typing. MRSA isolates belonging to ST239 were genotypically homogeneous, while those belonging to ST5 showed variations in spaA type, SCCmec type, and PFGE patterns. The rates of resistance of the MRSA isolates belonging to ST239 to trimethoprim, sulfamethoxazole, tobramycin, gentamicin, erythromycin, and tetracycline were significantly higher than those of the isolates belonging to ST5 (P < 0.05). This study demonstrated that the ST239 clone, while rarely detected in Korea, was prevalent and that the antimicrobial susceptibility of the ST239 clone was significantly different from that of the ST5 clone.
INTRODUCTION
Staphylococcus aureus is one of the most important pathogens in health care facility-associated infections and community-acquired infections (18). Since the first detection of methicillin-resistant S. aureus (MRSA) in 1961 (14), MRSA has spread worldwide and causes serious problems in clinical settings. MRSA bacteremia is associated with higher mortality than methicillin-susceptible S. aureus bacteremia (5). Moreover, MRSA strains are resistant to a wide range of antimicrobial agents (32). The overall frequency of health care facility-associated infections caused by MRSA has increased during the last decade in Korea, where the frequency of MRSA among health care facility-associated staphylococcal infections is estimated to be over 70% (15).
Multiple DNA band-based typing techniques such as pulsed-field gel electrophoresis (PFGE) (19), PCR-based typing (13), and staphylococcal chromosomal cassette mec (SCCmec) typing (21) have been applied to track MRSA strains, but they are limited due to differences between laboratories. DNA sequence-based approaches such as multilocus sequence typing (MLST) (9), spaA typing (11, 22, 29), and coa typing (28) have been verified as adequate methods for global epidemiological studies of MRSA strains. Accordingly, combinations of DNA band-based techniques with DNA sequence-based techniques are frequently used to differentiate between MRSA strains at the local and the international levels.
Phylogenetic studies by molecular typing techniques have shown that only a few epidemic MRSA clones are responsible for the large proportion of MRSA infections worldwide (3, 6, 8, 10, 23, 25, 30). Five major internationally spread epidemic MRSA clones, the Iberian (7, 12, 27), Brazilian (31), Hungarian (6, 22), New York/Japan (2, 24), and pediatric (26) clones, have been identified. In Asian countries, two epidemic clones, the Brazilian clone (sequence type 239 [ST239]-MRSA-IIIA) and the New York/Japan clone (ST5-MRSA-II), have been found to be prevalent and to possess unique geographic distributions (2, 3, 16). Although the ST5 clone was found to be prevalent in a hospital in Seoul, Korea (16), little is known about the MRSA clones in Korea. The aim of this study was to analyze the MRSA clones circulating in the Kyungpook National University Hospital (KNUH), Daegu, Korea, between 2001 and 2004 and to evaluate the antimicrobial susceptibilities of MRSA isolates on the basis of their clonal types.
MATERIALS AND METHODS
MRSA isolates. A total of 188 nonduplicate MRSA isolates were collected between February 2001 and August 2004: 28 from 2001, 61 from 2002, 45 from 2003, and 54 from 2004. MRSA isolates that were defined as being responsible for health care facility-associated infections (infections acquired 48 h after hospital admission) were included. The isolates originated from wounds (n = 69; 36.7%), blood (n = 45; 23.9%), pus (n = 18; 9.6%), the upper respiratory tract (n = 18; 9.6%), urine (n = 12; 6.4%), and other clinical sources (n = 26; 13.8%). S. aureus isolates were tested for phenotypic resistance to oxacillin by the salt agar dilution method, according to the guidelines of the CLSI (formerly the National Committee Clinical Laboratory Standards) (20). The presence of the mecA gene was confirmed by PCR, as described previously (17).
Antimicrobial susceptibility test. The MICs of the antimicrobial agents were determined by the agar dilution method with a Steers multiple inoculator, according to the guidelines of the CLSI (20). The antimicrobial agents tested included oxacillin, teicoplanin, vancomycin, gentamicin, tobramycin, trimethoprim, sulfamethoxazole, trimethoprim-sulfamethoxazole (SXT; 1:19), chloramphenicol, erythromycin, tetracycline, rifampin, and ciprofloxacin.
MLST, spaA typing, and SCCmec typing. Chromosomal DNA was extracted by using a Wizard genomic DNA preparation kit (Promega), with the addition of 0.5 mg/ml of lysostaphin and 0.3 mg/ml of RNase in the lysis buffer. MLST was performed by the method described by Enright et al. (9). PCR fragments of seven genes, arcC, aroE, glpF, gmk, pta, tpi, and yqiL, were directly sequenced in an ABI Prism 3100 analyzer (Applied Biosystems). The allele number and sequence type were determined with the use of a database accessible via http://saureus.mlst.net/. spaA typing was performed as described previously (29). Multiplex PCR was performed to identify the SCCmec structural types (21). Strains COL, N315, and ANS46 were included as controls for SCCmec type I, II, and III, respectively (21).
PFGE. The genomic DNA was digested with SmaI (Boehringer Mannheim) for 18 h and separated on a 1.0% agarose gel by using a contour-clamped homogeneous electric field apparatus (CHEF DRIII Systems; Bio-Rad Laboratories) in 0.5x TBE (Tris-borate-EDTA) buffer. The conditions for electrophoresis were 6 V/cm for 10 h with the pulse time increasing from 5 to 15 s, followed by 13 h with the pulse time increasing from 15 to 60 s. A bacteriophage lambda DNA ladder, comprised of 48.5-kb concatemers (Bio-Rad Laboratories), was used as the size standard. Digital images were stored electronically as TIFF files and were analyzed with GelCompar software (Applied Maths).
Statistical analysis. Statistical analysis was performed by the 2 test and Fisher's exact test. Differences with P values <0.05 were considered statistically significant. Statistical analyses were performed with SPSS version 12.0 software.
RESULTS AND DISCUSSION
Molecular typing of MRSA isolates. The 188 MRSA isolates were classified into seven ST types, nine spaA types, five PFGE patterns, and three SCCmec types with variants (Table 1). Based on the STs, the most prevalent types were ST239 and ST239 single locus variant (ST344), which accounted for 56% (n = 106) of the isolates. The second most prevalent types were ST5 and ST5 slv (ST221), which accounted for 34% (n = 64) of the isolates. The remaining three types, ST72, ST1, and undetermined ST (ST1 slv), were detected in 12, 5, and 1 isolates, respectively. To our knowledge, this is the first time that ST239 has been found to be prevalent in a Korean hospital. Ko et al. (16) recently reported that all MRSA isolates from Korea and Japan, with the exception of one isolate from Korea, belonged to ST5, while ST239 was prevalent in many other Asian countries, including China, India, Indonesia, Singapore, Sri Lanka, Thailand, and Vietnam. ST239 was also prevalent in Hong Kong (13). Although the exact reason for the discrepancies of the prevalent MRSA clones between hospitals in Korea is not clear, each prevalent MRSA clone may have certain advantages in terms of its survival and distribution in a health care setting. The discrepancies of the prevalent MRSA clones may be partly due to the fact that the hospitals are located 300 km apart in Korea. This finding suggests that different epidemic MRSA clones are prevalent in different hospitals in Korea.
MRSA isolates belonging to ST239 were genetically homogenous: an identical MLST type (2-3-1-1-4-4-3), an invariant spaA type (WGKAOMQ), and a uniform PFGE profile (pattern B). MRSA isolates belonging to ST239 were assigned to SCCmec types III (n = 85) and IIIA (n = 15), which are characteristics of the Hungarian (ST239-MRSA-III) and Brazilian (ST239-MRSA-IIIA) clones, respectively. The genetic uniformity of ST239 suggested that this clone was relatively recently introduced and spread in KNUH.
The second most frequent MRSA clone was ST5. Fifty-nine isolates were identified as the New York/Japan clone (ST5-MRSA-II). This clone has also been found to be predominant in Japan (2) and the United States (4, 9, 24). In contrast to ST239, MRSA isolates belonging to ST5 showed genetic diversities in terms of spaA type, PFGE profile, and SCCmec type. Among the 63 MRSA isolates belonging to ST5, 35 (56%) isolates carried genetic variations in spaA and/or SCCmec elements compared with the sequence of the New York/Japan clone. Twenty-three (37%) isolates showed deletion-insertion events in spaA, which generated four spaA types (Table 1), and 31 (49%) isolates carried SCCmec type II variants, as determined by a multiplex PCR strategy. MRSA strains carrying SCCmec type II displayed four bands of 381, 342, 284, and 209 bp (21). SCCmec type II variants were distinguished from SCCmec type II by the absence of 381 bp, which corresponded to the integration of pUB110, or the absence of 342 bp, which corresponded to the internal dcs region. Four MRSA isolates belonging to ST5 carried SCCmec type IVA. SCCmec type IVA was distinguished from SCCmec type IV by an extra band of 381 bp, which corresponded to the integration of pUB110. ST5-MRSA-IVA was first detected in the current study, although the pediatric clone (ST5-MRSA-IV) has spread worldwide. The genetic diversity of ST5 suggests that this clone is of a very old lineage.
The frequency of ST239 sharply increased from 32% in 2001 to 66% in 2002 and then gradually decreased to 58% in 2003 and 46% in 2004 (Fig. 1). ST5 constituted 36%, 23%, 33%, and 44% of the MRSA isolates identified in 2001, 2002, 2003, and 2004, respectively. All 6 isolates belonging to ST344 were isolated in 2001, while 12 MRSA isolates belonging to ST72 have been isolated since 2002. Accordingly, there was no striking change in the prevalence of MRSA clones between 2001 and 2004 in KNUH.
Antimicrobial susceptibility. The MRSA isolates were highly resistant to tobramycin (94%), gentamicin (92%), erythromycin (90%), ciprofloxacin (89%), and tetracycline (83%), while most of the MRSA isolates were susceptible to chloramphenicol (98%) and rifampin (94%) (Table 2). The overall rates of resistance to trimethoprim, sulfamethoxazole, and SXT among the MRSA isolates tested were 51%, 55%, and 52%, respectively. No isolate was resistant to vancomycin and teicoplanin. The majority (94%) of MRSA isolates were multiresistant to more than three of the non--lactam antimicrobial agents tested. The rates of resistance of the MRSA isolates belonging to ST239 were significantly higher than those of isolates belonging to ST5: trimethoprim, 100% versus 0% (P < 0.001); sulfamethoxazole, 93% versus 6% (P < 0.001); SXT, 96% versus 2% (P < 0.001); tobramycin, 100% versus 84% (P < 0.001); gentamicin, 99% versus 92% (P < 0.05); erythromycin, 100% versus 87% (P < 0.001); and tetracycline, 99% versus 71% (P < 0.001). Additionally, the rates of multiresistance were significantly higher among ST239 isolates than that among ST5 isolates (P < 0.05) (data not shown). These data suggested that the antimicrobial susceptibilities of MRSA isolates belonging to ST239 were quite different from those of MRSA isolates belonging to ST5.
MRSA isolates belonging to ST5 from Korea (16), Japan (16), and the United States (4) were all susceptible to SXT. Most of the MRSA isolates belonging to ST239 from India, Indonesia, Singapore, Sri Lanka, Taiwan, and Greece were resistant to SXT (1, 3, 16), while those from China, Vietnam, and the Czech Republic were susceptible (3, 16, 17). These data, combined with our current results, suggest that the susceptibility of MRSA isolates belonging to ST239 to SXT is different between the bacterial isolation sites.
Differences in the MICs of ciprofloxacin, trimethoprim, sulfamethoxazole, and SXT were also observed on the basis of SCCmec type (Table 3). MRSA isolates carrying SCCmec type II and II variants showed a high level of resistance to ciprofloxacin, while MRSA isolates carrying SCCmec type IV and IVA showed a low level of resistance to ciprofloxacin. A high level of resistance to trimethoprim, sulfamethoxazole, and SXT was observed in MRSA isolates carrying SCCmec type III and IIIA compared with MRSA isolates carrying SCCmec type II, II variants, IV, and IVA. Among the 22 MRSA isolates carrying SCCmec type IV and IVA, 18 isolates belonged to sporadic clones, ST72 (n = 12), ST1 (n = 5), and undetermined ST (n = 1). The majority (84%) of MRSA isolates belonging to ST239 and ST5 were resistant to more than five of the antimicrobial agents tested, while 83% of the sporadic clones were resistant to a maximum of three or fewer antimicrobial agents.
The current study demonstrated that the prevalent ST239 and ST5 clones are highly resistant to antimicrobial agents in comparison with the susceptibilities of the sporadic MRSA clones. The antimicrobial susceptibilities of MRSA isolates belonging to ST239 were quite different from those of the MRSA isolates belonging to ST5. Accordingly, we propose that different oral antimicrobial regimens are necessary for the treatment of MRSA infections on the basis of clonal type.
ACKNOWLEDGMENTS
This study was supported by a grant from the Korea Health 21 R&D Project, Ministry of Health & Welfare, Republic of Korea (03-PJ1-PG1-CH03-0002).
We thank Herminia de Lencastre, from the Laboratorio de Genetica Molecular, Instituto de Technologia Química e Biologica da Universiadade Nova de Lisboa, Lisbon, Portugal, for providing MRSA strains COL, N315, and ANS46.
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Institute of Infectious Diseases
Department of Internal Medicine, Kyungpook National University School of Medicine, Daegu, Korea
ABSTRACT
A total of 188 nonduplicate methicillin-resistant Staphylococcus aureus (MRSA) isolates obtained between 2001 and 2004 in a university hospital in Daegu, Korea, were analyzed for their clonal types by molecular typing techniques, including multilocus sequence typing, spaA typing, staphylococcal chromosomal cassette mec (SCCmec) typing, and pulsed-field gel electrophoresis (PFGE). They were examined for their antimicrobial susceptibilities. The majority (87%) of MRSA isolates belonged to sequence type 239 (ST239; n = 100; 53%) and ST5 (n = 63, 34%) on the basis of sequence typing. MRSA isolates belonging to ST239 were genotypically homogeneous, while those belonging to ST5 showed variations in spaA type, SCCmec type, and PFGE patterns. The rates of resistance of the MRSA isolates belonging to ST239 to trimethoprim, sulfamethoxazole, tobramycin, gentamicin, erythromycin, and tetracycline were significantly higher than those of the isolates belonging to ST5 (P < 0.05). This study demonstrated that the ST239 clone, while rarely detected in Korea, was prevalent and that the antimicrobial susceptibility of the ST239 clone was significantly different from that of the ST5 clone.
INTRODUCTION
Staphylococcus aureus is one of the most important pathogens in health care facility-associated infections and community-acquired infections (18). Since the first detection of methicillin-resistant S. aureus (MRSA) in 1961 (14), MRSA has spread worldwide and causes serious problems in clinical settings. MRSA bacteremia is associated with higher mortality than methicillin-susceptible S. aureus bacteremia (5). Moreover, MRSA strains are resistant to a wide range of antimicrobial agents (32). The overall frequency of health care facility-associated infections caused by MRSA has increased during the last decade in Korea, where the frequency of MRSA among health care facility-associated staphylococcal infections is estimated to be over 70% (15).
Multiple DNA band-based typing techniques such as pulsed-field gel electrophoresis (PFGE) (19), PCR-based typing (13), and staphylococcal chromosomal cassette mec (SCCmec) typing (21) have been applied to track MRSA strains, but they are limited due to differences between laboratories. DNA sequence-based approaches such as multilocus sequence typing (MLST) (9), spaA typing (11, 22, 29), and coa typing (28) have been verified as adequate methods for global epidemiological studies of MRSA strains. Accordingly, combinations of DNA band-based techniques with DNA sequence-based techniques are frequently used to differentiate between MRSA strains at the local and the international levels.
Phylogenetic studies by molecular typing techniques have shown that only a few epidemic MRSA clones are responsible for the large proportion of MRSA infections worldwide (3, 6, 8, 10, 23, 25, 30). Five major internationally spread epidemic MRSA clones, the Iberian (7, 12, 27), Brazilian (31), Hungarian (6, 22), New York/Japan (2, 24), and pediatric (26) clones, have been identified. In Asian countries, two epidemic clones, the Brazilian clone (sequence type 239 [ST239]-MRSA-IIIA) and the New York/Japan clone (ST5-MRSA-II), have been found to be prevalent and to possess unique geographic distributions (2, 3, 16). Although the ST5 clone was found to be prevalent in a hospital in Seoul, Korea (16), little is known about the MRSA clones in Korea. The aim of this study was to analyze the MRSA clones circulating in the Kyungpook National University Hospital (KNUH), Daegu, Korea, between 2001 and 2004 and to evaluate the antimicrobial susceptibilities of MRSA isolates on the basis of their clonal types.
MATERIALS AND METHODS
MRSA isolates. A total of 188 nonduplicate MRSA isolates were collected between February 2001 and August 2004: 28 from 2001, 61 from 2002, 45 from 2003, and 54 from 2004. MRSA isolates that were defined as being responsible for health care facility-associated infections (infections acquired 48 h after hospital admission) were included. The isolates originated from wounds (n = 69; 36.7%), blood (n = 45; 23.9%), pus (n = 18; 9.6%), the upper respiratory tract (n = 18; 9.6%), urine (n = 12; 6.4%), and other clinical sources (n = 26; 13.8%). S. aureus isolates were tested for phenotypic resistance to oxacillin by the salt agar dilution method, according to the guidelines of the CLSI (formerly the National Committee Clinical Laboratory Standards) (20). The presence of the mecA gene was confirmed by PCR, as described previously (17).
Antimicrobial susceptibility test. The MICs of the antimicrobial agents were determined by the agar dilution method with a Steers multiple inoculator, according to the guidelines of the CLSI (20). The antimicrobial agents tested included oxacillin, teicoplanin, vancomycin, gentamicin, tobramycin, trimethoprim, sulfamethoxazole, trimethoprim-sulfamethoxazole (SXT; 1:19), chloramphenicol, erythromycin, tetracycline, rifampin, and ciprofloxacin.
MLST, spaA typing, and SCCmec typing. Chromosomal DNA was extracted by using a Wizard genomic DNA preparation kit (Promega), with the addition of 0.5 mg/ml of lysostaphin and 0.3 mg/ml of RNase in the lysis buffer. MLST was performed by the method described by Enright et al. (9). PCR fragments of seven genes, arcC, aroE, glpF, gmk, pta, tpi, and yqiL, were directly sequenced in an ABI Prism 3100 analyzer (Applied Biosystems). The allele number and sequence type were determined with the use of a database accessible via http://saureus.mlst.net/. spaA typing was performed as described previously (29). Multiplex PCR was performed to identify the SCCmec structural types (21). Strains COL, N315, and ANS46 were included as controls for SCCmec type I, II, and III, respectively (21).
PFGE. The genomic DNA was digested with SmaI (Boehringer Mannheim) for 18 h and separated on a 1.0% agarose gel by using a contour-clamped homogeneous electric field apparatus (CHEF DRIII Systems; Bio-Rad Laboratories) in 0.5x TBE (Tris-borate-EDTA) buffer. The conditions for electrophoresis were 6 V/cm for 10 h with the pulse time increasing from 5 to 15 s, followed by 13 h with the pulse time increasing from 15 to 60 s. A bacteriophage lambda DNA ladder, comprised of 48.5-kb concatemers (Bio-Rad Laboratories), was used as the size standard. Digital images were stored electronically as TIFF files and were analyzed with GelCompar software (Applied Maths).
Statistical analysis. Statistical analysis was performed by the 2 test and Fisher's exact test. Differences with P values <0.05 were considered statistically significant. Statistical analyses were performed with SPSS version 12.0 software.
RESULTS AND DISCUSSION
Molecular typing of MRSA isolates. The 188 MRSA isolates were classified into seven ST types, nine spaA types, five PFGE patterns, and three SCCmec types with variants (Table 1). Based on the STs, the most prevalent types were ST239 and ST239 single locus variant (ST344), which accounted for 56% (n = 106) of the isolates. The second most prevalent types were ST5 and ST5 slv (ST221), which accounted for 34% (n = 64) of the isolates. The remaining three types, ST72, ST1, and undetermined ST (ST1 slv), were detected in 12, 5, and 1 isolates, respectively. To our knowledge, this is the first time that ST239 has been found to be prevalent in a Korean hospital. Ko et al. (16) recently reported that all MRSA isolates from Korea and Japan, with the exception of one isolate from Korea, belonged to ST5, while ST239 was prevalent in many other Asian countries, including China, India, Indonesia, Singapore, Sri Lanka, Thailand, and Vietnam. ST239 was also prevalent in Hong Kong (13). Although the exact reason for the discrepancies of the prevalent MRSA clones between hospitals in Korea is not clear, each prevalent MRSA clone may have certain advantages in terms of its survival and distribution in a health care setting. The discrepancies of the prevalent MRSA clones may be partly due to the fact that the hospitals are located 300 km apart in Korea. This finding suggests that different epidemic MRSA clones are prevalent in different hospitals in Korea.
MRSA isolates belonging to ST239 were genetically homogenous: an identical MLST type (2-3-1-1-4-4-3), an invariant spaA type (WGKAOMQ), and a uniform PFGE profile (pattern B). MRSA isolates belonging to ST239 were assigned to SCCmec types III (n = 85) and IIIA (n = 15), which are characteristics of the Hungarian (ST239-MRSA-III) and Brazilian (ST239-MRSA-IIIA) clones, respectively. The genetic uniformity of ST239 suggested that this clone was relatively recently introduced and spread in KNUH.
The second most frequent MRSA clone was ST5. Fifty-nine isolates were identified as the New York/Japan clone (ST5-MRSA-II). This clone has also been found to be predominant in Japan (2) and the United States (4, 9, 24). In contrast to ST239, MRSA isolates belonging to ST5 showed genetic diversities in terms of spaA type, PFGE profile, and SCCmec type. Among the 63 MRSA isolates belonging to ST5, 35 (56%) isolates carried genetic variations in spaA and/or SCCmec elements compared with the sequence of the New York/Japan clone. Twenty-three (37%) isolates showed deletion-insertion events in spaA, which generated four spaA types (Table 1), and 31 (49%) isolates carried SCCmec type II variants, as determined by a multiplex PCR strategy. MRSA strains carrying SCCmec type II displayed four bands of 381, 342, 284, and 209 bp (21). SCCmec type II variants were distinguished from SCCmec type II by the absence of 381 bp, which corresponded to the integration of pUB110, or the absence of 342 bp, which corresponded to the internal dcs region. Four MRSA isolates belonging to ST5 carried SCCmec type IVA. SCCmec type IVA was distinguished from SCCmec type IV by an extra band of 381 bp, which corresponded to the integration of pUB110. ST5-MRSA-IVA was first detected in the current study, although the pediatric clone (ST5-MRSA-IV) has spread worldwide. The genetic diversity of ST5 suggests that this clone is of a very old lineage.
The frequency of ST239 sharply increased from 32% in 2001 to 66% in 2002 and then gradually decreased to 58% in 2003 and 46% in 2004 (Fig. 1). ST5 constituted 36%, 23%, 33%, and 44% of the MRSA isolates identified in 2001, 2002, 2003, and 2004, respectively. All 6 isolates belonging to ST344 were isolated in 2001, while 12 MRSA isolates belonging to ST72 have been isolated since 2002. Accordingly, there was no striking change in the prevalence of MRSA clones between 2001 and 2004 in KNUH.
Antimicrobial susceptibility. The MRSA isolates were highly resistant to tobramycin (94%), gentamicin (92%), erythromycin (90%), ciprofloxacin (89%), and tetracycline (83%), while most of the MRSA isolates were susceptible to chloramphenicol (98%) and rifampin (94%) (Table 2). The overall rates of resistance to trimethoprim, sulfamethoxazole, and SXT among the MRSA isolates tested were 51%, 55%, and 52%, respectively. No isolate was resistant to vancomycin and teicoplanin. The majority (94%) of MRSA isolates were multiresistant to more than three of the non--lactam antimicrobial agents tested. The rates of resistance of the MRSA isolates belonging to ST239 were significantly higher than those of isolates belonging to ST5: trimethoprim, 100% versus 0% (P < 0.001); sulfamethoxazole, 93% versus 6% (P < 0.001); SXT, 96% versus 2% (P < 0.001); tobramycin, 100% versus 84% (P < 0.001); gentamicin, 99% versus 92% (P < 0.05); erythromycin, 100% versus 87% (P < 0.001); and tetracycline, 99% versus 71% (P < 0.001). Additionally, the rates of multiresistance were significantly higher among ST239 isolates than that among ST5 isolates (P < 0.05) (data not shown). These data suggested that the antimicrobial susceptibilities of MRSA isolates belonging to ST239 were quite different from those of MRSA isolates belonging to ST5.
MRSA isolates belonging to ST5 from Korea (16), Japan (16), and the United States (4) were all susceptible to SXT. Most of the MRSA isolates belonging to ST239 from India, Indonesia, Singapore, Sri Lanka, Taiwan, and Greece were resistant to SXT (1, 3, 16), while those from China, Vietnam, and the Czech Republic were susceptible (3, 16, 17). These data, combined with our current results, suggest that the susceptibility of MRSA isolates belonging to ST239 to SXT is different between the bacterial isolation sites.
Differences in the MICs of ciprofloxacin, trimethoprim, sulfamethoxazole, and SXT were also observed on the basis of SCCmec type (Table 3). MRSA isolates carrying SCCmec type II and II variants showed a high level of resistance to ciprofloxacin, while MRSA isolates carrying SCCmec type IV and IVA showed a low level of resistance to ciprofloxacin. A high level of resistance to trimethoprim, sulfamethoxazole, and SXT was observed in MRSA isolates carrying SCCmec type III and IIIA compared with MRSA isolates carrying SCCmec type II, II variants, IV, and IVA. Among the 22 MRSA isolates carrying SCCmec type IV and IVA, 18 isolates belonged to sporadic clones, ST72 (n = 12), ST1 (n = 5), and undetermined ST (n = 1). The majority (84%) of MRSA isolates belonging to ST239 and ST5 were resistant to more than five of the antimicrobial agents tested, while 83% of the sporadic clones were resistant to a maximum of three or fewer antimicrobial agents.
The current study demonstrated that the prevalent ST239 and ST5 clones are highly resistant to antimicrobial agents in comparison with the susceptibilities of the sporadic MRSA clones. The antimicrobial susceptibilities of MRSA isolates belonging to ST239 were quite different from those of the MRSA isolates belonging to ST5. Accordingly, we propose that different oral antimicrobial regimens are necessary for the treatment of MRSA infections on the basis of clonal type.
ACKNOWLEDGMENTS
This study was supported by a grant from the Korea Health 21 R&D Project, Ministry of Health & Welfare, Republic of Korea (03-PJ1-PG1-CH03-0002).
We thank Herminia de Lencastre, from the Laboratorio de Genetica Molecular, Instituto de Technologia Química e Biologica da Universiadade Nova de Lisboa, Lisbon, Portugal, for providing MRSA strains COL, N315, and ANS46.
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