1 Department of Laboratory Medicine, Yonsei University Wonju College of Medicine, Ilsan-dong 162, Wonju, Kangwon-do; 2 Department of Laboratory Medicine, Hallym College of Medicine, Chuncheon; 3 Department of Infectious Disease, Yonsei University Wonju College of Medicine, Wonju, South Korea
Received 7 November 2003; returned 19 January 2004; revised and accepted 2 March 2004
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Abstract |
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Methods: MICs of five antimicrobials were determined for 106 VGS isolated from blood cultures. The macrolide resistance mechanisms of erythromycin non-susceptible isolates were studied by the double-disc test and PCR.
Results: In all, 42.4% of the isolates were susceptible to penicillin. Nine of 61 penicillin non-susceptible isolates were fully resistant (MIC 4 mg/L). Rates of non-susceptibility to erythromycin, clindamycin and ceftriaxone were 33.9%, 17.9% and 9.4%, respectively. Twenty-two (61.1%) of 36 erythromycin non-susceptible isolates expressed constitutive resistance to macrolidelincosamidestreptogramin B antibiotics (a constitutive MLSB phenotype); 13 isolates (36.1%) expressed an M phenotype; and one isolate, a Streptococcus bovis isolate, had an inducible MLSB resistance phenotype. erm(B) was found in isolates with constitutive/inducible MLSB phenotypes, and mef(A) in isolates with the M phenotype. In three isolates (two isolates with a constitutive MLSB phenotype and in one isolate with the M phenotype), none of erm(A), erm(B), erm(C) or mef(A) was detected by PCR.
Conclusions: Penicillin non-susceptible VGS were more resistant to erythromycin, clindamycin and ceftriaxone than were penicillin-susceptible isolates. A constitutive MLSB phenotype associated with erm(B) was the predominant mechanism of macrolide resistance among erythromycin non-susceptible isolates from this Korean hospital.
Keywords: -haemolytic streptococci, erythromycin resistance, MLSB phenotype, erm(B), mef(A)
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Introduction |
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The objectives of the present study were to determine the incidence and patterns of antimicrobial resistance among VGS isolated from blood cultures in a Korean hospital and to clarify the macrolide resistance phenotypes and genotypes of erythromycin non-susceptible isolates.
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Materials and methods |
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Susceptibility to penicillin G, erythromycin, clindamycin, ceftriaxone (Sigma Chemical Co., St Louis, MO, USA) and vancomycin (Daewoong Lilly, Korea) was determined by an agar dilution method,2 and MICs were interpreted using NCCLS criteria.3 Macrolide resistance phenotypes of erythromycin non-susceptible isolates were determined using a double-disc test with erythromycin (15 µg) and clindamycin (2 µg) discs on MuellerHinton agar plates containing 5% sheep blood. Genomic DNA was extracted with an Easy-DNA kit (Invitrogen, Carlsbad, CA, USA) according to the manufacturers instructions. The presence of erm and mef class genes was determined by PCR amplification using previously described primers specific for erm(A), erm(B), erm(C) and mef(A).4,5
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Results |
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Among 36 erythromycin-resistant isolates, 22 isolates (61.1%) expressed a constitutive macrolidelincosamidestreptogramin B (MLSB) phenotype, 13 isolates (36.1%) expressed an M phenotype and one Streptococcus bovis isolate expressed an inducible MLSB phenotype. The erm(B) gene was detected in most isolates with constitutive/inducible MLSB phenotypes, while mef(A) was detected in most isolates with the M phenotype. None of erm(A), erm(B), erm(C) or mef(A) was detected in two isolates with a constitutive MLSB phenotype, and in one isolate with the M phenotype (Table 2). Since strains positive for erm genes were not further tested for mef genes, we do not know how many isolates might have had both resistance mechanisms.
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Discussion |
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Increased rates of macrolide resistance in blood isolates of the VGS have been reported in recent years, and the rate of erythromycin resistance noted in our study (34%) was similar to those reported among blood culture isolates in the USA (41%), Canada (38.1%) and northern Taiwan (35%).1,6,7 These rates limit the value of macrolides as prophylaxis in high-risk populations or as a treatment of viridans streptococcal bacteraemia or endocarditis.
The rate of clindamycin resistance in VGS has been determined mainly by the distribution of MLSB resistance phenotypes. In Taiwan, Teng et al.7 documented that all macrolide-resistant VGS were constitutively resistant. However, in studies reported by western countries, resistance rates to clindamycin were lower than that of erythromycin.1 In this study, a constitutive MLSB phenotype was the most frequent mechanism observed among erythromycin-resistant isolates (61.1%). The M phenotype was expressed by 36.1% of isolates; this rate is higher than that reported by Teng et al.,7 but lower than that reported by Ioannidou et al.8 This difference of distribution of MLSB phenotypes may be explained by geographical variation and source of the isolates.
In conclusion, we found high rates of non-susceptibility to penicillin and macrolides among VGS in Korea, and found that a constitutive MLSB phenotype associated with erm(B) was dominant. The high frequency of non-susceptibility to penicillin and macrolides among the VGS limits the use of these drugs as therapeutic or prophylactic agents for bacteraemias caused by these organisms. Our findings indicate clearly that clinical microbiology laboratories should carry out periodic surveillance of antibiotic susceptibility among various species of VGS. Further studies are needed to determine the suitable antimicrobial agents for chemoprophylaxis.
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Footnotes |
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References |
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2 . National Committee for Clinical Laboratory Standards. (2003). Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria that Grow AerobicallySixth Edition: Approved Standard M7-A6. NCCLS, Wayne, PA, USA.
3 . National Committee for Clinical Laboratory Standards. (2003). Performance Standards for Antimicrobial Susceptibility TestingThirteenth Informational Supplement M100-S13. NCCLS, Wayne, PA, USA.
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Kataja, J., Huovinen, P., Skurnik, M. et al. (1999). Erythromycin resistance genes in group A streptococci in Finland. Antimicrobial Agents and Chemotherapy 43, 4852.
5 . Sutcliffe, J., Grebe, T., Tait-Kamradt, A. et al. (1996). Detection of erythromycin-resistant determinants by PCR. Antimicrobial Agents and Chemotherapy 40, 25626.[Abstract]
6 . Doern, G. V., Ferraro, M. J., Brueggemann, A. B. et al. (1996). Emergence of high rates of antimicrobial resistance among viridans group streptococci in the United States. Antimicrobial Agents and Chemotherapy 40, 8914.[Abstract]
7 . Teng, L. J., Hsueh, P. R., Chen, Y. C. et al. (1998). Antimicrobial susceptibility of viridans group streptococci in Taiwan with an emphasis on the high rates of resistance to penicillin and macrolides in Streptococcus oralis. Journal of Antimicrobial Chemotherapy 41, 6217.[Abstract]
8 . Ioannidou, S., Tassios, P. T., Kotsovili-Tseleni, A. et al. (2001). Antibiotic resistance rates and macrolide resistance phenotypes of viridans group streptococci from the oropharynx of healthy Greek children. International Journal of Antimicrobial Agents 17, 195201.[CrossRef][ISI][Medline]