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 Original Articles |
| Detection of Vancomycin Heteroresistant Staphylococcus aureus Using Mu-3 Agar |
| Kyungwon Lee, M.D., Dongeun Yong, M.D., Kwang-il Park, M.D.,
Keonsoo Yi, M.D., Jeong Won Shin, M.D., and Yunsop Chong, Ph.D. |
| Department of Clinical Pathology and Research Institute of Bacterial Resistance,
Yonsei University College of Medicine, Seoul, Korea |
Vol.32 Num.5 (p349~356)
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Background:The aim of the study was to determine prevalence of potential heterogeneous vancomycin-resistant Staphylococcus aureus (h-VRSA) among methicillin-resistant S. aureus (MRSA) isolated in Korea by using Mu-3 agar and to determine the effect of in vitro vancomycin exposure on the resistance.
Methods:MRSAs isolated in 1980-1999 were screened for the presence of VISA or h-VRSA using Mu-3 agar. MIC of vancomycin was tested by NCCLS agar dilution and broth microdilution tests. Suspected h-VRSA were selected by vancomycin-containing media and change of resistance was determined by population analysis. A strain with Mu50 type growth was serially exposed to 8 μg/ml of vancomycin containing media and change of the vancomycin resistance was determined.
Results:Among the 455 MRSA isolates, 18 (3.9 %) grew on selective brain heart infusion agar (BHIA), and 354 (77.8%) on Mu-3 agar, 66 (14.5%) with Mu3 type growth and 78 (17.1%) with Mu50 type growth. MIC of vancomycin was 11 μg/ml for some of the isolates when inocula were approximately 106 CFU, but VISA was not present when tested by NCCLS broth microdilution test. Exposure of the isolates to vancomycin raised the MIC. Serial exposure once to 8 μg/ml of vancomycin resulted in significant decrease of cells susceptible to 8-12 μg/ml of vancomycin.
Conclusion:VISA was not present among the test isolates, but 34.2% were suspected to be potential h-VRSAs, suggesting possible emergence of VISA if vancomycin was administered prolonged period. It is considered that suitable screening media are vancomycin containing BHIA for VISA and Mu-3 agar for h-VRSA. The isolates showing Mu50 type growth on Mu-3 agar are not always VISA, but rather h-VRSA. |
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Keywords : Vancomycin resistance, Heteroresistance, Staphylococcus aureus |
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