Abstract
Recently, decreased susceptibility to vancomycin (VCM) among the clinical strains of methicillin resistant Staphylococcus aureus (MRSA) has been reported in Japan. To investigate VCM susceptibility among the clinical MRSA strains isolated in our hospital, we determined the MIC of VCM and performed a population analysis. A total of 116 strains, including 80 MRSA isolated from August 1996 to May 1997 (MRSA-A group), 16 MRSA isolated in 1990 when VCM was not available for MRSA infections (MRSAB group), and 20 methicillin-sensitive S. aureus (MSSA) isolated August 1996 to May 1997 (MSSA group), were analyzed. The MIC of VCM in all 96 MRSA strains was less than 4 μg/ml. The strains in the MRSA-A group tended to have a higher MIC than those in the MRSA-B group, although no statistically significant difference was seen. Population analysis revealed that 71 (88.8%) or 12 (15%) of 80 strains in the MRSA-A group, 15 (93.8%) or 4 (25%) of 16 strains in the MRSA-B group, and 19 (95%) or 6 (30%) of 20 strains in the MSSA group contained subclones that could grow on agar plates containing 4 or 8μg/ml of VCM, respectively. The strains in the MRSA-A group tended to form more colonies on agar plates containing VCM compared with those in the other two groups. The results of the present study suggest decreased susceptibility to VCM among MRSA strains in our hospital in recent years.
