Abstract
Vancomycin susceptibilities of three methicillin-esistant Staphylococcus aureus (MRSA) strains Mu 3, Mu 50, H-1 and a methicillin-usceptible S. aureus strain ATCC 6538 P were compared by determining MIC and population analysis, which showed that their MIC values were 1, 8, 0.5 and 0.5μg/ml, respectively, while the vancomycin concentrations required to inhibit 1×107 cells of these strains were 10, 13, 4, and 3μg/ml, respectively. Population analysis of Mu 3 showed a typical hetero-ype vancomycin resistance. The profiles of 14C-N-acetylglucosamine incorporation into the cell demonstrated that the incorporation to Mu 3 cells was 2.6 and 14.4-old higher than those to H-1 and ATCC 6538 P, respectively, and was 0.8-old lower than that to Mu 50 cells. In vitro synergy studies with a combination of vancomycin and aminobenzylpenicillin, cefpirome, ceftizoxime or aminobenzylpenicillin/sulbactam demonstrated an antagonism for the heterogeneous vancomycin-esistant S. aureus (hetero-VRSA) Mu 3, with a fractional inhibitory concentration (FTC) index of >2. Subsequently, a hetero-VRSA detection method was developed by means of detecting the antagonism vancomycin and ceftizoxime. 1, 673 clinical MRSA strains isolated from 1995 through 1996 in sixteen medical facilities all over Japan were screened for hetero-VRSA, which showed that the frequency of hetero-VRSA was 6.5% among the all MRSA clinical isolates.
