Search for Staphylococcus aureus heterogeneously resistant to vancomycin (hetero-VRSA) in MRSA strains isolated from clinical samples during 1990s

Hetero-VRSA was studied in 978 MRSA strains isolated from clinical samples during 1991 to 1998. Although no VRSA was detected, 23 strains (2.4%) were identified as hetero-VRSA by the vancomycin-resistance using MU3 agar plate. The frequency of hetero-VRSA was not increased in the course of time. MIC of the hetero-VRSA to vancomycin and teicoplanin was 1-2 micrograms/ml and 0.5-12 micrograms/ml, respectively. All of the hetero-VRSA strains were confirmed as a heterogeneous strain by a population analysis. Although 43% of the hetero-VRSA strains were coagulase type II, positive for TSST-1, and enterotoxin type C, others were various in the characteristics. In the gene analysis by pulse field gelelectrophoresis (PFGE), 4 sets of 2 strains were found to be identical among the 23 strains but the other 15 strains were genetically different. We speculate that hetero-VRSA strains were generated in 1991 secondary possibly by use of beta-lactam antibiotics.