[Prospective study of Staphylococcus aureus with reduced susceptibility to glycopeptides]

The emergence of clinical methicillin-resistant Staphylococcus aureus with reduced susceptibility to glycopeptides has recently been documented. Heterogeneous levels of glycopeptides susceptibility have been demonstrated in these isolates. Prospectively, we investigated the presence of S. aureus isolates with reduced susceptibility to vancomycin in our hospital from October 1997 to January 1998. The vancomycin and teicoplanin susceptibility of 248 consecutively methicillin-susceptible and -resistant S. aureus isolates from clinical and surveillance specimens was studied by broth microdilution and agar dilution methods. In addition, 108 CFU/mI of overnight cultures were plated onto BHI-agar plates containing 6 and 10 microg/ml of vancomycin and were incubated for 48 hours at 35 degrees C. Under these conditions selected colonies were subcultured onto vancomycin plates and vancomycin susceptibility (E-test) was determined again. Vancomycin MIC(90) values by the agar dilution technique for methicillin-susceptible and -resistant isolates were 2 and 4 microg/ml, respectively No selected colonies were observed on 10 microg/ml vancomycin plates. In contrast, 12 isolates, including 8 (3.9%) methicillin-susceptible and 4 (9. 1%) methicillin-resistant isolates, showed subpopulations at a frequency rate of 10(-6)-10(-7) on 6 microg/ml vancomycin plates. Nevertheless, a significant increase in MIC values for vancomycin was not observed in these S. aureus subpopulations.