Characterization of staphylococci with reduced susceptibilities to vancomycin and other glycopeptides

During the last several years a series of staphylococcal isolates that demonstrated reduced susceptibility to vancomycin or other glycopeptides have been reported, We selected 12 isolates of staphylococci for which the vancomycin MICs were greater than or equal to 4 mu g/ml or for which the teicoplanin MICs were greater than or equal to 8 mu g/ml and 24 control strains for which the vancomycin MICs were less than or equal to 2 mu g/ml or for which the teicoplanin MICs were less than or equal to 4 mu g/ml to determine the ability of commercial susceptibility testing procedures and vancomycin agar screening methods to detect isolates with reduced glycopeptide susceptibility. By PCR analysis, none of the isolates with decreased glycopeptide susceptibility contained known vancomycin resistance genes, Broth microdilution tests held a full 24 h were best at detecting strains with reduced glycopeptide susceptibility, Disk diffusion did not differentiate the strains inhibited by 8 mu g of vancomycin per mi from more susceptible isolates, Most of the isolates with reduced glycopeptide susceptibility were recognized by MicroScan conventional panels and Etest vancomycin strips, Sensititre panels read visually were more variable, although with some of the panels MICs of 8 mu g/ml were noted for these isolates, Vitek results were 4 mu g/ml for all strains for which the vancomycin MICs were greater than or equal to 4 mu g/ml. Vancomycin MICs on Rapid MicroScan panels were not predictive, giving MICs of either less than or equal to 2 or greater than or equal to 16 mu g/ml for these isolates, Commercial brain heart infusion vancomycin agar screening plates containing 6 mu g of vancomycin per mi consistently differentiated those strains inhibited by 8 mu g/ml from more susceptible strains, Vancomycin-containing media prepared in-house showed occasional growth of susceptible strains, Staphylococcus aureus ATCC 29213, and on occasion, Enterococcus faecalis ATCC 29212, Thus, strains of staphylococci with reduced susceptibility to glycopeptides, such as vancomycin, are best detected in the laboratory by nonautomated quantitative tests incubated for a full 24 h. Furthermore, it appears that commercial vancomycin agar screening plates can be used to detect these isolates.