Evidence for a continuum of decreased vancomycin susceptibility in unselected Staphylococcus aureus clinical isolates

Some Staphylococcus aureus isolates have glycopeptide minimal inhibitory concentrations (MICs) in the susceptible range but have subpopulations that grow on greater than or equal to4 mug/mL vancomycin. Clinical laboratory methods for determining susceptibility have proven to be inadequate for detecting these strains. Among methicillin-resistant S. aureus (MRSA) and methicillin-susceptible S. aureus (MSSA) clinical isolates, 149 (66.2%) of 225 and 17 (56.6%) of 30, respectively, grew on brain-heart infusion (BHI) medium containing 2 mug/mL vancomycin; 17 (7.5%) of the MRSA and 2 (6.6%) of the MSSA isolates grew on BHI screening plates containing 4 mug/mL vancomycin. One isolate grew on plates containing 6 mug/mL vancomycin. This isolate escaped detection by routine testing but had a vancomycin MIC of 6 mug/mL when tested in BHI medium. This isolate also had decreased Triton X-100-induced autolysis and killing when incubated in broth media containing vancomycin, properties accorded to glycopeptide-intermediate S. aureus isolates. These observations suggest that glycopeptide-intermediate-like S. aureus isolates are circulating undetected and that a continuum of decreased susceptibility exists in unselected isolates.