Reduced susceptibility to vancomycin of nosocomial isolates of methicillin-resistant Staphylococcus aureus

The MICs of vancomycin for 56 random nosocomial Staphylococcus aureus isolates homogeneously resistant to methicillin (homMRSA), 16 heterogeneously resistant isolates (hetMRSA) and 25 susceptible isolates (MSSA) were determined by a standard broth microdilution method. Representative isolates were also tested by an agar incorporation method, the Etest and population analysis. Although always in the susceptible range, MICs of vancomycin for homMRSA were significantly higher than those for hetMRSA or MSSA. Moreover, a homMRSA strain belonging to one of the major Greek MRSA clones contained a sub-population of cells that could grow in the presence of vancomycin 8 mg/L at a frequency of 6.7 x 10(-8).