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

The MICs of vancomycin for 56 random nosocomial Staphylococcus aureus isola tes homogeneously resistant to methicillin (homMRSA), 16 heterogeneously re sistant isolates (hetMRSA) and 25 susceptible isolates (MSSA) were determin ed by a standard broth microdilution method. Representative isolates were a lso tested by an agar incorporation method, the Etest and population analys is. Although always in the susceptible range, MICs of vancomycin for homMRS A were significantly higher than those for hetMRSA or MSSA. Moreover, a hom MRSA 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 a t a frequency of 6.7 x 10(-8).