Characteristics of Laboratory-Derived Vancomycin-Intermediate Staphylococcus aureus Strains

Four clinical vancomycin susceptible Staphylococcus aureus (VSSA) isolates were subjected to selection for the vancomycin-intermediate S. aureus (VISA) phenotype using increasing concentrations of vancomycin. The vancomycin MIC achieved for VISA strains was 7 µg/mL. Population analysis profiles of the bacteria revealed that almost 100% of population growing at 4 µg/mL of vancomycin while some subpopulations were found to be resistant to concentrations ranging from 6 to 9 µg/mL of vancomycin. The results correlated with the AUC ratios for VISA, which ranged from 3.02-3.3 in this study. In the absence of vancomycin in the assay buffer, all the laboratory-derived VISA strains exhibited reduced whole cell autolysis compared to that of their VSSA counterparts. Examination of cell wall morphologies revealed that almost all the laboratory-derived VISA strains had thicker and rougher cell wall surfaces compared to their parental strains. Vancomycin had no remarkable effects on the thickness and roughness of the cell wall of all the laboratory-derived VISA derivatives.