ACTIVATED CELL-WALL SYNTHESIS IS ASSOCIATED WITH VANCOMYCIN RESISTANCE IN METHICILLIN-RESISTANT STAPHYLOCOCCUS-AUREUS CLINICAL STRAINS MU3 AND MU50

We have previously reported methicillin-resistant Staphylococcus aureu s clinical strains, Mu50 and Mu3, representing two categories of vanco mycin resistance: Mu50 representing vancomycin-resistant S. aureus (VR SA) with MICs greater than or equal to 8 mg/L, and Mu3 representing he tero-VRSA with MICs less than or equal to 4 mg/L using standard MIC de termination methods. The mechanisms of vancomycin resistance in these strains were investigated. These strains did not carry the enterococca l vancomycin-resistance genes, vanA, vanB, or vanC1-3, as tested by PC R using specific primers. However, both strains produced three to five times the amount of penicillin-binding proteins (PBPs) 2 and 2' when compared with vancomycin-susceptible S. aureus control strains with or without methicillin resistance; the amounts of PBP2 produced in Mu3 a nd Mu50 were comparable to those in the vancomycin-resistant S aureus mutant strains selected in vitro. Incorporation of C-14-labelled N-ace tyl-glucosamine into the cell was three to 20 times increased in Mu50 and Mu3, and release of the radioactive cell wall material was increas ed in Mu3 (and also in Mu50, though to a lesser extent), compared with control strains. The amounts of intracellular murein monomer precurso r in these strains were three to eight times greater than those found in control strains. Transmission electron microscopy showed a doubling in the cell wall thickness in Mu50 compared with the control strains. Mu3 did not show obvious cell wall thickening. These data indicate th at activated synthesis and an increased rate of cell wall turnover are common features of Mu3 and Mu50 and may be the prerequisite for the e xpression of vancomycin resistance in S. aureus.