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Mechanism of vancomycin resistance in methicillin resistant Staphylococcus aureus.

A collection of laboratory mutants and clinical MRSA strains, additionally exhibiting resistance to glycopeptide antibiotics, was studied in detail. The nature of resistance to glycopeptides was found to be different from that existing in vancomycin resistant (VR) enterococci. The mutants produced abnormal murein in which the level of highly oligomeric muropeptides was drastically reduced. Biochemical and genetic analyses of Penicillin Binding Proteins (PBPs) showed inactivation of PBP4. Changes in other PBPs were not apparent, except for PBP2a that was inactivated in the highly VR mutant VM. Transposon inactivation of the pbpB gene and several other genes involved in synthesis of staphylococcal peptidoglycan all caused dramatic reduction of glycopeptide resistance in the staphylococcal mutants. While inactivation of PBP2a slightly increased the levels of glycopeptide resistance, a combination of vancomycin or teicoplanin with beta-lactam inhibitors, chosen on the basis of their relatively selective affinities for individual staphylococcal PBPs completely inhibited the expression of glycopeptide resistance in MRSA. Glycopeptide antibiotics caused a virtually complete inhibition of cell wall turnover and autolysis and massive overgrowth of cell wall material in the glycopeptide resistant mutants. Bacteria were able to remove quantitatively glycopeptide molecules from the growth medium, and sequestered antibiotic could be recovered in biologically active form from the purified cell walls. These observations and the results of the vancomycin binding studies suggest alterations in the structural organization of the mutants' cell wall such that access of glycopeptide molecules to the sites of wall biosynthesis is blocked by steric hindrance.