Inactivation of the methicillin resistance gene mecA in vancomycin-resistant Staphylococcus aureus.

Acquisition of high-level resistance to vancomycin in the laboratory mutant VM50 (vancomycin MIC increased from 1.5 to 100 microg/ml) was accompanied by the appearance of a heterogeneous phenotype and a virtual loss in methicillin resistance: in most cells of cultures of VM50 the methicillin MIC of the parental strain was reduced from 800 to 1.5 microg/ml with only a subpopulation (10(-5)) retaining methicillin resistance at near the parental level (MIC of 400 microg/ml). Interestingly, the vancomycin MIC of this subpopulation was less (25 microg/ml) than that of VM50 (100 microg/ml). A similar antagonism between methicillin and vancomycin resistance levels was observed upon introduction of an intact mecA into VM50 on a plasmid vector: methicillin resistance of the majority of cells increased from 1.5 to 100 microg/ml while the vancomycin MIC declined from 100 to 12/25 microg/ml. Membrane preparations from mutant VM50 showed no detectable penicillin-binding protein (PBP) 2A by the fluorographic assay. Sequencing of the mecA gene resident in mutant VM50 indicated the presence of a 19-bp duplication between nucleotide residues 280-298, leading to the generation of a stop codon TAA starting at nucleotide position 286.