ALTERED PRODUCTION OF PENICILLIN-BINDING PROTEIN 2A CAN AFFECT PHENOTYPIC-EXPRESSION OF METHICILLIN RESISTANCE IN STAPHYLOCOCCUS-AUREUS

Altered production of penicillin-binding protein 2a (PBP 2a) may affec t the phenotypic expression of resistance in methicillin-resistant Sta phylococcus aureus (MRSA). COL, an MRSA strain that constitutively pro duces PBP 2a, was transformed with a recombinant plasmid containing th e two beta-lactamase regulatory genes, blaI and blaR1, with either the beta-lactamase gene, blaZ, or a truncated blaZ. Both of the transform ed MRSA strains now produced an inducible PBP 2a, and the MICs of nafc illin, methicillin, and imipenem for these strains were similar to tho se for the parental strain. A mutation in blaR1 that resulted in the c omplete repression of PBP 2a production altered the phenotypic express ion of methicillin resistance in that strain, as evidenced by efficien cy-of-plating experiments. Rather than being homogeneously resistant l ike COL, the blaR1 mutant strain now appeared to have a small resistan t subpopulation. Gene products that regulate PBP 2a production may con tribute to the organism's expression of methicillin resistance, but ad ditional chromosomally located factors are required.