MASSIVE REDUCTION IN METHICILLIN RESISTANCE BY TRANSPOSON INACTIVATION OF THE NORMAL PBP2 IN A METHICILLIN-RESISTANT STRAIN OF STAPHYLOCOCCUS-AUREUS

Screening of a large transposon library constructed in the background of a highly and homogeneously methicillin-resistant Staphylococcus aur eus (MRSA) strain (methicillin MIC 1,600 mu g/ml) for Tn551 mutants wi th reduced resistance, identified mutant RUSA130 with a methicillin MI C of 12 mu g/ml. Cloning in E. coli followed by sequencing located the Tn551 insert Ohm 703 near the C-terminal of the PBP2 gene. Penicillin -binding assays with mutant RUSA130 showed the presence of normal amou nts of penicillin-binding protein 2A (PBP2A) but the absence of PBP2. These observations suggest that the mecA gene product PBP2A is not the sole catalyst of peptidoglycan synthesis in MRSA growing in the prese nce of beta-lactam antibiotics, since an intact PBP2 is also essential for the optimal expression of methicillin resistance in MRSA.