ROLE OF PENICILLINASE PLASMIDS IN THE STABILITY OF THE MECA GENE IN METHICILLIN-RESISTANT STAPHYLOCOCCUS-AUREUS

The stability of methicillin resistance (Mc(r)) in three independent clinical isolates, MR108, MR6, and MR61, of methicillin-resistant Staphylococcus aureus (MRSA) was studied. The Mc(r) phenotype was stably maintained in the progeny of all three MRSA clones carrying penicillinase plasmids. However, when the clones were tested after elimination of the plasmids, methicillin-susceptible (Mc(s)) subclones appeared at various frequencies. Seven Mc(s) subclones were classified into two groups based on their stabilities. Five Mc(s) subclones, which were derived from homogeneous strains MR108 and MR61, were stably susceptible. They lost penicillin-binding protein 2' production, and moreover, the mecA gene was deleted in four of five subclones. Two subclones were derived from heterogeneous strain MR6. They were very unstable, and more than half of their progeny were Mc(r) revertants. However, the remainder were stably Mc(s) and had lost penicillin-binding protein 2' and the mecA gene. We propose that penicillinase plasmids, which are present in most MRSA strains, play an important role in the stability and phenotypic expression of the mecA gene.