Emergence of methicillin-resistant clones from cephamycin-resistant Staphylococcus aureus

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Abstract
Staphylococcus aureus strains specifically resistant to cephamycin antibiotics have been found among recent clinical isolates. These strains formed penicillin-binding protein (PBP) 2′ and became phenotypically resistant to methicillin after induction with cefoxitin. Other cephamycin-type antibiotics also induced methicillin-resist-ance, whereas non-cephamycin-type cephalosporins such as cefmenoxime and cefti-zoxime did not do so. The clones that constitutively synthesized PBP 2′ arose from the cephamycin-resistant strains at a frequency of 10−5. They were indistinguishable from clinically isolated methicillin-resistant S. aureus (MRSA). Cephamycin-resistant S. aureus may be a source for emerging MRSA.