Plasmid-Encoded Transferable mecB-Mediated Methicillin Resistance in Staphylococcus aureus. SAmecB

Phenotypically categorized methicillin-resistant Staphylococcus aureus (MRSA) isolated during cefoxitin-based nasal screening was tested negative for the presence of the mecA and mecC genes as well as for the SCCmec-orfX junction region. The isolate was found to carry a mecB gene previously described for Macrococcus caseolyticus, but not for staphylococcal species. The gene is flanked by β-lactam regulatory genes similar to mecR, mecI, and blaZ and is part of an 84.6 kb multidrug-resistance plasmid that harbors genes encoding additional resistances to aminoglycosides (aacA-aphD, aphA, and aadK) as well as macrolides (ermB) and tetracyclines (tetS). By analogy with the widespread penicillin resistance, this further plasmid-borne β-lactam resistance mechanism harbors the putative risk of acceleration or reacceleration of the MRSA spread, resulting in broad ineffectiveness of β-lactams as a main therapeutic application against staphylococcal infections.