Recruitment of the mecA gene homologue of Staphylococcus sciuri into a resistance determinant and expression of the resistant phenotype in Staphylococcus aureus

Strains of methicillin-resistant Staphylococcus aureus (MRSA) have become t he most important causative agents of hospital-acquired diseases worldwide. The genetic determinant of resistance, mecA, is not a gene native to S. au reus but was acquired from an extraspecies source by an unknown mechanism. We recently identified a close homologue of this gene in isolates of Staphy lococcus sciuri, a taxonomically primitive staphylococcal species recovered most frequently from rodents and primitive mammals. In spite of the close sequence similarity between the mecA homologue of S. sciuri and the antibio tic resistance determinant mecA of S. aureus, S. sciuri strains were found to be uniformly susceptible to beta -lactam antibiotics. In an attempt to a ctivate the apparently "silent" mecA gene of S. sciuri, a methicillin-resis tant derivative, K1M200 (for which the MIC of methicillin is 200 mug/ml), w as obtained through stepwise exposure of the parental strain S. sciuri K1 ( methicillin MIC of 4 mug/ml) to increasing concentrations of methicillin. D NA sequencing of the mecA homologue from K1M200 revealed the introduction o f a point mutation into the -10 consensus of the promoter: the replacement of a thymine residue at nucleotide 1577 in the susceptible strain K1 by ade nine in the resistant strain K1M200, which was accompanied by a drastic inc rease in transcription rate and the appearance of a new protein that reacte d with monoclonal antibody prepared against the penicillin-binding protein 2A (PBP2A), i.e., the gene product of S. aureus mecA. Transduction of mecA from K1M200 (cloned into a plasmid vector) into a methicillin-susceptible S . aureus mutant resulted in a significant increase of methicillin resistanc e (from a methicillin MIC of 4 mug/ml to 12 and up to 50 mug/ml), the appea rance of a low-affinity PBP detectable by the fluorographic assay, and the production of a protein that reacted in a Western blot,vith monoclonal anti body to PBP2A. Antibiotic resistance and the protein products disappeared u pon removal of the plasmid-borne mecA homologue. The observations support t he proposition that the mecA homologue ubiquitous in the antibiotic-suscept ible animal species S. sciuri may be an evolutionary precursor of the methi cillin resistance gene mecA of the pathogenic strains of MRSA.