Recruitment of the mecA Gene Homologue of Staphylococcus sciuri into a Resistance Determinant and Expression of the Resistant Phenotype in Staphylococcus aureus

Abstract

Strains of methicillin-resistant Staphylococcus aureus (MRSA) have become the most important causative agents of hospital-acquired diseases worldwide. The genetic determinant of resistance, mecA, is not a gene native to S. aureus but was acquired from an extraspecies source by an unknown mechanism. We recently identified a close homologue of this gene in isolates of Staphylococcus 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 antibiotic resistance determinant mecA of S. aureus, S. sciuri strains were found to be uniformly susceptible to ␤-lactam antibiotics. In an attempt to activate the apparently "silent" mecA gene of S. sciuri, a methicillin-resistant derivative, K1M200 (for which the MIC of methicillin is 200 g/ml), was obtained through stepwise exposure of the parental strain S. sciuri K1 (methicillin MIC of 4 g/ml) to increasing concentrations of methicillin. DNA sequencing of the mecA homologue from K1M200 revealed the introduction of 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 adenine in the resistant strain K1M200, which was accompanied by a drastic increase in transcription rate and the appearance of a new protein that reacted 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 resistance (from a methicillin MIC of 4 g/ml to 12 and up to 50 g/ml), the appearance of a low-affinity PBP detectable by the fluorographic assay, and the production of a protein that reacted in a Western blot with monoclonal antibody to PBP2A. Antibiotic resistance and the protein products disappeared upon removal of the plasmid-borne mecA homologue. The observations support the proposition that the mecA homologue ubiquitous in the antibiotic-susceptible animal species S. sciuri may be an evolutionary precursor of the methicillin resistance gene mecA of the pathogenic strains of MRSA.The emergence and worldwide spread of methicillin-resistant Staphylococcus aureus (MRSA) between the early 1960s and the late 1990s have begun to pose serious threats to the chemotherapy of staphylococcal diseases worldwide. The genetic determinant of methicillin resistance in MRSA is the acquired gene mecA, which encodes the low-affinity penicillinbinding protein 2A (PBP2A), which, according to current theory, can function as a surrogate transpeptidase in the presence of high concentrations of ␤-lactam antibiotics that inactivate the four high-affinity PBPs native to S. aureus (5). The mecA gene and the associated large (40-to 60-kb) mec element (9,10,13,15,21,27) are not native to S. aureus but were acquired from an extraspecies source by an unknown mecha...

MRSA and MSSA: The Mechanism of Methicillin Resistance and the Influence of Methicillin Resistance on Biofilm Phenotype of Staphylococcus aureus

“…But S. sciuri containing mecA homolog is susceptible to methicillin. This supported that MRSA strains are descendents of ancestral strains in evolutionary process [35,36]. Staphylococcus haemolyticus (S. haemolyticus) genome carries intact IS1272 element, whereas the gene of S. aureus and S. epidermidis carries IS1272 element deleted [37].…”

Methicillin resistance in Staphylococcus aureus requires glycosylated wall teichoic acids

“…Polyvalent scaffolding by glycopolymers (glycolipids and glycoproteins) is a known mechanism for organizing proteins in a variety of biological processes (38,39). Whereas there are no genes of high homology to tarS in Staphylococcus sciuri, the organism from which PBP2a was speculated to derive (40,41), all deposited S. aureus strain sequences in the National Center for Biotechnology Information contain tarS. It is possible that PBP2a may exploit β-OGlcNAcylated WTAs by binding to them directly or by interacting with other cellular factors that are scaffolded by these polymers.…”

A comparative evaluation of methicillin-resistant staphylococci isolated from harness racing-horses, breeding mares and riding-horses in Italy

“…It has been reported that many Staphylococcus epidermidis and other CoNS from human clinical isolates are resistant to methicillin [28] . An evolutionary theory proposed that S. aureus could have acquired mecA gene from S. sciuri, a species frequently isolated from animals, which harbours a close structural homologue of mecA gene [29] . These bacteria can serve as reservoirs of resistance determinants in the community, which could lead to the emergence of novel MRSA or MRS strains.…”