Determinants of high-level oxacillin resistance in MRSA strain USA300.

Methicillin-resistant Staphylococcus aureus (MRSA) strains carry either a mecA- or a mecC-mediated mechanism of resistance to beta-lactam antibiotics, but the phenotypic expression of resistance shows much more extensive strain-to-strain variation. In recent communications, we identified genetic determinants associated with the stringent stress response that were shown to play a major role in the antibiotic resistant phenotype of the historically earliest “archaic” clone of MRSA and in the mecC carrying MRSA strain LGA251. Here, we sought to test whether or not the same genetic determinants also contribute to the resistant phenotype of highly and homogeneously resistant (H*R) derivatives of a major contemporary MRSA clone USA300. We found that the resistance phenotype was linked to six genes (fruB, gmk, hpt, purB, prs, and relA), which were most frequently targeted among the analyzed 20 H*Rs (one mutation per clone in 19 out of the 20 H*Rs). Besides the strong parallels with our previous findings (five out of the six genes matched), all repeatedly targeted genes were found to be linked to guanine metabolism pointing to the key role that this pathway plays in defining the level of antibiotic resistance independent of the clonal type of MRSA