Driving Forces of Mechanisms Regulating Oxacillin-Resistance Phenotypes of MRSA: Truly Oxacillin-Susceptible mecA-Positive Staphylococcus aureus Clinical Isolates also Exist

As MRSA are considered Staphylococcus aureus isolates with oxacillin minimum inhibitory concentration (MIC) of ≥4 mg/L or harboring the mecA gene. However, the presence of mecA does not necessarily lead to oxacillin resistance and mecA gene-carrying isolates may have oxacillin MIC within the susceptible range (≤2 mg/L). During the last few years it has become apparent that oxacillin-susceptible (OS) mecA-positive S. aureus isolates (commonly called OS-MRSA) are rather commonly detected worldwide and may remain undiagnosed using phenotypic susceptibility testing methods. This review will summarize the current reports on OS-MRSA isolations and the underlying mechanisms regulating the expression of oxacillin resistance and also oxacillin susceptibility in mecA-positive S. aureus isolates. As MRSA commonly cause severe infections against which effective therapies are limited, understanding of these mechanisms could enable the identification of new targets for the treatment or reversion of the MRSA phenotype.