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.
Keywords: FemXAB, OS-MRSA, heterogeneity, community-associated MRSA, PBP2a, molecular detection.
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