Is That Staphylococcus aureus Isolate Really Methicillin Susceptible?

The increased use of molecular diagnostic methods for the detection of methicillin-resistant Staphylococcus aureus (MRSA) has led to more frequent reports of mecA-positive, oxacillin-susceptible S. aureus (OS-MRSA) strains. This poses a dilemma for the laboratory, which must choose between reporting phenotypic or genotypic results as the best indicator for clinical response. Recent studies have demonstrated that phenotypic expression of oxacillin resistance can be “induced” in some OS-MRSA strains, either by a true induction mechanism or through the selection of mutations in a heterogeneously oxacillin-resistant strain, which results in an increased percentage of cells expressing oxacillin resistance. Thus, additional testing may be warranted when phenotypic and genotypic methods for oxacillin resistance disagree to avoid treating patients with antimicrobial agents that may prove ineffective.