Low prevalence of borderline oxacillin resistant Staphylococcus aureus (BORSA) in a tertiary care hospital in South Carolina

Prompt treatment for Staphylococcus aureus bloodstream infections is often dependent on known diagnostic testing modalities to differentiate between methicillin-susceptible S. aureus (MSSA) and methicillin-resistant S. aureus (MRSA). Borderline-oxacillin resistant S. aureus (BORSA), a rare, non-mecA mediated phenotype, has unclear resistance mechanisms but potentially significant consequences as it is frequently misidentified as MSSA but behaves more like MRSA. A retrospective analysis was performed of MSSA bloodstream infections to determine the prevalence of BORSA. Our institution found BORSA prevalence of 0.1%, consistent with literature. Though prevalence is low, due to unclear mechanisms and unreliable detection methods, BORSA may pose a therapeutic and epidemiological threat.