Stealth MRSA: a diagnostic pitfall in invasive Staphylococcus aureus infection

Methicillin-resistant Staphylococcus aureus (MRSA) is commonly identified by phenotypic antimicrobial susceptibility testing or detection of the mecA gene encoding penicillin-binding protein 2a (PBP2a). A subset of isolates, referred to as oxacillin-susceptible MRSA (OS-MRSA) or "stealth MRSA," demonstrates phenotypic susceptibility to oxacillin despite harboring mecA, creating a diagnostic and therapeutic challenge.

We describe a case of invasive infection caused by a USA300 OS-MRSA strain in a patient with a soft tissue abscess and infective endocarditis. Repeated phenotypic testing demonstrated oxacillin susceptibility, while molecular assays detected the mecA gene. Additional phenotypic and genotypic analyses confirmed OS-MRSA. The patient was successfully treated with MRSA-active antimicrobial therapy following recognition of the genotype-phenotype discrepancy.

This case demonstrates the limitations of phenotypic susceptibility testing alone for detection of methicillin resistance in S. aureus. Supplemental testing beyond routine phenotypic susceptibility may be needed for accurate classification and appropriate management of OS-MRSA, particularly in invasive infections where failure to recognize resistance may lead to suboptimal therapy.