Stealth MRSA: a diagnostic pitfall in invasive <i>Staphylococcus aureus</i> infection

ABSTRACT Background 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. Case Summary 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. Conclusion 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.