A Staphylococcus aureus isolate from a wound infection has an oxacillin MIC of 8 mg/L and the cefoxitin screen disk shows a zone of 18 mm. This isolate is best classified as:

• A Methicillin-susceptible Staphylococcus aureus (MSSA)
• B Methicillin-resistant Staphylococcus aureus (MRSA) ✓
• C Borderline oxacillin-resistant Staphylococcus aureus (BORSA) — no mecA
• D Heterogeneous VISA (hVISA) requiring population analysis

Explanation

MRSA is defined by the presence of the mecA gene (or mecC) encoding an altered penicillin-binding protein PBP2a. Phenotypically, a cefoxitin disk screen zone ≤21 mm for S. aureus predicts mecA-mediated resistance with high sensitivity and specificity and is the preferred surrogate marker per CLSI guidelines. Oxacillin MIC ≥4 mg/L also confirms resistance. This isolate satisfies both criteria; BORSA is typically oxacillin MIC 1–8 mg/L without mecA, and hVISA has normal oxacillin susceptibility but reduced vancomycin susceptibility.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.