A blood culture bottle flags positive at 14 hours. Gram stain shows Gram-positive cocci in clusters. Rapid identification by MALDI-TOF MS identifies Staphylococcus aureus. The report also includes a Cefoxitin disc diffusion result showing an inhibition zone of 14 mm. How should this be interpreted?
- A MSSA (methicillin-sensitive S. aureus) — cefoxitin ≥22 mm indicates susceptibility
- B Intermediate — repeat MIC by broth microdilution needed
- C MRSA (methicillin-resistant S. aureus) — cefoxitin breakpoint ≤21 mm indicates resistance ✓
- D Cannot interpret; must use oxacillin disc instead
Correct answer: C. MRSA (methicillin-resistant S. aureus) — cefoxitin breakpoint ≤21 mm indicates resistance
Explanation
For Staphylococcus aureus, CLSI breakpoints for cefoxitin disc diffusion (30 µg disc): susceptible (MSSA) ≥22 mm; resistant (MRSA) ≤21 mm. A zone of 14 mm falls in the resistant category, indicating MRSA. Cefoxitin is used as a surrogate for mecA gene detection because it is a stronger inducer of mecA expression than oxacillin, giving more reliable results. MRSA should be reported as resistant to ALL beta-lactams (except ceftaroline and ceftobiprole). Treatment for serious MRSA infections: IV vancomycin or daptomycin.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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