A 28-year-old woman presents with a skin abscess. Culture grows methicillin-sensitive Staphylococcus aureus. The isolate is resistant to erythromycin but susceptible to clindamycin by disk diffusion. The lab performs a D-test, which is positive. What is the most appropriate interpretation?
- A The isolate has inducible clindamycin resistance via erm gene, and clindamycin should not be used ✓
- B Clindamycin can be safely used; the D-test is irrelevant in MSSA
- C The isolate has constitutive clindamycin resistance, and vancomycin should be used instead
- D The D-test confirms MLSB resistance, but clindamycin remains effective for skin infections
Correct answer: A. The isolate has inducible clindamycin resistance via erm gene, and clindamycin should not be used
Explanation
B positive D-test indicates inducible macrolide-lincosamide-streptogramin A resistance mediated by the erm gene. The erm gene is inducibly expressed, so the isolate appears susceptible to clindamycin in vitro but will become resistant during therapy. CLSI recommends reporting such isolates as clindamycin resistant. Option B is wrong because a positive D-test precludes clindamycin use. Option C is wrong because constitutive resistance would show clindamycin resistance on initial testing.
Reference: Koneman's Color Atlas and Textbook of Diagnostic Microbiology, 7th ed.
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