A Staphylococcus aureus isolate from a diabetic foot ulcer shows an erythromycin inhibition zone of 6 mm and a clindamycin zone of 21 mm on routine disk diffusion. A D-zone test performed by placing both disks 15 mm apart shows flattening of the clindamycin zone adjacent to the erythromycin disk, giving a D shape. How should the laboratory report clindamycin susceptibility?
- A Susceptible, since the clindamycin zone exceeds the susceptible breakpoint
- B Intermediate, pending repetition of the test after overnight incubation in carbon dioxide
- C Resistant, because the isolate carries erm-mediated inducible MLSb resistance that may become constitutive during therapy ✓
- D Susceptible only if the patient has no history of prior macrolide exposure
Explanation
Flattening of the clindamycin zone toward the erythromycin disk (positive D-test) demonstrates erm-encoded inducible macrolide-lincosamide-streptogramin C resistance. CLSI instructs laboratories to report clindamycin as resistant because the erm regulator can mutate to constitutive expression during monotherapy, producing sudden clinical failure. Option A ignores this induction phenomenon entirely. Carbon dioxide incubation is irrelevant here; it is mentioned in guidelines only for testing certain organisms such as streptococci, not for interpreting a positive D-test.
Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 11th ed.
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