A Staphylococcus aureus wound isolate shows an erythromycin zone of 8 mm and a clindamycin zone of 26 mm. On the same plate, the clindamycin zone is flattened on the side facing the erythromycin disk, giving a D shape. What does this indicate?

• A Beta-lactamase hyperproduction causing antagonism between the two drugs
• B True clindamycin susceptibility, so clindamycin can be reported without restriction
• C Efflux-mediated macrolide resistance that also confers clindamycin resistance
• D Inducible clindamycin resistance due to erm-encoded ribosomal methylation, so clindamycin should be reported as resistant ✓

Explanation

The A-test detects inducible macrolide-lincosamide-streptogramin B (MLSb) resistance. Erythromycin in the nearby disk induces expression of erm methylase genes, which alter the 23S rRNA target shared by clindamycin, producing the characteristic flattening of the clindamycin zone. CLSI advises reporting clindamycin as resistant or commenting on likely treatment failure. Efflux (msrA) affects macrolides only and produces no A-zone.

Reference: Koneman's Color Atlas and Textbook of Diagnostic Microbiology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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