A wound isolate of Staphylococcus aureus shows resistance to erythromycin (zone 6 mm) and an apparent zone of inhibition around clindamycin (zone 22 mm). A clindamycin disk is placed adjacent to the erythromycin disk, and flattening of the clindamycin zone on the side facing erythromycin is observed (D-shaped zone). How should clindamycin be reported?

• A Susceptible, because the standalone zone is within the susceptible range
• B Resistant, because the D-test detects inducible macrolide-lincosamide-streptogramin B resistance ✓
• C Intermediate, pending repeat testing on a fresh subculture
• D Susceptible only if the isolate is methicillin-susceptible

Explanation

The D-test demonstrates inducible MLSB resistance: erythromycin induces expression of erm-encoded 23S rRNA methylase, which alters the ribosomal binding site shared by macrolides, lincosamides and streptogramin B. Clindamycin must therefore be reported as resistant, since clinical failure occurs when the erm gene is expressed in vivo under lincosamide pressure. The size of the clindamycin zone alone does not predict this phenotype, and methicillin susceptibility status is irrelevant to the interpretation.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.