A Staphylococcus aureus isolate from a soft tissue abscess tests susceptible to erythromycin and clindamycin by routine MIC methods. The laboratory performs a D-zone test, which shows flattening of the clindamycin zone adjacent to the erythromycin disk. What is the correct interpretation and therapeutic implication?

• A True susceptibility to both drugs; either may be used
• B Efflux-mediated macrolide resistance via mef gene; clindamycin remains fully reliable
• C Inducible macrolide-lincosamide-streptogramin B resistance via erm methylase; clindamycin should not be used ✓
• D Beta-lactamase hyperproduction; all protein synthesis inhibitors will fail

Explanation

The D-test detects inducible resistance encoded by erm genes, which encode a ribosomal RNA methylase that alters the 23S rRNA target shared by macrolides, lincosamides and streptogramin C. Erythromycin induces expression of this enzyme, so clindamycin appears susceptible on routine testing but fails clinically. A positive D-test mandates reporting clindamycin as resistant. The mef efflux pump confers resistance to macrolides only and would give a negative D-test, leaving clindamycin usable.

Reference: Manual of Clinical Microbiology, 12th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.