Resistance to clindamycin can be induced in vitro by exposure to which antibiotic?

• A. Ampicillin
• B. Erythromycin
• C. Gentamicin
• D. Penicillin

Answer: (B)

Exposure to a macrolide antibiotic can induce clindamycin resistance through inducible MLSB (macrolide-lincosamide-streptogramin B) resistance. Bacteria carrying the erm gene produce a methylase that methylates the 23S rRNA in the 50S ribosomal subunit. This methylation alters the antibiotic binding site, so clindamycin can no longer bind effectively, rendering the bacteria resistant when induction occurs. Erythromycin acts as the inducer, triggering the expression of this resistance mechanism. Ampicillin, penicillin, and gentamicin do not induce this erm-mediated switch in expression, though they may select for resistant strains by other pathways. Clinically, inducible resistance is often detected with a D-test, where a specific pattern near erythromycin and clindamycin discs reveals the inducible resistance.