An erythromycin-resistant Streptococcus pneumoniae isolate is found to carry the erm gene. The precise molecular mechanism conferred by this gene is:
- A Methylation of adenine on 23S ribosomal RNA, reducing macrolide binding affinity ✓
- B Ribosomal protection by a protein that displaces the drug from its binding site
- C Efflux of the drug through membrane transporters
- D Enzymatic hydrolysis of the macrolide lactone ring
Correct answer: A. Methylation of adenine on 23S ribosomal RNA, reducing macrolide binding affinity
Explanation
The erm genes encode ribosomal methyltransferases that methylate an adenine residue on 23S rRNA within the peptidyl transferase loop. This alters the shared binding site of macrolides, lincosamides, and streptogramins B, producing cross-resistance called the MLSb phenotype. Efflux is mediated by mef/msr genes, and ribosomal protection is the tet(M) mechanism seen with tetracycline resistance, not macrolide resistance.
Reference: Katzung Basic and Clinical Pharmacology, 15th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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