What is the mechanism of action of macrolide antibiotics?

• A. Inhibit DNA gyrase
• B. Bind to the 50S ribosomal subunit and inhibit translocation
• C. Inhibit cell wall synthesis
• D. Block folate synthesis

Answer: (B)

Macrolide antibiotics bind to the 50S ribosomal subunit, specifically interacting with the 23S rRNA, and block the translocation step of protein synthesis. This prevents the ribosome from moving along the mRNA after a peptide bond forms, halting elongation of the growing polypeptide. Without elongation, bacterial protein production stops, leading to a bacteriostatic effect in many cases. This mechanism is distinct from other antibiotic actions: DNA gyrase inhibitors disrupt DNA replication, cell wall synthesis inhibitors prevent building the cell wall, and folate synthesis blockers disrupt production of essential cofactors. So, binding to the 50S subunit and inhibiting translocation best describes how macrolides work.