What is the mechanism of action of macrolide antibiotics, and are they typically bacteriostatic or bactericidal?

• A. Inhibit 50S ribosomal subunit; typically bacteriostatic.
• B. Inhibit DNA gyrase; bactericidal.
• C. Inhibit cell wall synthesis; bactericidal.
• D. Bind to 30S ribosomal subunit; bactericidal.

Answer: (A)

Macrolide antibiotics act by binding to the 50S ribosomal subunit in bacteria, near the site where the growing peptide chain would move during elongation. This interaction blocks translocation, effectively halting protein synthesis. Because turning off protein production prevents bacterial growth rather than immediately killing the cells, macrolides are typically bacteriostatic.

Inhibiting DNA gyrase (the mechanism of fluoroquinolones), inhibiting cell wall synthesis (like beta-lactams), and binding to the 30S ribosomal subunit (as with aminoglycosides or tetracyclines) describe other antibiotic classes with different actions and often bactericidal effects.