Which antibiotic class inhibits protein synthesis at the 30S ribosomal subunit?

• A. Macrolides
• B. Tetracyclines
• C. Lincosamides
• D. Aminoglycosides

Answer: (D)

The key idea is that certain antibiotics halt protein synthesis by binding to the small subunit of the bacterial ribosome. The class that fits this mechanism best binds irreversibly to the 30S subunit and causes errors in decoding mRNA, plus they block the formation of the initiation complex. This leads to the production of faulty proteins and, importantly, to bacterial death. That bactericidal effect, along with their requirement for oxygen to enter the bacterial cell, makes them particularly effective against aerobic Gram-negative bacteria and distinguishes them from many other protein-synthesis inhibitors.

Tetracyclines also target the 30S subunit, but they primarily prevent aminoacyl-tRNA from entering the A site, which is typically bacteriostatic rather than bactericidal. In contrast, macrolides and lincosamides act on the 50S subunit, not the 30S, and interfere with translocation or peptide bond formation rather than decoding.