Which statement best describes the action of penicillin?

• A. It inhibits RNA polymerase
• B. It disrupts cell membrane permeability
• C. It inhibits cell wall synthesis by binding PBPs
• D. It inhibits DNA gyrase

Answer: (C)

Penicillin works by blocking cell wall synthesis. It does this by binding to penicillin-binding proteins (PBPs), the enzymes that cross-link the peptidoglycan mesh that gives the bacterial wall its strength. The beta-lactam ring of penicillin fits into the active site of these PBPs and acylates them, inactivating their transpeptidase function. Without proper cross-linking, the cell wall becomes weak, and bacteria that are actively growing will lyse from osmotic pressure. This is why penicillin is bactericidal in susceptible organisms.

This action is different from inhibiting RNA polymerase (which would stop transcription), disrupting membrane permeability (as some polymyxins do), or inhibiting DNA gyrase (as fluoroquinolones do).