What is the main mechanism of action of Penicillins?

• A. Inhibit transpeptidase
• B. Inhibit RNA synthesis
• C. Inhibit DNA gyrase
• D. Block folate synthesis

Answer: (A)

Penicillins kill bacteria by inhibiting transpeptidase enzymes, the penicillin-binding proteins that cross-link peptidoglycan in the bacterial cell wall. By blocking this cross-linking, the wall becomes weak and unable to withstand osmotic pressure during growth, leading to bacterial lysis. This bactericidal effect relies on the drug’s beta-lactam ring, which mimics the normal substrate and acylates the active site of PBPs. Other antibiotic classes work through different targets—RNA synthesis inhibitors disrupt transcription, DNA gyrase inhibitors affect DNA supercoiling, and folate synthesis blockers prevent nucleotide production—so those mechanisms are distinct from how penicillins act.