What is the mechanism of action of beta-lactam antibiotics?

• A. Inhibit protein synthesis at 50S.
• B. Disrupt cytoplasmic membrane by pore formation.
• C. Inhibit bacterial cell wall synthesis by binding penicillin-binding proteins and activating autolysins.
• D. Inhibit DNA gyrase.

Answer: (C)

Beta-lactam antibiotics kill bacteria by blocking the synthesis of the peptidoglycan cell wall. They do this by binding to penicillin-binding proteins, the enzymes that cross-link (transpeptidate) the wall strands. When these enzymes are inhibited, the cell wall cannot be properly formed, so the wall weakens and the bacterium lyses as it attempts to grow. Some beta-lactams also promote autolysin activity, enzymes that break down peptidoglycan, accelerating wall destruction. Because they target active cell wall synthesis, these drugs are typically bactericidal against growing bacteria. Other mechanisms—like inhibiting protein synthesis at the ribosome, disrupting membranes, or inhibiting DNA gyrase—belong to different antibiotic classes.