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

• A. Inhibit bacterial protein synthesis at the 50S ribosomal subunit
• B. Inhibit bacterial cell-wall synthesis by binding penicillin-binding proteins
• C. Inhibit bacterial DNA replication by inhibiting DNA polymerase
• D. Disrupt bacterial cell membranes by detergents

Answer: (B)

Beta-lactam antibiotics kill bacteria by blocking cell-wall synthesis. They bind to penicillin-binding proteins, the enzymes that cross-link peptidoglycan strands in the cell wall. By inactivating these enzymes, the mesh-like structure cannot form properly, weakening the wall. In actively growing bacteria, this leads to osmotic instability and cell lysis, making these drugs bactericidal. Resistance can arise from beta-lactamases that hydrolyze the drug’s beta-lactam ring or from altered PBPs that reduce binding. This mechanism contrasts with agents that inhibit protein synthesis at the 50S ribosomal subunit, inhibit DNA replication, or disrupt membranes, which are actions of other antibiotic classes.