What is the main mechanism by which beta-lactam antibiotics exert bactericidal effects?

• A. Inhibit protein synthesis by binding ribosomes
• B. Inhibit cell wall synthesis by binding PBPs
• C. Inhibit DNA replication by inhibiting DNA gyrase
• D. Inhibit folate synthesis

Answer: (B)

Beta-lactam antibiotics kill by stopping the synthesis of the bacterial cell wall. They bind to penicillin-binding proteins, the enzymes that catalyze the cross-linking of peptidoglycan strands during cell-wall construction. This binding mimics the normal substrate and irreversibly inhibits the transpeptidation step, so cross-links cannot form. Without a strong, covalently cross-linked wall, the cell cannot maintain integrity as it grows, and osmotic pressure causes the bacterium to swell and burst—a bactericidal effect. This action is most effective when bacteria are actively dividing and building new wall material. Other antibiotic classes work by different pathways, such as blocking protein synthesis at ribosomes, inhibiting DNA replication, or preventing folate synthesis, but those mechanisms are separate from the cell-wall–synthesis inhibition described here.