Which statement best describes penicillin's mechanism of action?

• A. They inhibit cell wall synthesis
• B. They inhibit protein synthesis
• C. They disrupt DNA replication
• D. They inhibit RNA transcription

Answer: (A)

Penicillin works by blocking the synthesis of the bacterial cell wall. It binds to penicillin-binding proteins, the enzymes that cross-link peptidoglycan strands (transpeptidases) during wall formation. Without this cross-linking, the cell wall cannot withstand the internal turgor pressure as the bacterium grows, leading to cell lysis and death. This selective toxicity comes from humans lacking peptidoglycan in their cells, so penicillin targets bacteria specifically. This mechanism is different from attackers that inhibit protein synthesis, DNA replication, or RNA transcription, which act on ribosomes, DNA gyrase, or RNA polymerase, respectively. Because penicillin is most effective when bacteria are actively growing and building their walls, it is bactericidal under those conditions. Some bacteria resist by beta-lactamases breaking down the drug or by altering the target PBPs, which is why certain infections require combinations or alternative agents.