Why do penicillins work only on bacteria that are actively reproducing?

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Penicillins function by targeting the enzymes responsible for cell wall synthesis. Because this synthesis occurs exclusively during the process of microbial replication, the drug is only effective against bacteria that are actively reproducing and building new cell walls.1

By inhibiting these specific enzymes and simultaneously activating others that break down the microorganism's protective wall, penicillin compromises the structural integrity of the bacterium. This mechanism explains why the antibiotic is ineffective against dormant or non-replicating bacteria, as they are not actively synthesizing the cell wall components that the drug targets. Furthermore, because human cells lack cell walls entirely, they remain unharmed by this specific process.1