Mechanism of Action: Penicillins inhibit bacterial enzymes because the actions described are capable of directly killing bacteria. Because penicillin dose not act on mammalian cells, these drugs are relatively free of toxicity.Resistance to the action of penicillins occurs in bacteria that produce penicillinases or beta-lacta-mases. These enzymes destroy the penicillin molecule before the drug can fatally disrupt the bacterial cell wall. Less common mechanisms of resistance do not involve beta-lactamases or penicillinases but arise from intrinsic resistance of the targets of penicillins action in the bacteria.