Mechanism of Action: Penicillins inhibit bacterial enzymes the substance that gives rigidity to the bacterial cell wall. Without rigid support, the cell wall is unablr to withstand osmotic forces from within the cell, and the bacteria can rupture. Penicillins also alter the composition of the cell wall, futher weakening the structure. Penicillins are considered bactericidal 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.