Why do beta-lactam antibiotics (e.g., penicillins) show selective toxicity toward bacteria but not human cells?

Because human cells lack a lipid bilayer membrane and therefore cannot be affected by beta-lactams that only integrate into membranes and cause depolarization without interacting with any enzymatic machinery.

Because beta-lactams inhibit mitochondrial ribosomes identical to bacterial 70S ribosomes, but mitochondria compensate by increasing glycolysis so host toxicity is never observed.

Because beta-lactams inhibit bacterial penicillin-binding proteins that catalyze peptidoglycan cross-linking, a structure humans lack entirely, so human cells have no analogous targets and are spared at therapeutic doses.

Because human cells produce peptidoglycan only in the context of cell division and beta-lactams target a stage of peptidoglycan formation that is absent in fully differentiated human cells, making them selectively toxic.