Why do penicillins harm bacteria without harming human cells?

Penicillins are able to harm bacteria without damaging human cells because they specifically target the synthesis of the bacterial cell wall, a structure that human cells fundamentally lack.1

The mechanism of action for penicillins involves a unique chemical structure known as a $\beta$-lactam ring. This ring inhibits specific bacterial enzymes responsible for the cross-linking of peptidoglycan strands, which provides the cell wall with its rigid stability. By blocking this final assembly step, penicillins weaken the protective wall of replicating microorganisms, leading to their death.2314

Because human cells do not possess cell walls or the specific enzymes required for peptidoglycan synthesis, they remain unaffected by the drug's primary mechanism. This selective toxicity makes penicillins among the safest of all antibiotics, with the most common adverse reactions being related to hypersensitivity rather than direct cellular damage.14