Penicillin is an irreversible inhibitor of the transpeptidases, enzymes that cross-link components of the bacterial cell wall. The cell wall is thus fragile and the bacteria die.
Penicillin and its derivatives are structural analogs of the natural substrates of these enzymes.
Why doesn't penicillin normally kill humans, unless a severe allergic reaction (anaphylaxis) develops? How does penicillin inhibit transpeptidase (what kind of inhibitor is it)?