Penicillin is an antibiotic that works by inhibiting the transpeptidase enzyme involved in bacterial cell wall biosynthesis (Figure 16.21). Penicillin is thought to react irreversibly with the transpeptidase that catalyzes the cross-linking. Unfortunately, resistance to penicillin can be acquired by bacteria that synthesize lactamase, an enzyme that hydrolyzes the lactam ring of penicillin and destroys its ability to interfere with peptidoglycan synthesis.