A class of antibiotics known as β-lactams which include penicillin, amoxicillin, carbapenems and cephalosporins undergo nucleophilic acyl substitution reactions.  The nucleophile is a hydroxyl group on a transpeptidase enzyme expressed by the bacteria.  Transpeptidase is required by bacteria to construct and maintain their cell walls.  When the penicillin below reacts with the transpeptidase, it deactivates the enzyme preventing the bacteria from growing and proliferating.  A β-lactam is a 4-membered ring cyclic amide as shown below.