β-lactamases are enzymes which catalyse the hydrolysis of β-lactams.  There are about fifty different known types.  The production of β-lactamases by bacterial cells is the most important contributing factor to the development of penicillin-resistant strains of bacteria.

By hydrolysing the labile β-lactam ring bond, the penicillin can no longer react with the transpeptidase enzyme.  Thus the antibacterial properties of the penicillin have been deactivated.7