Beta-lactamases target the four-atom beta-lactam ring found in beta-lactam antibiotics. In the most used classification system, Ambler classification, beta-lactamases are grouped into four classes (A–D) based on amino acid sequence homologies.
Ambler class A beta-lactamases include enzymes that are active on extended-spectrum cephalosporins (extended-spectrum beta-lactamases, ESBLs).
Ambler class B enzymes rely on zinc for their activity and are also known as metallo-beta-lactamases.
Ambler class D beta-lactamases were originally known as oxacillinases, owing to their rapid action on oxacillin, and are also known as OXA beta-lactamases. OXA enzymes have emerged as an important cause of carbapenem resistance, initially in Acinetobacter baumannii. OXA-type ESBLs have since spread to other bacterial families, including Enterobacteriaceae.