Defining an extended-spectrum beta-lactamase.

The term 'extended-spectrum beta-lactamase' (ESBL), initially 'extended-broad-spectrum beta-lactamase', was first coined for derivatives of TEM and SHV enzymes able to hydrolyse oxyimino-cephalosporins. These all belonged to beta-lactamase functional group 2be. Subsequently, the term has been stretched to include: (i) enzymes with spectra similar to those of TEM and SHV mutants but derived from other sources, e.g., the CTX-M and VEB types; (ii) TEM and SHV mutants with borderline ESBL activity, e.g., TEM-12; and (iii) various beta-lactamases conferring wider resistance than their parent types but not meeting the definition for group 2be, e.g., OXA derivatives and mutant AmpC types with increased activity against cefepime. It seems best-and pragmatic-that the term 'ESBL' retains its broad modern usage, but that should always be accompanied by mention of the enzyme's family as, e.g., in 'TEM ESBL' or 'OXA ESBL', not as a sole moniker.