Although there is no consensus of the precise definition of ESBL, three kinds of ESBL definitions have been proposed. First, the classical definition includes variants derived from TEM-1, TEM-2, or SHV-1; K1 (KOXY) of Klebsiella oxytoca. Second, the broadened definition has stretched the classical definition of ESBL to include: (1) β-lactamases (CTX-M-ESBLs, GES-ESBLs, and VEB-ESBLs), with spectra similar to those of TEM and SHV variants (designated as TEM- and SHV-ESBLs, respectively) but derived from other sources; (2) TEM and SHV variants with borderline ESBL activity; e.g., TEM-12; and (3) various β-lactamases conferring wider resistance than their parent types but not meeting the definition for group 2be; e.g., OXA-types (OXA-ESBLs) and mutant AmpC-types (AmpC-ESBLs), with increased activity against oxyimino-cephalosporins and with resistance to clavulanic acid. Third, the all-inclusive definition includes: (1) ESBL(A) (named for class A ESBLs); (2) ESBL(M) (miscellaneous ESBLs), which has been subdivided into ESBL(M-C) (class C; plasmid-mediated AmpC) and ESBL(M-D) (class D); and (3) ESBL(CARBA) (ESBLs with hydrolytic activity against carbapenems), which has been subdivided into ESBL(CARBA-A) (class A carbapenemases), ESBL(CARBA-B) (class B carbapenemases), and ESBL(CARBA-D) (class D carbapenemases). The consensus view about the ESBL definition is that the classical ESBL definition must be expanded to class A non-TEM- and non-SHV-ESBLs (CTX-M-, GES-, VEB-ESBLs, etc.). However, these three definitions evoke rational debate on the question "Which would be included in the category of ESBLs among AmpC-ESBLs, OXA-ESBLs, and/or carbapenemases?" Therefore, there is a great need for consensus in the precise definition of ESBL.
© 2010 Wiley Periodicals, Inc.