Enterobacteriaceae are capable of producing β-lactamases that hydrolyze penicillins and cephalosporins, including the extended spectrum cephalosporins (cefoxime, ceftriazone, ceftizoxime, and ceftazidime). These enzymes are referred to as ESBLs. A chromogenic agar has been developed for the detection of ESBLs. The agar chrom ID ESBL (bioMerieux, Marcy l’Etolle, France) uses cefpodoxime as a substrate to increase the recovery and sensitivity of CTX-M type ESBL isolates. Some limitations must be considered in the use of this medium, including hyperproducing AmpC (Enterobacter and Citrobacter spp.) and hyperproducing penicillinase (K. oxytoca) false positives. In addition, both Vitek 2 (bioMerieux, Durham, North Carolina) and Phoenix (Becton Dickinson, Sparks, Maryland) have ESBL panels, with expert interpretation available for clinical diagnostic use. Table 1 presents an example of an ESBL pattern from a clinical isolate that may require technical interpretation and correction before the results are reported.

Table1.  Extended Beta-Lactamase Antibiotic Resistance Pattern Based on Vitek 2 Gram-Negative Susceptibility AST-GN24 of an  E. coli Isolate

ESBLs can occur in bacteria other than Klebsiella spp., E. coli, and Proteus mirabilis. The Clinical and Laboratory Standards Institute and (CLSI) has created guide lines (CLISI document M-100 and M100-S23) for the minimum inhibitory concentration (MIC) and disk diffusion breakpoints for aztreonam, cefotaxime, cefpodoxime, ceftazidime, and ceftriaxone for E. coli, Proteus, and Klebsiella spp., as well as for cefpodoxime, ceftazidime, and cefotaxime for P. mirabilis. The sensitivity of the screening increases with the use of more than a single drug. ESBLs are inhibited by clavulanic acid; therefore, this property can be used as a confirmatory test in the identification process. In addition, with regard to cases in which moxalactam, cefonicid, cefamandole, or cefoperazone is being considered to treat infection caused by E. coli, Klebsiella spp., or Proteus spp., it is important to note that interpretive guidelines have not been evaluated, and ESBL testing should be performed. If isolates test ESBL positive, the results of the antibiotics listed should be reported as resistant.

CLSI has revised the interpretive criteria for cephalosporins (cefazolin, cefotaxime, ceftazidime, ceftizoxime, and ceftriaxone) and aztreonam. Using the new interpretive guidelines, routine ESBL testing is no longer necessary, and it is no longer necessary to edit results for cephalosporins, aztreonam, or penicillins from susceptible to resistant. ESBL testing will remain useful for epidemiologic and infection control purposes.

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