1. Double-disk diffusion and Etest ESBL detection tests.
Double-disk diffusion and Etest ESBL detection tests. (A) The double-disk diffusion ESBL detection test as suggested by Jarlier et al. is shown (76). A disk containing amoxicillin-clavulanate (AMC) is placed in proximity to a disk containing ceftazidime (CAZ) or another oxyimino-cephalosporin. The clavulanate in the amoxicillin-clavulanate disk diffuses through the agar and inhibits the β-lactamase surrounding the ceftazidime disk. Enhancement of the zone of the ceftazidime disk on the side facing the amoxicillin-clavulanate disk is interpreted as a positive test. (B) Etest ESBL strip (AB Biodisk, Solna, Sweden). The zone of inhibition is read from two halves of the strip containing ceftazidime alone (TZ) or ceftazidime plus clavulanate (TZL). A reduction in the MIC of ceftazidime of ≥3 dilutions in the presence of clavulanate is interpreted as a positive test. (C) The Etest ESBL strip is sometimes difficult to interpret with weak enzyme producers such as the strain expressing TEM-12 shown in this panel. The clavulanate from the ceftazidime plus clavulanate half of the strip diffuses into the agar and interferes with the reading of the MICs for the half of the strip containing ceftazidime alone.
Patricia A. Bradford Clin. Microbiol. Rev. 2001; doi: /CMR