Antimicrobial Susceptibility Testing and Rapid Diagnostics Practice Test 2026 - Free Practice Questions and Study Guide

Detecting ESBL producers phenotypically hinges on how these enzymes are inhibited by clavulanic acid. In a synergy-based disk test, you compare a cephalosporin disk alone to another disk containing the same cephalosporin plus clavulanate. If an ESBL is present, the clavulanate blocks the enzyme near the cephalosporin, so the inhibition zone around the combination disk expands toward the cephalosporin disk, producing a “synergy” or keyhole effect. This larger zone diameter with the clavulanate combination indicates ESBL production.

Other approaches either look for the gene itself (PCR) or aren’t specific to ESBLs—growth on media with carbapenems won’t distinguish ESBLs from other resistance mechanisms, and standard disk diffusion without inhibitors won’t reveal the enzymatic activity.