Which test is typically used phenotypically to detect ESBL production in Gram-negative rods?

Which test is typically used phenotypically to detect ESBL production in Gram-negative rods?

• A. Synergy test using clavulanic acid (e.g., combined disk or double-disk with clavulanate) showing an increased zone diameter
• B. PCR detection of bla genes
• C. Growth on media containing carbapenems
• D. Standard disk diffusion without inhibitors

Answer: (A)

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.

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.