Extended-Spectrum Beta-Lactamase (ESBL) Activity

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The page below is a sample from the LabCE course Case Studies in Clinical Microbiology. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Extended-Spectrum Beta-Lactamase (ESBL) Activity

  • Illustrated in the image is the surface of a disk diffusion test including a 30 µg ceftazidime disk (left) and a combination 30/10 µg ceftazidime/clavulanic acid disk (right).
  • Observe in the image that the zone of inhibition around the the combination ceftazidime/clavulanic acid disk (right) is at least 5 mm larger than around the ceftazidime disk (left).
  • This observation that the presence of clavulanic acid, a beta-lactamase inhibitor, has resulted in such a large increase in the zone of inhibition indicates that an extended-spectrum beta-lactamase (ESBL) is being produced.
  • When an organism is producing an ESBL, the susceptibility to individual cephalosporins cannot be predicted, thus requiring that each drug must be tested individually.