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