Use of β-lactamase inhibitors in disk tests to detect plasmid-mediated AmpC β-lactamases

Seeking a simple disk test for detection of organisms producing plasmid-mediated AmpC beta-lactamases, we evaluated the diagnostic utility of the beta-lactamase inhibitors 48-1220 (Ro 48-1220) and LN-2-128. Using NCCLS disk methodology, inhibition zone diameters were determined for five beta-lactam antibiotics tested alone and in combination with 20 mug of either 48-1220 or LN-2-128. Using an increase of 2:4 mm in zone diameter in the presence of an inhibitor as a positive test, cefotetan with LN-2-128 and 48-1220 was adequate for the detection of organisms producing plasmid-mediated AmpCs (specificity of 90% and sensitivity of 100%).