Evaluation of two methods for rapid testing for beta-lactamase production in Bacteroides and Fusobacterium

A total of 978 strains of Bacteroides and Fusobacterium were tested for beta-lactamase production by a disk test (Cefinase) and a microtiter nitrocefin assay. In 83% of strains both tests were positive and in 14.8% both were negative. In 1.7% of strains the disk test was positive and the microtiter test negative, and in 0.4% the disk test negative but the microtiter test positive. The disk test was less discriminatory in detecting amoxicillin-resistant strains. The microtiter test was less sensitive than the disk test, but more discriminatory if results were read within 1 h for Fusobacterium spp., within 8 h for the Bacteroides fragilis group, and within 2 h for other Bacteroides spp. Neither test should be used clinically at present.