Disk diffusion susceptibility of Branhamella catarrhalis and relationship of beta-lactam zone size to beta-lactamase production

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Abstract
We tested 231 isolates of Branhamella catarrhalis for beta-lactamase production and drug susceptibility by the National Committee for Clinical Laboratory Standards disk diffusion method. The nitrocephin disk (Cefinase) identified beta-lactamase in 98% of the enzyme-producing strains, and a zone diameter of inhibition of less than or equal to 29 mm for penicillin correctly predicted the presence of beta-lactamase in 99% of the isolates. No resistance to erythromycin, tetracycline, trimethoprim-sulfamethoxazole, or amoxicillin-clavulanic acid was observed.