Detection of beta-lactamase activity among clinical isolates of Branhamella catarrhalis with six different beta-lactamase assays.

A total of 74 different clinical isolates of Branhamella catarrhalis were examined for their ability to produce beta-lactamase by six different beta-lactamase assays. These included a conventional tube and disk test, in which the chromogenic cephalosporin nitrocefin was used as a substrate; a disk procedure, in which pyridinium-2-azo-p-dimethylanaline cephalosporin was used as a substrate; broth and disk acidometric methods; and a conventional tube iodometric assay. A total of 58 of the study isolates produced beta-lactamase. In all cases, positive results were obtained with the nitrocefin tube and disk assays after 1 min. With the pyridinium-2-azo-p-dimethylanaline cephalosporin disk test, 57 of the 58 beta-lactamase-producing strains yielded a positive reaction in 1 min; the remaining strain was positive after 10 min. None of the beta-lactamase-producing strains produced positive reactions by either the broth or disk acidometric methods after 1 min. With the broth test, 10 min was required for positive test results for 42 strains; 30 min was necessary for 16 strains. By the disk acidometric procedure, all 58 strains were positive after 10 min. Of 58 beta-lactamase-producing strains, 30 were positive by the iodometric assay after 1 min, 13 strains required 10 min, and 4 strains were detected as being beta-lactamase positive only after 30 min. One beta-lactamase-producing strain remained negative by the iodometric method. Among the 16 strains of B. catarrhalis that lacked beta-lactamase that were examined in this study, no false-positive results were obtained by any of the six assays.