Antibiotic susceptibility of beta-lactamase-producing strains of Branhamella (Neisseria) catarrhalis.

All 11 clinically significant isolates of Branhamella catarrhalis examined in this study were found to produce beta-lactamase. The enzyme was apparently not plasmid associated since extrachromosomal deoxyribonucleic acid was not detected in any of the strains. The beta-lactamase activity of all strains was significantly depressed by the beta-lactamase inhibitors clavulanic acid and CP 45899. Based on comparisons of relative susceptibility to various beta-lactam antibiotics, it was inferred that the beta-lactamase of B. catarrhalis was significantly more active against penicillin congeners than against cephalosporin congeners. Most strains were not inhibited by readily achievable serum concentrations of the pencillinase-sensitive penicillins, penicillin G, ampicillin, and amoxicillin. Methicillin was equally ineffective. With rare exceptions, most strains of B. catarrhalis were inhibited by achievable serum concentrations of seven cephalosporins (cephalothin, cephapirin, cephaloridine, cephalexin, cephamandole, cefaclor, and cefuroxin) and one cephamycin (cefoxitin). All strains were uniformly resistant to clindamycin but were inhibited by achievable serum concentrations of erythromycin, tetracycline, chloramphenicol, and trimethoprim-sulfamethoxazole. Comparison of geometric mean minimum inhibitory concentrations of all antimicrobial agents tested suggested that B. catarrhalis was most susceptible to cefoxitin, erythromycin, and tetracycline.