Susceptibility to beta-lactam antibiotics and production of beta-lactamase inBacteroides fragilis

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Abstract
Using the agar dilution technique, 231 strains ofBacteroides fragilis, isolated during a 2-year period from human infections, were identified at subspecies level and were tested for susceptibility to 13Β-lactam antibiotics. The penicillins were benzylpenicillin, ampicillin, carbenicillin, cloxacillin, and the recently described penicillin derivatives cyclacillin, ticarcillin, and PC-904. The following cephalosporin derivates were tested: cephaloridine, cephalothin, cephalexin, cefamandole and cefuroxime. The cephamycin C derivative cefoxitin was also included in the study. Cefoxitin was the most effective drug tested since more than 80% of the strains were inhibited by 8Μg/ml or less, and no strain had a minimal inhibitory concentration (MIC) of more than 64Μg/ml. There was no marked difference in sensitivity among the subspecies with exception of subspeciesvulgatus, which was slightly more sensitive to all antibiotics tested. The size of the inoculum was an important factor for obtaining reproducible results in the sensitivity tests. Increased inocula resulted in markedly higher MICs for cephaloridine and cefuroxime. Production ofΒ-lactamase was performed on all isolates by a chromogenic cephalosporin substrate and about 90% of the strains were found to beΒ-lactamase producers.