Posters presented at the Third Anaerobe Discussion Group Symposium, 1983

Fifty recent clinical isolates of the B.fragilis group were tested to determine their susceptibility to ampicillin, carbenicillin, mezlocillin, azlocillin and piperacillin using an agar dilution technique. The influence of clavulanic acid on the penicillin susceptibility of these strains was determined by incorporating 2 mg/1 of clavulanic acid into test media. The production of beta-lactamase by the strains was determined by the nitrocefin test and their isoelectric points by focusing on Polyacrylamide gel. Piperacillin was generally the most active of the penicillins tested and carbenicillin the least active with MIC50 values of 8 and 32 respectively. Although mezlocillin was slightly more active than piperacillin against highly resistant strains (MIC 64 mg/1 ampicillin) it was less active against strains with moderate resistance (16–32 mg/1 ampicillin). The majority of resistant strains became at least fivefold more susceptible to ampicillin and carbenicillin in the presence of clavulanic acid, while this effect was less marked for the acylureidopenicillins. In 18% of strains the susceptibility to all five penicillins tested was virtually unaffected by the presence of clavulanic acid.