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The effect of beta-lactam antibiotics on the growth curves of Bacteroides fragilis, obtained with the MS-2 system.

The effect of different concentrations of beta-lactam antibiotics on the growth curves of B. fragilis NICC 9343 was studied in transfer cartridges in an Abbott MS-2 system. Anaerobic conditions were ensured in an anaerobic chamber. Moxalactam and cefoxitin inhibited the strain, without regrowth, at concentrations of 1 and 2 mg/l respectively. No regrowth was seen at a concentration of 16 mg/l of piperacillin. Penicillin G, ceftazidine and carbenicillin inhibited growth at concentrations of 64, 32 and 32 mg/l, respectively. In the case of cefotaxime, cefoperazone, cefazolin and cefuroxime, concentrations of 125 mg/l and more were required to inhibit the culture, without regrowth in 22 h. This pilot study showed that (a) growth curves can be obtained in multichamber cartridges which provide a more detailed documentary approach to the mode of action of the beta-lactams on B. fragilis, and (b) in spite of the rapid growth of this organism, the early reading of results (e.g. at 6 h), might give erroneous results (regrowth due to beta-lactamase mediated drug destruction).