Effect of choice of medium on the results of in vitro susceptibility testing of eight antibiotics against the Bacteroides fragilis group.

The susceptibility of isolates of the Bacteroides fragilis group to eight antibiotics was determined by the agar dilution method on three media. At intermediate breakpoints established by the National Committee for Clinical Laboratory Standards (NCCLS), only minimal differences in the susceptibility of each species to any of the antibiotics were documented among the three media. At NCCLS susceptible breakpoints, greater differences among media were found for cefoxitin and cefotetan than for the other drugs tested. Only minimal differences among the three media were evident in comparisons of MIC50, MIC90, and geometric mean MIC values for all antibiotics tested. Thus, use of any one of these three media does not significantly affect agar-dilution MIC results for the B. fragilis group.