Comparison of in vitro antibiograms of Bacteroides fragilis group isolates: differences in resistance rates in two institutions because of differences in susceptibility testing methodology.

With 120 clinical isolates of the Bacteroides fragilis group, a comparison of rates of resistance to selected antimicrobial agents by using two susceptibility tests was performed in two medical institutions. The broth microdilution method produced MICs significantly lower than those determined by the agar dilution method. With ceftizoxime and cefoxitin, 88 and 18%, respectively, of the MICs were greater than or equal to 2 twofold dilutions apart. These differences in MIC results produced major interpretive discrepancies for ceftizoxime and cefoxitin, whereas no significant differences in resistance rates were noted for clindamycin and metronidazole.