Major methodology-dependent discordant susceptibility results for Bacteroides fragilis group isolates but not other anaerobes.

Two standardized susceptibility test methods, a broth microdilution (BMD) and agar dilution (AD) method were performed on a total of 441 clinical isolates of anaerobes with ceftizoxime, cefotaxime, ceftriaxone, cefoxitin, piperacillin, and metronidazole. Against the 339 strains of the Bacteroides fragilis group BMD minimum inhibitory concentration (MIC) values were lower than those from AD testing for all the beta-lactams. Overall for the B. fragilis group and the beta-lactams, the mode MIC values were two- to 64-fold lower, and the MIC50 values two- to eightfold lower. Resistance rates were 11%-28% higher overall with AD results and were higher especially for non-B. fragilis species. For non-Bacteroides anaerobes no major discrepancies were noted for Prevotella species, Peptostreptococcus species, and Viellonella parvula. With Clostridium species and Eubacterium species, some differences were noted with ceftizoxime because of differences in cut-off points. These data illustrate the magnitude of differences in results produced by the two methods using essentially the same test medium for the B. fragilis group. Fortunately, such major discordant results were not widely noted with other groups of anaerobes.