MAJOR METHODOLOGY-DEPENDENT DISCORDANT SUSCEPTIBILITY RESULTS FOR BACTEROIDES-FRAGILIS GROUP ISOLATES BUT NOT OTHER ANAEROBES

Two standardized susceptibility test methods, a broth microdilution (B MD) and agar dilution (AD) method were performed on a total of 441 cli nical isolates of anaerobes with ceftizoxime, cefotaxime, ceftriaxone, cefoxitin, piperacillin, and metronidazole. Against the 339 strains o f the Bacteroides fragilis group BMD minimum inhibitory concentration (MIC) values were lower than those from AD testing for all the beta-la ctams. Overall for the B. fragilis group and the beta-lactams, the mod e MIC values were two- to 64-fold lower, and the MIC(50) values two- t o eightfold lower. Resistance rates were 11%-28% higher overall with A D results and were higher especially for non-B. fragilis species. For non-Bacteroides anaerobes no major discrepancies were noted for Prevot ella species, Peptostreptococcus species, and Viellonella parvula. Wit h Clostridium species and Eubacterium species, some differences were n oted with ceftizoxime because of differences in cutoff points. These d ata illustrate the magnitude of differences in results produced by the two methods using essentially the same test medium for the B. fragili s group. Fortunately, such major discordant results were not widely no ted with other groups of anaerobes.