Susceptibility Results for the Bacteroides fragilis Group: Comparison of the Broth Microdilution and Agar Dilution Methods

The antimicrobial susceptibilities of members of the Bacteroides fragilis group were compared using the agar dilution and broth microdilution methods. A total of 455 B. fragilis group isolates were tested against 10 antibiotics. Significant disparity in susceptibility results for most antibiotics was observed between the two methods. Broth microdilution susceptibility results were most similar to agar dilution results when a twofold lower breakpoint was used. In addition, broth microdilution failed to detect resistance to some antibiotics when the recommended agar dilution breakpoint was used. MIC50, MIC90, and geometric mean MIC values for broth microdilution were consistently twofold to fourfold less than those for agar dilution. Susceptibilities of Bacteroides that are determined with use of broth microdilution may more accurately correspond to those determined with use of agar dilution if lower breakpoints are used for interpretation.