Evaluation of the E-test for susceptibility testing of the Bacteroides fragilis group. Danish study group

The susceptibilities of 100 clinical isolates belonging to the Bacteroides fragilis group to 9 antibiotics, i.e. ampicillin, piperacillin, ceftriaxone, cefotaxime, cefoxitin, imipenem, erythromycin, clindamycin, and metronidazole, were tested using a standard agar dilution method and the E-test. Overall, 81% of the E-test MICs were within one log2 dilution step of the agar dilution MICs and 95% were within two log2 dilution steps. The E-test showed significantly lower MIC values than the agar dilution method for ampicillin, cefotaxime, and imipenem, and significantly higher MIC values for piperacillin, ceftriaxone, erythromycin, clindamycin, and metronidazole. However, this only resulted in minor discrepancies that did not change the susceptibility status. The E-test is easy to perform and read, and the MIC values correlated well with the MICs obtained by the agar dilution method when testing susceptibility of the Bacteroides fragilis group.