Comparison of the E test and microdilution for detection of beta-lactam-resistant mutants that are stably derepressed for type I beta-lactamase.

The activities of cefotaxime, ceftazidime, piperacillin, and aztreonam were compared in the E test and broth microdilution test against 30 gram-negative bacterial mutants derepressed for type I beta-lactamases. The results demonstrated complete agreement between 24-h MICs of 80 to 83% and essential agreement between 24-h MICs of 90 to 97%. When sufficient growth was present for the E test to be read at 6 h, the essential agreement between 6- and 24-h E-test MICs was 100% for ceftazidime, piperacillin, and aztreonam and 85% for cefotaxime.