Accuracy of microdilution and the AutoMicrobic System in detection of beta-lactam resistance in gram-negative bacterial mutants with derepressed beta-lactamase.

Microdilution and the AutoMicrobic System (AMS) were compared with macrodilution for accuracy in detecting beta-lactam resistance in 16 isogenic pairs of gram-negative wild-type bacterial strains and mutant strains with derepressed class I beta-lactamase; an additional 12 gram-negative derepressed mutants were also tested. Of a total of 352 organism-beta-lactam combinations resulting in 840 determinations of minimum inhibitory concentration, the overall rates of very major discrepancy were 3% between macrodilution and microdilution and 2% between macrodilution and AMS. The corresponding rates of very major discrepancies with the derepressed mutants were both 2.7%. All but three of the wild-type strains were susceptible to all beta-lactam drugs tested but cefoxitin, while nearly 90% of derepressed mutants were resistant to these antibiotics. When careful attention was given to inoculum size and incubation time, microdilution and the AMS yielded results comparable to those obtained by macrodilution.