Accuracy of Microdilution and the AutoMicrobic System in Detection of -Lactam Resistance in Gram-Negative Bacterial Mutants with Derepressed -Lactamase

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Abstract
Microdilution and the AutoMicrobic System (AMS) were compared with macrodilution for accuracy in detecting β-lactam resistance in 16 isogenic pairs of gram-negative wild-type bacterial strains and mutant strains with derepressed class I β-lactamase; an additional 12 gram-negative derepressed mutants were also tested. Of a total of 352 organism-β-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 β-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.

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