title: Accuracy of microdilution and the AutoMicrobic System in detection of beta-lactam resistance in gram-negative bacterial mutants with derepressed beta-lactamase.
abstract: 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.
authors.affiliation: Department of Microbiology, Cleveland Clinic Foundation, Ohio 44195-5140.
authors.forename: J A
authors.initials: JA
authors.lastname: Washington
authors.forename: C C
authors.initials: CC
authors.lastname: Knapp
authors.forename: C C
authors.initials: CC
authors.lastname: Sanders
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D004352:Drug Resistance, Microbial; D006090:Gram-Negative Bacteria / Q000187:drug effects* / Q000201:enzymology / Q000235:genetics; D008826:Microbial Sensitivity Tests; D009154:Mutation; D001618:beta-Lactamases / Q000378:metabolism*; D047090:beta-Lactams
chemical_list: D000900:Anti-Bacterial Agents; D047090:beta-Lactams; D001618:beta-Lactamases