The incidence of vancomycin resistance among enterococci is increasing in the United States and elsewhere in the world, but automated susceptibility testing methods have difficulty detecting resistance expressed by certain strains. The agar screening method described by Willey et al. (B.

An agar dilution method for susceptibility testing of anaerobic bacteria was evaluated to determine whether results obtained would be consistent enough to recommend it as a reference method. The study was conducted in 10 laboratories where the minimum inhibitory concentrations of six antibiotics (carbenicillin, cefoxitin, chloramphenicol, clindamycin, penicillin G, and tetracycline) were determined against 10 bacterial strains on Wilkins-Chalgren agar prepared by three manufacturers. Minimum inhibitory concentrations falling on the modes varied from 57 to 80% of all determinations and on the mode or within ±1 log2 dilution of the mode from 87 to 100% within each laboratory. When data from all laboratories were pooled, minimum inhibitory concentrations from each laboratory agreed with the overall mode 48 to 71% of the time, with an overall agreement at ±1 log2 dilution of 96%. This degree of reproducibility allows for recommendation of the procedure as a reference method. Results with three of the test strains were very consistent, and these strains are recommended as control strains: Clostridium perfringens ATCC 13124, Bacteroides fragilis ATCC 25285 and Bacteroides thetaiotaomicron ATCC 29741. The minimum inhibitory concentrations for these strains were on the mode or within ±1 log2 dilution of the mode 98, 99, and 99% of the time, respectively. The remaining anaerobic bacteria are recommended as reference strains.

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The minimal inhibitory concentrations and minimal bactericidal concentrations of seven antibiotics were determined by broth dilution tests for 50

The ability of the Alamar microdilution MIC system to detect vancomycin resistance in enterococci was evaluated by comparing the results with an agar dilution screen method. Of 100 strains tested, 41 were resistant and 47 were susceptible by both tests. Five strains were intermediate and one was resistant by the Alamar MIC system but susceptible by the agar screen. Three strains each were susceptible or intermediate by the Alamar MIC system but resistant by the agar screen. The predictive values for the Alamar MIC system were 94% (susceptible) and 88% (combined intermediate and resistant). The Alamar MIC system does not appear to have sufficient accuracy for the detection or confirmation of vancomycin resistance in enterococci.