Evaluation of two media for antibiotic susceptibility testing of anaerobic bacteria using the receiver operating characteristic procedure.

Wilkins-Chalgren agar and Meat-Yeast agar were evaluated as media for antibiotic susceptibility testing using 112 anaerobic bacterial strains. The results obtained with the two media using the diffusion method were compared with those obtained by the dilution method as reference method. The results were analyzed by the receiver operating characteristic (ROC) procedure allowing a graphic representation of sensitivity and specificity of the technique for each cut-off value. The area under the ROC curves was calculated to compare the accuracy of the two methods. Six antibiotics were tested including amoxicillin, cefoxitin, piperacillin, doxycycline and clindamycin. For amoxicillin and clindamycin, the two methods showed a high and identical discriminative power for distinguishing susceptible bacteria from the others. Diffusion in Wilkins-Chalgren agar appeared better than diffusion in Meat-Yeast agar for separating resistant bacteria from bacteria of intermediate susceptibility (amoxicillin p less than 0.005; clindamycin p less than 0.04). For other drugs, diffusion in Wilkins-Chalgren agar always had a discriminative power higher than that obtained with diffusion in Meat-Yeast agar for separating susceptible bacteria from the others (cefoxitin p less than 0.0005; piperacillin p less than 0.02; doxycycline p less than 0.05). The Wilkins-Chalgren agar medium thus appeared superior to the Meat-Yeast agar medium using the ROC evaluation method, which would deserve wider utilization in the field of microbiology.