European collaborative study of reproducibility of quantitative sensitivity testing of anaerobes

Minimum inhibitory concentrations (MICs) of ampicillin, cefoxitin, cefbuperazone, latamoxef, metronidazole, clindamycin and chloramphenicol were determined for 15 different anaerobic bacteria including Bacteroides spp., anaerobic cocci and Clostridium spp., in 18 European laboratories, who used their own methodology. The degree of intra- and inter-laboratory reproducibility was surprisingly good--87% of results fell on the modal MIC or were within one dilution of it and only 4.4% of the results differed by three or more dilutions. Results for clindamycin were the least reproducible, as were those for clostridia. Of the organisms that we tested Bacteroides fragilis ATCC 25285, NCTC 9343 emerged as the most suitable for use in quality control, and Peptococcus variabilis ATCC 14956 the most appropriate if a control for more slowly-growing species is required.