Collaborative Evaluation of a Proposed Reference Dilution Method of Susceptibility Testing of Anaerobic Bacteria

Abstract

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.

Susceptibility testing of anaerobic bacteria: myth, magic, or method?

“…The susceptibility of 228 strains of the B. fragilis group isolated from fecal samples in Brazil was tested by an agar dilution method, using brain heart infusion broth and an inoculum of 3 x 104 CFU/ml (37). (The method is that used in the surveys at the Tufts University Medical Center mentioned above [29,105] [103]). In this study, no metronidazole-resistant B. fragilis group strains were found, and resistance to cefoxitin (i.e., MIC of >32 jig/ml) and clindamycin (i.e., MIC of >4 ,ug/ml) was 2 and 10%, respectively.…”

Cefotetan, a new cephamycin: comparison of in vitro antimicrobial activity with other cephems, beta-lactamase stability, and preliminary recommendations for disk diffusion testing

“…Both of these modified dilution tests have been reported earlier (1,25). Anaerobic isolates were tested by using Wilkins-Chalgren agar according to the reference agar dilution method (National Committee for Clinical Laboratory Standards) with incubation at 35°C for 48 h in GasPak (BBL Microbiology Systems) jars (11,13,22). Appropriate control organisms were run with all tests.…”

Análise sazonal do potencial antimicrobiano e teores de flavonoides e quinonas de extratos foliares de Aloe arborescens Mill., Xanthorrhoeaceae

“…Foram utilizadas cepas-padrão de alta concentração (Sutter et al, 1979), avaliando-se inicialmente a Concentração Inibitória Mínima (CIM). Em tubos de ensaio contendo 5 mL de meio de cultura, foram depositadas concentrações exponenciais (8 a 2048 µg/mL) dos extratos e 100 µL do inóculo padronizado em 1,5 x 108 UFC/ mL, quantificado segundo 0,5 da escala de McFarland (Koneman et al, 2001).…”