Suitability of the broth-disk elution test for evaluating susceptibility of obligate anaerobes to trimethoprim-sulfonamides

Department of Veterinary Microbiology and Immunology, School of Veterinary Medicine, University of California, Davis 95616.

Species of anaerobic bacteria isolated from clinical veterinary specimens were tested for susceptibility to trimethoprim-sulfonamides by the broth-disk elution technique. Three different media were used for each organism: prereduced anaerobically sterilized (PRAS) brain-heart infusion broth (BHI), thioglycollate broth, and a semidefined PRAS medium. Susceptibility results from these media were compared with those determined by interpreting the minimal inhibitory concentration obtained using an agar dilution technique. Results from broth-disk testing in semidefined medium agreed in 68.7% of the cases, in 53.7% for thioglycollate broth, and in 36.9% for BHI. The greatest deviation between techniques occurred with isolates belonging to the genus Bacteroides, followed by those of the genus Clostridium and those of the genus Fusobacterium. This deviation was directly proportional to increasing concentrations of thymidine in the BHI and thioglycollate broths but not with the semidefined medium. We conclude that the broth-disk elution method for measuring susceptibility of obligate anaerobes to trimethoprim-sulfonamides is unsuitable.