Rapid identification of Bacteroides fragilis with bile and antibiotic disks

A simple screening test is described for separating Bacteroides fragilis from other anaerobic gram-negative bacilli. The test utilizes filter paper disks impregnated with 25 mg of oxgall (Difco), tested in conjunction with antibiotic identification disks. The bile disks and antibiotic disks are placed on a supplemented brucella blood agar plate which has been inoculated by swabbing with a standardized cell suspension. After 24 h at 35 degrees C in a GasPak jar, resistance to kanamycin and bile is taken as a presumptive identification of B. fragilis. Susceptibility to one or both disks indicates the need for further identification and additional biochemical tests are required. Those strains that produce insufficient growth within 24 h are not likely to be B. fragilis. The reliability of the bile disk method was tested by comparing results with 100 clinical isolates versus results with bile in thioglycolate broth, peptone-yeast extract-glucose broth, and tryptic soy agar. All four bile test methods gave equilvalent results, but the broth media required much longer periods of incubation.