Effect of bile and desoxycholate on gram-negative anaerobic bacteria

Lysis of halobacteria in bacto-peptone by bile acids

All tested strains of halophilic archaebacteria of the genera Halobacterium, Haloarcula, Haloferax, and Natronobacterium lysed in 1% Bacto-Peptone (Difco) containing 25% NaCl, whereas no lysis was observed with other strains belonging to archaebacteria of the genera Halococcus, Natronococcus, and Sulfolobus, methanogenic bacteria, and moderately halophilic eubacteria. Substances in Bacto-Peptone which caused lysis of halobacteria were purified and identified as taurocholic acid and glycocholic acid. High-performance liquid chromatography analyses of peptones revealed that Bacto-Peptone contained nine different bile acids, with a total content of 9.53 mg/g, whereas much lower amounts were found in Peptone Bacteriological Technical (Difco) and Oxoid Peptone. Different kinds of peptones can be used to distinguish halophilic eubacteria and archaebacteria in mixed cultures from hypersaline environments.

New medium for selection and presumptive identification of the Bacteroides fragilis group

A medium, Bacteroides fragilis bile-esculin (BBE) agar, was designed for the selection and, presumptive identification of the B. fragilis group. BBE agar contains bile, esculin, ferric ammonium citrate, hemin, and gentamicin in a Trypticase soy agar base. Growth in the presence of 20% bile and esculin hydrolysis, detected by blackening of the medium, provide presumptive evidence for the identification of the B. fragilis group. In addition to stimulating the growth of many strains of the B. fragilis group, hemin provides the option of testing isolates for catalase production. Gentamicin and bile prevent the growth of most organisms other than the esculin-positive bacteroides that can tolerate bile. Of 160 clinical isolates of the B. fragilis group tested on BBE agar, 159 grew well on the medium and 157 blackened it. Other anaerobes, Enterobacteriaceae, and enterococci either failed to grow on BBE agar or did not produce the characteristic morphology and blackening associated with isolates of the B. fragilis group. In a clinical laboratory trial, 687 specimens from patients were inoculated onto BBE agar plates. The B. fragilis group was recovered from 81 (11.8%) of these specimens in 24 to 48 h. Use of BBE agar in the clinical laboratory enables earlier recovery and identification of this important pathogen.

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.

Tryptic soy bile-kanamycin test for the indentification of Bacteroides fragilis

A simple and practical test for the identification of Bacteroides fragilis is described. It utilizes two well-known properties of this species, i.e., stimulation of growth by bile and resistance to kanamycin. The test media are a tryptic-soy bile agar plate and a supplemented blood agar plate on which a kanamycin 1,000-mug/ml disk is placed. Incubation is for 24 h at 37 C in GasPak. The results of screening 190 strains, mostly clinical isolates, indicate that B. fragilis can be easily and reliably distinguished from other Bacteroides and from Fusobacterium species by its growth on tryptic-soy bile agar and resistance to kanamycin.