Bacteroides Bile Esculin Agar Base (BBE)

Principle And Interpretation

Bacteroides is the most common member of the normal gut flora and can cause serious infections if the normal GI mucosal barrier is breached. In the bloodstream, the organism can be carried to virtually any organ of the body. The Bacteroides fragilis group is more resistant to antimicrobial agents than most other anaerobes (1) and therefore immediate identification and prompt treatment is of vital importance. Bacteroides Bile Esculin Agar formulated by Livingston, Kominos and Yee (2) is used as a primary isolation medium for the selective and presumptive identification of Bacteroides fragilis group (2). Accompanying gram-negative organisms can be totally inhibited due to the presence of oxgall and gentamicin, with the latter added as a supplement. The medium is differential for B. fragilis group because of esculin hydrolysis test. B. fragilis hydrolyses the esculin present in the medium to esculetin and dextrose. The esculetin thus produced reacts with ferric ammonium citrate, present in the medium to form a brown-black coloured complex that is deposited around the colonies as a black halo. The medium contains highly nutritious Tryptone and Soya peptone which provide nitrogen and carbon source, long chain amino acids, vitamins and other essential nutrients. Hemin, which support growth of fastidious anaerobic bacteria like Bacteroides species. Bile inhibits almost all anaerobic gram-negative bacilli except Bacteroides fragilis (3). Gentamicin inhibits most organisms other than esculin positive Bacteroides that can tolerate bile. The minimum inhibitory concentration of 80 mcg/ml or greater is required for Bacteroides fragilis group organisms (4). The medium can be directly inoculated with the specimen. Since the medium is highly selective, a non-selective medium should be also inoculated with the specimen. B. fragilis group grow well on this medium. However, B. vulgatus usually does not hydrolyze esculin thus no discoloration to the surrounding medium is observed (5).