Differentiation and identification of human faecal anaerobic bacteria producing β-galactosidase (a new methodology)

Bacterial glycosidase activities, specially β-galactosidase (β-gal), are reduced in faeces from patients with Crohn's disease (CD). In order to test the hypothesis that an alteration of the colonic flora might be responsible for decreased faecal β-gal activity in CD patients, we developed a new methodology designed to differentiate and enumerate anaerobic bacteria producing β-gal. We used an enriched Columbia agar medium modified by addition of 3-bromo-4-chloro-5-indolyl-β-D-galactopyranoside (X-gal) and tested it on different faecal bacterial strains. The presence of blue diffuse zones with various sizes surrounding bacterial colonies clearly differentiated Bifidobacterium, Ruminococcus, Bacteroides and one of five strains of Lactobacillus tested from other β-gal positive bacteria. When the X-gal medium was applied to the analysis of normal human faecal flora, 57% ± 29 (mean ± S.D.) of the colonies growing on this medium were surrounded by blue haloes. Among them, all colonies, characterized by a large blue diffuse ring, were identified as bifidobacteria.