Diversity of sulfate-reducing bacteria, their substrates in the intestine, metabolism of molecular hydrogen, and their trophic relationship with other microorganisms were characterized. Mucosal bacteria of the colon and the role of intestinal microbiota in disease development were described. Microbial communities and hydrogen sulfide concentration in large intestine of healthy people and patients with ulcerative colitis were compared. A characterization of physiological and biochemical properties of intestinal sulfate-reducing bacteria, and their mechanisms of dissimilatory sulfate reduction in the colon were described. The conception of the functional role of sulfate-reducing bacteria in the humans and animals intestine was created.