Structural analysis of human and rat gastrointestinal microbial communities revealed their general similarity. The structure of microbial biocenosis in ileum parietal mucin appears to be highly sensitive to nutritional factors. Inadequate nutrition leads to destruction of microbial microassociations in parietal mucin, a calcium-deficient diet has similar effect in feces. Fiber-rich diets stimulates build-up of indigenous communities while artificial nutrients and calcium-enriched diets promote formation of mixed indigenous-transient microbial associations. Bacteria themselves prove to be weak modifiers of the observed effects.