The research proposed in this application includes a study in laboratory mice and rats of the mechanisms and significance in the gastrointestinal ecosystem of the associations of certain indigenous microorganisms with mucosal epithelia. In addition, it includes efforts to determine which of various types of indigenous microbes are particularly important in the ecosystem in influencing certain biological properties of the animals. Microbiological culture techniques including methods for culturing strictly anaerobic bacteria, frozen-section histology, immunofluorescence techniques, and electron microscopy are used to characterize the epithelial associations. The mechanisms of the associations are examined in ex-germfree mice associated with the layer-forming microbes or in germfree mucosae exposed to the microbes in tissue culture. The experiments include efforts to prevent the associations with antibodies specific for certain microbial surface components, or biochemicals able to destroy or block the surface components. The microbial culture and microscopic techniques are also used along with biochemical techniques in experiments on the types of microbes important in influencing host cecal size, bile acid composition, intestinal cell migration rate, and intestinal alkaline phosphatase activity. These properties are examined in adult and baby specific pathogen-free mice and ex-germfree mice associated with particular indigenous microbes. The biochemical techniques include thin-layer and gas-liquid chromatography for differentiating bile acids, and enzymatic assays and acrylamide gel electrophoresis for detecting activity levels and molecular species of alkaline phosphatase. These microbes tested are selected because they have anaerobic metabolism, colonize the animals early in life, maintain high population levels throughout life, and associate with specific epithelia.