Enteric infections remain a leading cause of morbidity and mortality in children under the age of five. Pathogens affecting this age group commonly use lectin-like adhesins to adhere to host glycoproteins and glycolipids expressed on small intestinal enterocytes and/or M cells. The expression patterns of these glycoconjugates can thus impact the host's underlying susceptibility to infection. A variety of factors including age, hormones, diet, and colonization with specific commensal microflora impacts epithelial glycoconjugate expression. We have shown that colonization of adult germ-free (gnotobiotic) with the commensal Bacteroides thetaiotaomicron induces a mature pattern by up-regulating enterocyte-specific expression of a 1,2 fucosyltransferase. Fucosyltransferases link fucose to terminal galactose and N-acetyl-glucosamine residues, and thereby mask potential ligands on epithelial surfaces. These data suggest that colonization with select commensals could be used to therapeutically stimulate beneficial changes in epithelial glycoconjugate expression in susceptible populations. This proposal will test the hypothesis that select microflora, notably B. thetaiotaomicron, stimulate the development of glycoconjugates on the apical surfaces of enterocytes and M cells that reduces the capacity of pathogenic bacteria, viruses and/or toxins to infect/intoxicate the intestinal epithelium. Aims 1 and 2 will determine the full impact of B. thetaiotaomicron on expression and accessibility of epithelial glycoconjugates in the small and large intestines. These aims are directly responsive to RFA HD 08-004 (Item 5) "Surveying glycoconjugates on the surface of enterocytes to discover oligosaccharides that may serves as ligands for pathogenic and non-pathogenic bacteria." Aim 3 will specifically test the proposed hypothesis using well-established mouse models of cholera toxin, ricin toxin, reovirus and Salmonella typhimurium infection.