Bacteroides thetaiotaomicron is a prominent member of our normal adult intestinal microbial community and a useful model for studying the foundations of human-bacterial mutualism in our densely populated distal gut microbiota. A central question is how members of this microbiota sense nutrients and implement an appropriate metabolic response. B. thetaiotaomicron contains a large number of glycoside hydrolases not represented in our own proteome, plus a markedly expanded collection of hybrid two-component system (HTCS) proteins that incorporate all domains found in classical two-component environmental sensors into one polypeptide. To understand the role of HTCS in nutrient sensing, we used B. thetaiotaomicron GeneChips to characterize their expression in gnotobiotic mice consuming polysaccharide-rich or -deficient diets. One HTCS, BT3172, was selected for further analysis because it is induced in vivo by polysaccharides, and its absence reduces B. thetaiotaomicron fitness in polysaccharide-rich diet-fed mice. Functional genomic and biochemical analyses of WT and BT3172-deficient strains in vivo and in vitro disclosed that alpha-mannosides induce BT3172 expression, which in turn induces expression of secreted alpha-mannosidases. Yeast two-hybrid screens revealed that the cytoplasmic portion of BT3172's sensor domain serves as a scaffold for recruiting glucose-6-phosphate isomerase and dehydrogenase. These interactions are a unique feature of BT3172 and specific for the cytoplasmic face of its sensor domain. Loss of BT3172 reduces glycolytic pathway activity in vitro and in vivo. Thus, this HTCS functions as a metabolic reaction center, coupling nutrient sensing to dynamic regulation of monosaccharide metabolism. An expanded repertoire of HTCS proteins with diversified sensor domains may be one reason for B. thetaiotaomicron's success in our intestinal ecosystem.