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Recent advances through studies on the human microbiome have shown that microbial communities associated with humans play significant roles in the physiology of the host. The beneficial roles include maturation and development of the mucosal and systemic immune system and protection against pathogens by competitive exclusion, elicited by producing antimicrobial compounds. The interest in the human and its microbiome in my laboratory, however, is how the microbes living in the large intestine, including the colon, impact the health and nutrition of the host. Human foods, based on cereal grains such as rye, wheat, barley, oats, rice and sorghum are known to contain arabinoxylan, a polysaccharide composed mostly of the pentose sugars xylose and arabinose. Humans lack the enzymes required for breakdown of arabinoxylans. Therefore, these complex polysaccharides escape degradation in the stomach and the small intestine and flow into the large intestine, where they are fermented by the resident microbes to various metabolites. The major microbial metabolites in the large intestine are acetate (C2), propionate (C3) and butyrate (C4). Butyrate serves as the preferred energy source for proliferation and differentiation of the colonocytes and propionate is metabolized by the liver cells by incorporating into gluconeogenesis. Acetate, on the other hand, is absorbed into the blood and together with absorbed butyrate are used in lipid biosynthesis, although other gut microbes such as Eubaterium rectale and Roseburia intestinalis are reported to metabolize acetate to butyrate through microbial cross-feeding. Furthermore, butyrate and propionate produced by other bacteria can be degraded into acetate by sulfate- and nitrate-reducing acetogenic bacteria, and therefore illustrating the dynamic interactions of short chain fatty acids production in the human gut. Arabinoxylan has been reported to have several beneficial effects on the human host, and aside from the effects derived from short chain fatty acids as outlined above, human meals supplemented with arabinoxylan-rich fiber lowered postprandial (after meal) glucose and insulin responses in healthy persons, suggesting also its potential impact on type-2 diabetic patients. The antioxidant properties of arabinoxylans, derived from its associated phenolic compounds and exerted through diverse mechanisms, such as free radical scavenging, metal chelation, blocking the free radical chain reactions and modulation of enzymatic activities, have also been reported in the literature. In an effort to understand how the human gut microbiome degrades arabinoxylan to elicit its health and nutritional properties, we have been studying arabinoxylan degradation by a Bacteroides intestinalis strain, isolated from a fecal sample of a Japanese. This bacterium appears to be endowed with a large number of genes for degradation of dietary arabinoxylans. In my presentation, I will demonstrate how we have used transcriptomic, biochemical, and structural analyses to gain important insights into a highly fine-tuned molecular machinery for sensing and degradation of both simple and complex arabinoxylans in B. intestinalis and its related species.