A long journey to the colon: The role of the small intestine microbiota in intestinal disease.

The small intestine represents a complex and understudied gut niche with significant implications for human health. Indeed, many infectious and non-infectious diseases center within the small intestine and present similar clinical manifestations to large intestinal disease, complicating non-invasive diagnosis and treatment. One major neglected aspect of small intestinal diseases is the feedback relationship with the resident collection of commensal organisms, the gut microbiota. Studies focused on microbiota-host interactions in the small intestine in the context of infectious and non-infectious diseases are required to identify potential therapeutic targets dissimilar from those used for large bowel diseases. While sparsely populated, the small intestine represents a stringent commensal bacterial microenvironment the host relies upon for nutrient acquisition and protection against invading pathogens (colonization resistance). Indeed, recent evidence suggests that disruptions to host-microbiota interactions in the small intestine impact enteric bacterial pathogenesis and susceptibility to non-infectious enteric diseases. In this review, we focus on the microbiota's impact on small intestine function and the pathogenesis of infectious and non-infectious diseases of the gastrointestinal (GI) tract. We also discuss gaps in knowledge on the role of commensal microorganisms in proximal GI tract function during health and disease.