The mucosal lining of the gastrointestinal tract is the route through which ingested nutrients are absorbed. It also serves to separate potentially toxic luminal contents and flora. These functions appear to be mutually incompatible, but are achieved by regional specialisations in epithelial structure and organ function. Enteric bacteria interact with enterocytes by influencing cellular electrolyte transport and tight junction permeability in the colon. The products of bacterial metabolism are essential for colonocyte nutrition.