Felix Sommer

Interactions between the host and its associated microbiota differ spatially and the local cross talk determines organ function and physiology. Animals and their organs are not uniform but contain several functional and cellular compartments and gradients. In the intestinal tract, different parts of the gut carry out different functions, tissue structure varies accordingly, epithelial cells are differentially distributed and gradients exist for several physicochemical parameters such as nutrients, pH, or oxygen. Consequently, the microbiota composition also differs along the length of the gut, but also between lumen and mucosa of the same intestinal segment, and even along the crypt‐villus axis in the epithelium. Thus, host‐microbiota interactions are highly site‐specific and the local cross talk determines intestinal function and physiology. Here we review recent advances in our understanding of site‐specific host‐microbiota interactions and discuss their functional relevance for host physiology.