The intestinal microbiota can determine functional differentiation of mucosal T helper (Th) cells, licensing them to promote or protect from intestinal inflammation. The central aim of this project is to determine the molecular crosstalk between distinct pro- and anti-inflammatory bacteria of the microbiota and the mucosal Th cells, and the role the intestinal epithelial cells (IEC) play in this. We will identify the bacteria licensing T-bet-deficient Th cells to induce colitis and determine how these bacteria affect the IEC barrier. Furthermore, we will determine how bacteria of the genus Anaeroplasma pass or affect the IEC barrier, induce TGF-β in mucosal Th cells and potentially ameliorate colitis.