In particular, Manca et al. showed that various small and large intestinal sections of GF and ABX mice exhibit higher levels of expression of CB1 and PPARα, two potentially beneficial receptors in colitis [46] and lower levels of GPR55 and GPR18, for which, instead a role in inflammation and intestinal immune response development, respectively, have been suggested [11, 47]. Here, PPARA is linked to colitis.