In particular, colonization of Oscillibacter and Clostridium producing short chain fatty acids (Naseribafrouei et al., 2014; Yu et al., 2017), which is known to influence the expression of tryptophan hydroxylase (TPH), 5-HT synthesis, release and cause depression and anxiety (Spohn and Mawe, 2017). Here, TPH1 is linked to major depressive disorder.