When a fecal microbiota transplantation from depressed patients into microbiota-depleted rats was carried out, it induced behavioral and physiological features characteristic of depression in the recipient animals, including anhedonia and anxiety-like behaviours, as well as increased metabolism of tryptophan into kynurenine, probably as a consequence of IDO1 activation, together with increases in the acute phase protein CRP. This evidence concerns the gene IDO1 and major depressive disorder.