It has been demonstrated that the reason why MDD patients with relatively high abundance of some gut flora (e.g., Blautia, Coprococcus, and Bifidobacterium), which are associated with the production of SCFAs, responded to selective serotonin reuptake inhibitors (SSRIs) antidepressants may be that SCFA maintains high levels of 5-hydroxytryptamine synthesized precursors by upregulating the expression of Tryptophan hydroxylases 1 in vitro, thereby enhancing the antidepressant-like effects of SSRIs antidepressants [42, 43]. The gene discussed is TPH1; the disease is major depressive disorder.