The FMT findings in our study revealed that fecal microbiota from patients with BD depression were sufficient to elicit depression-like behavior in mice, which may link to activated neuroinflammation and an elevated expression of TRANK1. It is noteworthy that TRANK1 overexpression resulted in decreased dendritic density in primary neurons, which may comprise neuronal morphological basis of depressive episodes in BD. The gene discussed is TRANK1; the disease is major depressive disorder.