BDNF and major depressive disorder: Since Sudo et al. showed that hippocampal BDNF levels decreased in germ-free mice compared with SPF mice [10], the relationship between BDNF and the gut microbiota has been investigated in numerous studies, using germ-free, antibiotic-treated, depression model, and fecal microbiota-transplanted mice [43,44,45,46,47], while impaired hippocampal neurogenesis has been reported in germ-free mice [48].