BDNF and major depressive disorder: Animal experiments have confirmed that the expression of BDNF in the hippocampus is downregulated in chronic and unpredictable stress-induced depression [10–13]; bacteria that confer a health benefit, such as Lactobacillus casei and Bifidobacterium can improve depression-like behavior in rats with chronic unpredictable mild stress-induced depression through the BDNF/TrkB signal pathway [14], which has become a research hotspot in the field of depression in recent years.