BDNF and major depressive disorder: Our results show that repeated administration of the BLLL formula, a newly developed probiotic consisting of B. breve, L. plantarum, L. paracasei, and L. helveticus, can reverse chronic stress-induced depression-like behavior in mice, which is achieved by reducing neuroinflammation and oxido-nitrosative stress, and by increasing BDNF levels in the brain (Figure 6).