Recent evidence reported that Lactobacillus gasseri NK109 alleviated colitis and gut dysbiosis, leading to psychiatric disorders such as depression and memory deficits induced by exposure to Escherichia coli K1 by inhibiting neuroinflammatory NF-κB signaling and IL-1β expression and increasing BDNF levels in the hippocampus of mice via the gut–vagus nerve–brain axis [232]. This evidence concerns the gene BDNF and major depressive disorder.