Thus, psychobiotics can directly influence many of the pathophysiological mechanisms involved in MDD, modulating the HPA axis, inflammation, neurotransmitter production (monoamines, GABA, glutamate, and acetylcholine), BDNF levels, as well as host metabolism and systemic actions via the vagus nerve or through critical microbial metabolites such as SCFAs [154]. Here, BDNF is linked to major depressive disorder.