The microbial community in the body influences the brain through systemic and communication and direct communication. Systemically, cytokines, neurotransmitters, and other small signaling molecules are produced by the microbiome either directly or indirectly. These go on to manifest direct consequences for brain function. An example of an indirect and systemic effect the microbiome has on brain function is an increase in BDNF, a protein produced in the hippocampus and involved in cognition, being elevated in those with microbes compared to germ free individuals in mice studies. Aside from systemic effects, the gut is also directly connected to the brain via the vagus nerve and communication occurs through that pathway. This is evidenced by microbiome effects that were demonstrated in mice being non-reproducible following severance of the vagus nerve. The gut microbiome-brain relationship is bidirectional. The brain is shown to modulate gut microbiome via corticosteroids and neurotransmitters, which alter function. The brain also initiates changes in behavior which lead to effects on the microbiome.
This newly discovered interplay allows us to approach health from a different perspective. Probiotics and prebiotics can now be considered by those treating disorders such as autism or depression, ones typically thought to be separate from hard medical science.