Many neurotransmitters are produced by microbes. Without these microbes, neurotransmitters would not exist and there would be detrimental effects to your body. Vice versa, the release of certain neurotransmitters can influence what microbiota exist in the body.
Mental health is a large determinant of the types and amounts of microbiota that are in the body. For example, if you are very stressed out, your body becomes stressed and reacts accordingly and can cause dysbiosis which is when the number of organisms is severely decreased. This itself causes increased susceptibility to diseases that affect the gastrointestinal system like leaky gut syndrome. There is also a connection between autism, a mental health disease, and numerous forms of gastrointestinal diseases such as IBD which suggests that there may be a connection between the health of the gut with the brain. Differences have been found in the numbers of bacteria in the gut of people with ASD and people who do not have ASD, which also supports the previous hypothesis that there is a connection between the health of the gut and the brain.
