Bacteria are not “dumb bugs”
It is uncertain why bacteria have these features, though neuroactive compounds may have had a role in signaling from one bacterial cell to another. However, the fact that these systems have been conserved through evolution presents an opportunity for distant phylogenetic kingdoms to communicate with each other and influence each other’s behavior. Our common neurochemical “language” means that the gut microbiome affects the host, and what the host does affects the gut microbiome.
Microbiology, neurology and immunology are linked through a common language of neurochemistry
A simple example is that oral drugs reach our intestinal flora before they have the chance to act on us, and many non-antibiotics have antibiotic-like effects on human gut bacteria. Antipsychotics, for example, might change the microbiome.3
In short, bacteria are not “dumb bugs”: they are neuroendocrine organisms; and this realization has given rise to the field of microbial endocrinology. Neurochemicals are not restricted to us, or even to our microbiome. They are found in the food we eat. Bananas contain dopamine and norepinephrine, and tomatoes dopamine and tyramine, while the beans from certain legumes contain physiologically active amounts of L-dopa.4
Along with physical health, our mental health is therefore likely to be influenced by complex interactions between our microbiome, our diet, and host factors such as stress. The complex interplay of factors is shown in Figure 1.