The intestinal microbiome has been shown to influence cognition and emotion, affecting moods and the state of psychiatric disorders, and even information processing.
Experiments like these could even identify pathways of communication between the brain and the microbiome that could be targeted by treatments. “That’s always the big hope from these mouse experiments, that we’re getting close to interventions”.
On an evolutionary timescale, human microbiomes have changed as more people have come to live in cities, and brain disorders have become increasingly prominent. The swarms of microbes inhabiting each of us have evolved with our species, and it’s vital that we understand how they impact both physical and mental health. Our environments may affect our nervous systems by way of the microbiome, adding new layers of complexity to the study of health and disease in the brain.

https://www.quantamagazine.org/how-microbiomes-affect-fear-20191204/