Describe how microbial communities in the body could influence brain and mental health states. Then, describe how brain and mental health states could influence microbial communities in the body. In what ways might these promote health and/or disease?
The proposed mechanism of the interaction between the brain and the gut may provide key insight into the relationship between microbes and mental health. Altered microbiota composition may signal for serotonin (5-HT) release by enteroendocrine cells, cytokine release, increased gut epithelium permeability for bacterial molecules (fatty acids, GABA, serotonin precursors), and increased afferent vagal input. All of these molecules play different roles within the brain. For example, serotonin is a neurotransmitter active in constricting smooth muscle and contributing to overall wellbeing and happiness (serotonergic pathway). Mental health disorders, such as depression, are highly reliant on serotonergic pathway functioning. Cytokines, or immune system chemical messengers, facilitate the immune response. GABA is an inhibitory neurotransmitter and decreases activity in the nervous system. With regard to stress and anxiety, germ-free mice have been shown to have an exaggerated stress response which could be rescued by Bifidobacterium. Microbial communities in the body, specifically the gut, may influence the brain and mental health with respect to Autism spectrum disorder (ASD); people with ASD often have IDB or other gastrointestinal problems as they are a common comorbidity in ASD patients.
The brain and mental states may influence and alter the gut microbiome through the communication with the efferent adrenergic nerve which signals for the release of noradrenaline, which has been shown to enhance growth of bacteria. Also, the enteric nervous system comprises a largely efferent network of neurons that spread throughout the gut tissue and may be influential in cellular response to microbes and other cells. Specifically, these neurons have the ability to influence motility, secretion, and nutrient delivery which can affect microbial balance.
