Production of Psychoactive Metabolites by Gut Bacteria.

The gut microbiome plays a vital role in numerous aspects of physiology, including functions related to metabolism, the immune system, behaviour, brain structure and function. Furthermore, it is now becoming increasingly clear that alterations in microbial composition or diversity are implicated in several disease states, including anxiety, depression, autism spectrum disorder (ASD), Alzheimer's disease (AD), Parkinson's disease (PD), obesity, and diabetes. Therefore, therapeutic targeting of the gut microbiota has the potential to be useful in the treatment of both stress-related disorders and metabolic diseases. An important method by which the gut microbiome can influence the gut-brain axis is through microbial production of psychoactive metabolites. Several bacteria have been shown to produce metabolites which can impact host health, such as short-chain fatty acids, conjugated linoleic acid, antimicrobials, exopolysaccharides, and vitamins. Furthermore, several molecules with neuroactive functions, including serotonin, gamma-aminobutyric acid, catecholamines, and acetylcholine, have been isolated from bacteria within the human gut. This review aims to explore the psychoactive metabolites reported to be produced by gut bacteria, particularly those of relevance to stress-related disorders. Screening methods for psychoactive metabolite production, as well as the challenges and limitations of this research, will also be addressed. Finally, the implications of metabolite production for neuropsychiatric disorders such as depression, anxiety, and stress, behavioural disorders such as ASD, and neurodegenerative disorders such as AD and PD will be discussed.