The gut flora (microbiota) plays an important role in human health through their metabolic function in breaking down ingested food to provide nutrients to cells and organs of the body. Among the metabolic products that are generated by the gut flora, proteins are critical for the function of the nervous system, such as serotonin, which is involved in managing stress and anxiety .
No Gut Flora, No Serotonin
Serotonin is a monoamine neurotransmitter that modulates mood, cognition, learning, memory, depression, anxiety, social phobia, schizophrenia, obsessive‐compulsive, panic disorders, and numerous physiological processes such as vomiting and vasoconstriction. It is mainly produced by the gut flora and involves bacteria such as Bifidobacterium and Lactobacillus.
When there is an imbalance in the gut flora composition, the production of serotonin is also affected, and therefore, its low level and availability in the body, significantly affect our mood, levels of stress, and anxiety.
Interestingly, probiotics and prebiotics have been shown to increase the production level of serotonin, suggesting a direct link between the gut flora and our capacity to manage stress and anxiety  .
Prebiotics and probiotics have been shown to reverse behavioral deficits, adjust the composition of gut microbiota, rise the peripheral levels of the serotonin precursor tryptophan, and alter 5-hydroxyindoleacetic acid (5-HIAA) and dihydroxyphenylacetic acid (DOPAC) levels in the brain .
To reduce stress and anxiety that could be associated with an unbalance in the Gut flora (Dysbiosis), intake of pre-and probiotics can help prevent dysbiosis, and thus, promote the production of serotonin by the gut flora, while excessive use of antibiotics, may promote dysbiosis and consequently enhances the stress and anxiety disorders.
 Marcinkiewcz, C.A., Mazzone, C.M., D’Agostino, G., Halladay, L.R., Hardaway, J.A., DiBerto, J.F., Navarro, M., Burnham, N., Cristiano, C., Dorrier, C.E. and Tipton, G.J., 2016. Serotonin engages an anxiety and fear-promoting circuit in the extended amygdala. Nature, 537(7618), pp.97-101.
 Li, H., Wang, P., Huang, L., Li, P. and Zhang, D., 2019. Effects of regulating gut microbiota on the serotonin metabolism in the chronic unpredictable mild stress rat model. Neurogastroenterology & Motility, 31(10), p.e13677.