Gut microbiota and depression

Abstract

The human gut microbiota, a well-known factor influencing health and well-being, contains a massive amount of genetic material, which can be viewed collectively as an “alternate” genome that could be manipulated by external interventions. The impact of the gut microbiota on mood and depression has attracted significant attention from the research and medical communities over the past decade. Results of landmark studies in the field have revealed the existence of a bidirectional influence between the gut microbiota and mental health, including depression. The effect of depression on gut microbiota composition is mainly mediated through (1) stress, (2) the change in release of signaling molecules such as neurotransmitters in the gut lumen, and (3) the change in immune response and increase in inflammatory cytokines. In turn, the effect of gut microbiome on depression is mediated through (1) increase or reduction in neurotransmitters synthesis by bacteria, (2) induction or inhibition of inflammatory cytokines synthesis, and finally (3) hypothalamic-pituitary-adrenal axis. Current evidences have mostly been obtained from animal studies; however, despite their observational and mostly correlative nature, human clinical data are also instrumental to complete our understanding of the mechanisms and physiological interactions at play. Here, we review the current mechanisms proposed in the field to explain the inner workings of the microbiome-gut-brain (MGB) axis in depression.