Gut dysbiosis (bacterial imbalance) and compositional differences have been identified in people with affective disorders, compared to healthy individuals, and these abnormalities have been linked to symptom severity [7,8,9,10], increases in pro-inflammatory cytokines and cortisol, and decreased in levels of brain-derived neurotrophic factor (BDNF) [11,12,13,14], among other factors, which are prominent biomarkers of depression. Here, BDNF is linked to major depressive disorder.