BDNF and major depressive disorder: Figure 1 illustrates the efficacy of probiotics, particularly butyrate-producing bacteria, to alleviate the physiological causes of MDD while also structurally modulating the intestinal flora. This treatment method is used to treat MDD disorders (4, 17, 18). New hypotheses regarding the development of depression have emerged, including the changes in the hypothalamus-pituitary-adrenal (HPA) axis, which is associated with the stress response, as well as the dysfunction in neurogenesis and neuroplasticity, which involves brain-derived neurotrophic factors (BDNF) (19).