Figure 1.

The role of the gut microbiota and communication with the brain. A healthy gut contains large fractions of the phyla Firmicutes and Bacteroidetes, including the genera Prevotella, Bacteroides, and Ruminococcus followed by Verrucomicrobia and Actinobacteria, but contains a low number of Proteobacteria phyla members (Mowry and Glenn, 2018). The gut microbiota is related to GI function, but also involved in several complex modulatory processes, inflammation and immune response, and peripheral (enteric) and central neurotransmission, as well as synthetize and secrete essential substances. The gut microbiota contributes to important homeostatic processes and is essential for the homeostasis of intestinal intraepithelial lymphocytes (Liu L. et al., 2019). The gut microbiota can affect brain function and bioactivity through gut–brain axis via several pathways: (1) the neural regulating pathway, in which vagus nerve links between the gut and the spinal cord (autonomic nervous system; Bonaz et al., 2018). The gut microbiota can secrete and regulate neurotransmitters of the CNS. (2) The endocrine pathway. The HPA axis can release glucocorticoids, etc., after stimulations by stress or other factors, which can alter gut microbiota composition and increase gut epithelium permeability and immune responses in gut (Ait-Belgnaoui et al., 2012; Park et al., 2013; Bellavance and Rivest, 2014). (3) The immune-regulating pathway via lymphocyte, cytokines, chemokines, and antigen-presenting effect of SCFAs communicating with the brain. (4) The blood circulation pathway (Logsdon et al., 2018). The immune and endocrine molecules, such as cytokines and hormones, can pass BBB and intestinal mucosa to influence both gut and brain functions (Zac-Varghese et al., 2010). BBB, blood–brain barrier; CNS, central nervous system; HPA axis, hypothalamic–pituitary–adrenal axis; SCFA, short-chain fatty acids; APC, antigen-presenting cells.