Figure 1.

The immune pathway. Along the immune pathway, the gut microbiota regulates the brain by the bacteria themselves, microbial-derived metabolites (e.g., SCFAs, secondary bile acids, and amino acid metabolites), bacterial cell wall components (e.g., peptidoglycan, LPS), as well as microbial-intrigued immune cells and their secretory factors. The gut microbiota modulates enteric immunity in terms of the intestinal barrier, peripheral immune cells, and cytokines. For example, SCFAs derived by the gut microbiota seem to maintain a symbiotic relationship with the host by suppressing immune responses and protecting commensal bacteria from elimination, while also enhancing enteric barriers and reduce gut permeability to prevent invasion by harmful microorganisms. Proofs are that SCFAs have restrictive effects on neutrophil chemotaxis and mononuclear phagocyte system, promoting effect on regulatory T cells (Treg cells) and SCFAs can alleviate gut epithelium injury and regulate tight junctions. Meanwhile, in the CNS, microglia act as an important agent in neurodevelopment through their functions of synaptic pruning, neural progenitor cells (NPCs) pool supervision, neurogenesis regulation, etc. The gut microbiota affects microglia from different ways, exerting considerable effects on the neurodevelopmental process.