The Short Version
Evidence from numerous peer-reviewed reviews and experimental studies supports a pathway in which pathogenic or dysbiotic gut bacteria activate immune cells to release pro-inflammatory cytokines; these cytokines can penetrate the brain, promote neuroinflammation, and are associated with heightened anxiety-, depression-, and threat-related responses. Most data come from animal work and human correlational studies, and effects are clearest in dysbiosis or chronic stress, so universality and direct causation in healthy individuals remain uncertain.