Impact of the gut–brain axis on neurodegenerative diseases and potential benefits of probiotics

Gut microbiota are an essential community for human health. They produce different bioactive components and short-chain fatty acids for physiological benefits. Intestinal flora elicits an immune response and maintains a balance between inflammatory and anti-inflammatory cytokines during the inflammatory response. The intricate relationship between the activities of gut microbes and brain functions creates the gut–brain axis. Microbial metabolites of the gut reach the brain through blood and lymphatic circulation. The bioactive components from gut microorganisms regulate oxidative stress, cytokine profiles, neuroinflammation, apoptotic reactions and neurodegeneration. Dysbiosis alters the activity of gut microbiota, which causes gastrointestinal disorders, neurodegenerative diseases, metabolic syndrome and cancers. Probiotic bacteria (Lactobacillus spp., Bifidobacterium spp.) have different health promotional effects and exhibit a significant role in immunomodulation. These bacteria display some protective role against various diseases, including neurodegenerative disorders.