The brain and other organs require a wide range of chemicals, which the gut bacteria creates like a virtual organ. The link between humans and microorganisms is symbiotic; we feed the microbes, and they in turn provide us necessary chemicals. Around 1000 species of bacteria have been found in the human gut, and the Bacteroidetes and Firmicutes phyla make up around 80% of the entire microbiota in the human gut. Diet, exercise, stress, illness, and medicine are the primary factors determining microbiota structure in adults. Given the high prevalence of these diseases, we investigate the role of the gut microbiota in multiple sclerosis, autism, Parkinson's disease, and Alzheimer's disease in this narrative review. We concentrate on preclinical research that advances knowledge of disease pathogenesis.
