THE MICROBIOTA-GUT-BRAIN AXIS IN NEUROLOGICAL DISEASEA REVIEW

The microbiota–gut–brain axis is a bidirectional communication network connecting the intestinal tract, the enteric nervous system, and the central nervous system through neural, endocrine, immune, and metabolic pathways. Converging evidence implicates gut dysbiosis in the onset and progression of autism spectrum disorder (ASD), Parkinson’s disease (PD), and Alzheimer’s disease (AD). This research-style review synthesizes mechanistic insights on barrier
dysfunction, neuroinflammation, microbial metabolites, and neuromodulators that bridge intestinal ecology with brain structure and function. Particular emphasis is placed on organismlevel microbial signatures and clinically tractable biomarkers, including fecal calprotectin, short-chain fatty acids (SCFAs), hydrogen sulfide, sulfite, lipopolysaccharide (LPS), trimethylamine N-oxide (TMAO), tryptophan–kynurenine intermediates, glial markers (GFAP, Sox-10),
and alpha-synuclein (?-syn) species. We integrate dietary and microbial therapeutics—vegetarian, Mediterranean, low-protein high-carbohydrate, ketogenic, probiotic, prebiotic, synbiotic,
antibiotic, and fecal microbiota transplantation—linking them to distinct microbial shifts such as increases in Akkermansia muciniphila, Bifidobacterium, and Lactobacillus, and decreases
in SCFA-producers including Lachnospiraceae, Roseburia, Faecalibacterium, Blautia, Coprococcus, and Butyrivibrio observed across disorders. Together, these data support a testable
model in which intestinal dysbiosis and barrier failure trigger inflammatory and neuroendocrine cascades that exacerbate proteinopathy, synaptic failure, and circuit-level dysfunction.
The review concludes with future directions for personalized diagnostics and nutrition-based interventions.

KEYWORDS: Gut–brain axis; Microbiota; Autism spectrum disorder; Parkinson’s disease; Alzheimer’s disease; Biomarkers; SCFAs; TMAO; ?-synuclein; Lactobacillus; Bifidobacterium; Akkermansia muciniphila; Lachnospiraceae; Faecalibacterium; Diet; Probiotics; Prebiotics; FMT