The Role of the Microbiota-Gut-Brain Axis in Psychiatric and Neurological Disorders

The human gut microbiota, comprising trillions of microorganisms, plays a pivotal role not only in digestion and immunity but also in modulating central nervous system (CNS) function through the microbiota–gut–brain (MGB) axis. This bidirectional network integrates neural pathways, endocrine signalling, immune mechanisms, and microbial metabolites, enabling dynamic communication between the gut and brain. Emerging evidence indicates that disruptions in microbial homeostasis, or dysbiosis, contribute to psychiatric and neurological disorders, including depression, anxiety, autism spectrum disorder, schizophrenia, Parkinson’s disease, Alzheimer’s disease, multiple sclerosis, epilepsy, and stroke. Mechanistically, microbial alterations influence neurodevelopment, neurotransmitter synthesis, tryptophan metabolism, neuroinflammation, and the integrity of the blood–brain and intestinal barriers. Preclinical models using germ-free animals, faecal microbiota transplantation, and probiotic interventions demonstrate causal links between gut microbiota and behavioural or cognitive changes, while clinical studies increasingly highlight the therapeutic potential of psychobiotics, prebiotics, synbiotics, and diet-based strategies. Additionally, short-chain fatty acids, bile acids, and microbial-derived indoles exert neuroactive effects, modulating synaptic plasticity and immune responses. Despite these advances, translation into clinical practice is limited by methodological heterogeneity, interindividual variability, and the challenge of distinguishing causation from correlation. Keywords: microbiota–gut–brain axis, psychiatric disorders, neurological disorders, psychobiotics, dysbiosis, neuroinflammation