The gut-brain axis (GBA) is a complex two-way communication system that links the gastrointestinal tract and the central nervous system (CNS) through neural, immune, hormonal, and microbial pathways. The microbiota-gut-brain axis (MGBA), a more specific concept, focuses on how gut microorganisms, including bacteria, viruses, and other microbes, modulate this communication and influence neurological health. This comprehensive review examines the intricate mechanisms through which gut microorganisms modulate neural function and contribute to neurological health and disease pathogenesis. The gut microbiota, comprising bacteria, viruses, fungi, and bacteriophages, produces essential neuroactive compounds including neurotransmitters- Gamma-Aminobutyric Acid (GABA), serotonin (5-HT), dopamine (DA), short-chain fatty acids (SCFAs), and metabolites that directly influence brain physiology through vagal, hormonal, and immunological pathways. Dysbiosis of the gut microbiota has been implicated in various neurological disorders, including Alzheimer's disease, Parkinson's disease, autism spectrum disorders, and schizophrenia. In healthy conditions, beneficial bacterial strains such as Lactobacillus species synthesize GABA and regulate mood, while SCFA-producing bacteria like Fecalibacterium prausnitzii maintain blood-brain barrier integrity and exert neuroprotective effects. Conversely, pathological states demonstrate altered microbial compositions, reduced bacterial diversity, and compromised production of beneficial metabolites. Emerging evidence highlights the previously underexplored role of the gut virome, particularly bacteriophages, in regulating bacterial populations and influencing neurodevelopment. Viral dysbiosis correlates with cognitive impairment and neurodegenerative processes through modulation of bacterial metabolism and inflammatory responses. Understanding these complex host-microbiome-virome interactions provides novel therapeutic opportunities for neurological disorders through targeted interventions including probiotics, fecal microbiota transplantation, and phage-based therapies, representing a paradigm shift toward microbiome-centered approaches in neurological medicine.
