The Microbiome–Neurodegeneration Interface: Mechanisms, Evidence, and Future Directions

The gut microbiota has emerged as a central regulator of the gut–brain axis, profoundly influencing neural, immune, and metabolic homeostasis. Increasing evidence indicates that disturbances in microbial composition and function contribute to the onset and progression of neurodegenerative diseases (NDs) through mechanisms involving neuroinflammation, oxidative stress, and impaired neurotransmission. Gut dysbiosis is characterized by a loss of microbial diversity, a reduction in beneficial commensals, and an enrichment of pro-inflammatory taxa. These shifts alter intestinal permeability and systemic immune tone, allowing microbial metabolites and immune mediators to affect central nervous system (CNS) integrity. Metabolites such as short-chain fatty acids (SCFAs), tryptophan derivatives, lipopolysaccharides (LPS), and trimethylamine N-oxide (TMAO) modulate blood–brain barrier (BBB) function, microglial activation, and neurotransmitter synthesis, linking intestinal imbalance to neuronal dysfunction and cognitive decline. Disruption of this gut–brain communication network promotes chronic inflammation and metabolic dysregulation, key features of neurodegenerative pathology. SCFA-producing and tryptophan-metabolizing bacteria appear to exert neuroprotective effects by modulating immune responses, epigenetic regulation, and neuronal resilience. The aim of this work was to comprehensively explore the current evidence on the bidirectional communication between the gut microbiota and the CNS, with a focus on identifying the principal molecular, immune, and metabolic mechanisms supported by the strongest and most consistent data. By integrating findings from recent human studies, this review sought to clarify how microbial composition and function influence neurochemical balance, immune activation, and BBB integrity, ultimately contributing to the onset and progression of neurodegenerative processes. Collectively, these findings position the gut microbiota as a dynamic interface between the enteric and CNS, capable of influencing neurodegenerative processes through immune and metabolic signaling.