Preface to the Special Issue: "Microbiota and Gut-Brain Axis".

The microbiota-gut-brain axis has emerged as a central framework for understanding how microbial communities influence nervous system development, function, and disease. This Special Issue highlights three interconnected themes: microbial metabolites and neurochemistry, developmental and neuroimmune mechanisms, and translation to neurological and psychiatric disease. The articles in the collection examine how microbial metabolites modulate neuroinflammatory and neurochemical pathways, how stress, development, immune signaling, and gut health shape gut-brain communication across the lifespan, and how these mechanisms contribute to disorders including autism spectrum disorder, depression, Parkinson's disease, spinal cord injury, and addiction. Together, these contributions advance a mechanistic and translational understanding of microbiota-gut-brain interactions and identify important directions for future research.