Gut-brain axis system updates: Optimizing microbiome biomarkers for psychiatry and neurology

Technological advances in nucleic acid sequencing in recent years effectively launched and propelled a new age of the microbiome. A mechanism known as the gut-brain axis has been adopted by psychiatric and neurological researchers, as another possible way to reconcile gene-by-environmental and other hypotheses of how serious brain disorders develop. Toward this end, the microbiome, may represent an informative biomarker, or series of biomarkers, of cellular, molecular, and metabolic pathways that are altered in disease. As with any field projected to be an enormous influence on the future of medicine, early studies of the microbiome are numerous and sometimes afflicted with study design and data quality issues. Some progress in these areas is underway, and here we provide a brief update of the current state of the microbiome with a focus on clinical studies and the quests to better understand mechanistically what functional outcomes in neuropsychiatric disorders might be mediated by microbes. We also review the concept of the microbiome biomarker as a dynamic and evolving measure that keeps improving as technology flourishes. In vitro gut systems that merge innervation and vascularization of epithelial organoids exemplify next-generation microbiome biomarkers.