Moni Sivakumar S., Hussien Abouelhag
Psychiatric neuroscience is undergoing a paradigm shift: the gut microbiome is no longer a correlational curiosity but a causal hub in the pathogenesis of anxiety, major depressive disorder (MDD), and the therapeutic action of antidepressants. Moving beyond broad taxonomic surveys, this review assembles a mechanistic atlas of the “psychobiome”—the specific bacterial strains and their neuroactive metabolites that directly modulate microglial activation, vagus nerve firing, and blood–brain barrier (BBB) integrity. We delineate three principal metabolite highways: (1) the tryptophan/kynurenine/indole axis, through which strains such as Lactobacillus plantarum 299v, Bifidobacterium longum NCC3001, and Clostridium sporogenes calibrate microglial phenotype and BBB tight junctions; (2) the GABAergic circuit, exemplified by Lactobacillus rhamnosus JB‑1 and Bifidobacterium adolescentis, which signals to the brain via vagal afferents and intracerebral GABA receptor modulation; and (3) short‑chain fatty acids (acetate, propionate, butyrate), produced by Faecalibacterium prausnitzii and Clostridium butyricum, that epigenetically program microglial homeostasis and reinforce the BBB. We further dissect the emerging evidence that selective serotonin reuptake inhibitors (SSRIs) require an intact microbiome to stimulate hippocampal neurogenesis and clinical response, repositioning gut bacteria as obligatory co‑mediators of psychopharmacology. Finally, we propose that rationally designed, multi‑strain microbial consortia—targeting all three metabolite pathways—represent a new class of psychiatric drug, with the potential to correct the endophenotypes of depression and anxiety at their molecular roots. By cataloging strain‑level mechanisms with actionable molecular precision, this review provides a translational blueprint for the next generation of psychobiotics.