IL6 and major depressive disorder: Extending this, large population-level data from the Flemish Gut Flora Project identified butyrate-producing Faecalibacterium and Coprococcus as positive correlates of quality of life and inverse correlates of depression scores, where butyrate inhibits HDACs, activates FFAR2/FFAR3, tightens epithelial junctions, and suppresses IL-6/TNF-α, establishing these genera as reproducible, population-scale candidates for biomarker panels (165).