IDO1 and neoplasm: Beyond E. coli, engineered Clostridium butyricum (L-Trp CB) targets the TME by colonizing hypoxic tumor regions to deliver two complementary metabolite-mediated effects: it produces butyrate to inhibit IDO—an enzyme that depletes Trp to generate immunosuppressive Kyn—and releases Trp to fuel CD8+ T cell metabolic demands.