Scientists have identified 3 bacteriophages, 47 bacterial species, and 50 fecal metabolites that were significantly more or less abundant in people with major depressive disorder (MDD) compared with healthy controls, according to a study in 311 individuals. The findings provide evidence that MDD may be characterized by gut microbiome disturbances. Jian Yang and colleagues also developed a marker panel based on the bacterial, viral, and metabolic MDD signatures they uncovered, which effectively differentiated between patients with MDD and controls.