Bacteria Making Serotonin

About 90 percent of your body's serotonin is produced not in your brain but in your gut. That does not mean gut serotonin simply travels upstairs to change your mood — the brain has its own supply — but specific bacterial species can stimulate intestinal cells to make serotonin and feed into the gut-brain signaling network.

When researchers transplanted gut bacteria from anxious mice into calm mice, the calm mice showed more anxiety-like behavior within days. The reverse also worked in some experiments: bacteria from calmer mice made anxious mice act bolder. The cautious lesson is not that personality is written by microbes, but that gut bacteria can influence behavior-related pathways.