Factopus

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LESSON 2 · The Microbe Within

PAUSE TO THINK

What happens to gut serotonin in mice raised with no bacteria at all?

It drops by roughly 60 percent

Germ-free mice make about **60 percent less** serotonin in the gut than mice with normal bacteria. Add the missing microbes back, and serotonin production climbs again — proof the bacteria are doing the building.

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