How do bacteria in a mouse’s gut change the way it behaves?

Those mice responded differently to stress, releasing more stress hormone than ordinary ones. Transplanting bacteria from normal mice into them closed part of that gap, which is what turns a correlation into a cause. The animals also differ from ordinary mice in more than their bacteria, so the result holds for mice and does not travel further on its own.

To see why this is possible at all, start with the numbers. Trillions of bacteria live in the gut, carrying many times more genes than we do. They make small molecules that reach the bloodstream, including some that resemble neurotransmitters.

The difficulty is causation. Anxious people really do differ in gut bacteria, but anxiety changes diet, sleep and gut motility, so the arrow could point either way. Breaking that circle takes an intervention rather than an observation.

The extreme intervention is raising germ-free mice, with no bacteria at all. Those animals responded differently to stress and released stress hormones differently. Then came the decisive part: transplanting bacteria from ordinary mice closed part of the gap.

That establishes cause, in a mouse. A sterile animal is not a state that exists in nature, and it differs from an ordinary mouse in far more than its bacteria. In people most findings along the gut-brain axis remain correlational, and nothing shows that swapping bacteria changes mood. What it did do is open a field, and widen the argument about diet and antibiotics beyond digestion.