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What controls our emotions? The same two people might see the same tragic newspaper headline, but one is much more emotionally affected than the other. Why is that? According to an international team led by UCLA researchers, these emotions may be partially driven by an unlikely source: our gut bacteria. In fact, the bacteria that colonize our intestines may influence the architecture of the brain itself.

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The experiment examining the relationship between brain development and the gut microbiome was conducted with mice. Specific strains of mice, one that lacked a complete gut microbiome and one that had a normal microbiome, were compared in terms of their brain development and behavior.

Interestingly, the mice with the “normal” microbiome had more anxiety and moved less than the mice without the microbiome. When exposed to a microbiome early in life, the active, calm mice began to act more anxious and reduced their movements. Microbiota evidently had this influence by affecting hormonal expression and building particular pathways in the brain associated with anxiety. The influence on brain development apparently occurred at a particular point early in the mouse’s life, so it’s possible early exposure to a proper microbiome is important for normal development. The researchers speculate that the microbiome helps the mice develop an appropriate sense of caution.

The influence of microorganisms is not limited to brain development. While no conclusive link has been established, a few studies have detected abnormalities in the gut microbiomes of children with Autism Spectrum Disorders compared to non-affected children. Nor is microorganism influence limited to gut bacteria. A separate study in hyenas (also awaiting conclusive proof) suggests that individual hyenas use a specific cocktail of stinky bacteria in their scent markers left as a message to other hyenas. Even some fungus can get in on the act, hijacking ants and forcing the unfortunate insects to relocate to a place favorable for a fungus to grow. The ant does not survive, but it makes great fungus food.

Despite a growing body of evidence from other animals, until the UCLA study there was not much direct research on how the microbiome affects humans. But given its potential importance, hopefully there will soon be more research along these lines. Going with your gut, indeed!


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Proceedings of the National Academy of Sciences of the United States of America, Vol. 108, No. 7 (February 15, 2011), pp. 3047-3052
National Academy of Sciences
Proceedings of the National Academy of Sciences of the United States of America, Vol. 111, No. 6 (February 11, 2014), pp. 2051-2053
National Academy of Sciences