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The Personality Transplant

How much is your personality a product of your gut microbiome?

There is an information super highway including hundreds of millions of neurons that connects our brain and our gut called the brain-gut axis. This axis informs our brain if we are hungry, whether we are experiencing stress, or if we have recently ingested a disease-causing microbe. But what about all the bacteria that live in our gut, our microbiota, are they wired into the brain-gut axis? Is that sudden craving for an afternoon snack our own idea, or was that a seed planted by our gut microbes? Recent evidence indicates that our microbiota is playing a much larger role in influencing our brain and behavior than most would have anticipated.

A few years ago a group of researchers noticed that the two different strains of laboratory mice they were studying had quite different personalities. One type seemed extremely anxious, the Woody Allen of lab mice. The other was more outgoing and gregarious, much like the Italian actor/director Roberto Benigni. The scientists could even quantify these personality differences by placing the mice on an elevated platform and seeing how long it took the mice the step off. The Woody Allen mice spent up to five minutes carefully inching their way off the platform, whereas the Roberto Benigni mice confidently leapt off within seconds. The scientists then transferred the microbiota from the Woody Allen mice to the Roberto Benigni mice and vice versa. (This procedure, known as a fecal transplant, has gained much attention recently as an effective method for curing the potentially fatal infection caused by the pathogenic bacterium C. difficile. And if you are wondering whether a fecal transplant is what it sounds like, it is.) Amazingly after this transplant, the Woody Allen mice with the Roberto Benigni microbiota became much more bold, stepping off the elevated platform a full minute sooner than before. Similarly the Benigni mice that received the Woody Allen microbiota were now much more cautious getting off the platform. By exchanging microbiotas, these scientists had not only performed a fecal transplant, but a personality transplant as well. They could even link the changes in microbiota to differences in the brain chemistry of the mice. Clearly the gut microbes were “communicating” with the brain in a profound way. This study and others like it are laying the foundation for the burgeoning field of science connecting the microbiota to the brain-gut axis. Perhaps our gut microbes are playing a larger role in our behavior and moods than we appreciate presently.

But, before you use your gut microbes as a scapegoat for your less desirable personality traits, it is important to put studies like these into perspective. These experiments were performed on mice, which are not humans. And while the microbiota transplant did shift their behavior it didn’t completely flip-flop them. There was more to the nature of these mice than their microbes. But what this study does show is the far-reaching impact that microbes living down in the depths of the gut can have on the functioning of the most complex organ at the top of the skull. Scientists are discovering that these microbes are wired into so much more than just digestion. These bacteria are communicating with our brain and it’s clear that the messages they’re sending are a lot more nuanced than just “feed me”.


Bercik, P., et al. “The Intestinal Microbiota Affect Central Levels of Brain-Derived Neurotropic Factor and Behavior in Mice.” Gastroenterology 141.2 (2011): 599–609, 09.e1-3. Print.

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