Quilted Science

Patchwork thoughts on psychology, neuroscience, and human behavior.

Bacteria and Your Brain

Gut flora affect brain function and might play important role in brain disorders

If you count cells, we are not even close to being a majority human. 

In our bodies, bacteria far outnumber human cells, 10:1. The number of bacterial cells living inside the human body even exceeds the number of neurons in the brain (and that’s roughly 80 billion neurons).

Before you get disgusted at being a living Petri dish, realize that these bacteria are beneficial.  They have a symbiotic relationship with our bodies, one that is vital to physical and mental health.

Recent research on the bacteria in our intestines, which is also called gut flora or microbiota, really makes a strong case that we should suspend judgment that what goes on in our intestines is gross.  I am convinced that gut flora is actually awesome, and I’m going to talk about a few reasons why.

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Gut flora plays an important role in digestion and health.  All gut flora is not created equal, and recently the FDA has approved fecal microbiota transplants to treat debilitating gastrointestinal conditions, such as hard-to-cure infections and autoimmune diseases like Crohn’s and colitis.  (Mercifully, scientists are working on a pill that aims to do the same thing as a fecal transplant.)

But—and this is the awesome part—gut flora also affects how our brain works. 

Cross-disciplinary research spanning neuroscience and gastroenterology has started identifying the biological mechanisms behind the bidirectional connection between the gut and brain.

For example, scientists were able to make anxious mice non-anxious by transplanting gut flora from non-anxious mice into the intestines of anxious mice.  They were also able to do the reverse, making non-anxious mice anxious via fecal microbiota transplant, which is the same general procedure described above in humans.

And experiments in healthy humans show that just eating over-the-counter probiotic yogurt had widespread effects on the brain.

Published last week, a study in mice suggests a relationship between gut flora and some behaviors seen in autism spectrum disorder. This provocative study used an animal model of autism to examine how different populations of gut flora affected the animals.

The mice used in the study had gastrointestinal problems that are seen in certain subpopulations of autistic humans, and the mice also displayed analogs of behaviors seen in some autistic humans.

The researchers treated the mice with probiotics, which altered their gut flora in a controlled way.  The scientists then examined how probiotics changed both gastrointestinal malfunctions and behavior.

Probiotic treatment alleviated gastrointestinal issues, specifically leaky gut, present in the mice.  The authors point out that a recent study in humans identified that autistic children with leaky gut were missing the specific bacteria included in the mouse experimental probiotic.

But what is really interesting is that giving the mice probiotics improved the behaviors associated with autism in humans.  Changing the bacteria in the gut changed the brain and therefore behavior.  

The authors summarize their results as “supporting emerging evidence for a gut-brain link in modulating neurodevelopmental disorders.”

Though this study used an animal model for autism, the finding of gut flora composition affecting behavior applies to other brain disorders as well.  “… the behavioral abnormalities characteristic to human ASD can be individually seen in other neurological diseases such as schizophrenia, obsessive compulsive disorder, Angelman syndrome, and Prader-Willi syndrome.”

So instead of thinking of the all bacteria you play host to as icky or gross, give your gut flora the respect it deserves.  Gut flora is pretty awesome.

Kimberlee D’Ardenne, Ph.D. is a neuroscientist by training, science writer by choice.

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