Moody Microbes

How are you feeling right now? Happy or sad? Angry or content? The microbes living inside of you, particularly those in your large intestine, are partially responsible for your state of mind right now.

Bacteria from the genus Lactobacillus, a genus of bacteria often found in the intestine.
A creative commons image. Source.  

Microbes—tiny creatures that aren’t even part of the human body—influence the brain?

Yes, intestinal bacteria, and the food we feed them, play critical roles in how we feel. 

Diagram of the large intestine, where our gut bacteria live,
 A creative commons image. Source: Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436

Here’s how bacteria and diet influence the brain:

Microbes talk to the brain through the immune system. Dendritic cells—immune system cells in gut lining—constantly monitor microbes for misbehavior. If the dendritic cells sense that the intestinal bacteria are acting up, they trigger the release of cytokines—compounds that cause gut inflammation.

Cytokines communicate with the brain in two ways: as hormones and as signals sent via the vagus nerve—a thick nerve connecting the gut to the brain. These hormonal and nervous system signals make the brain reduce energy levels, and increase pain sensitivity. Cytokines can even induce feelings of sadness often felt during a stomach ache or gut infection.

It doesn’t require a full-blown gut infection to induce gut inflammation—simply eating a diet high in animal fat can cause the release of cytokines—and the subsequent mood drop.

Bacteria from the genus Klebsiella, a genus often found living in the gut.
A creative commons image. Source.

Microbes also signal the brain through metabolites—byproducts of microbial digestion. The metabolites microbes produce change based on what food we feed them. Metabolites travel through the bloodstream and act as hormones in the brain to influence our moods.

Bacteria from the genus Lactobacillus, a genus of bacteria often found in the intestine.
A creative commons image. Source.

 Generally, food that is good for humans also is good for our bacteria and causes them to release beneficial metabolites. Some metabolites, like those released after eating whole grains and vegetables, trigger release of serotonin, a neurotransmitter which is linked to improved mood. Fatty, unhealthy food on the other hand is likely to cause gut inflammation and the depressed mood that comes with that condition.

Illustration of a human brain. A public domain image. Source.

Gut microbes are so vital to our emotional state and thought process that scientists are starting to think of the gut, the brain, and the human microbiome as a connected system rather than separate entities. This system is often called the gut/brain/microbiome axis. Next time you are in a good mood remember to thank your intestinal microbes!

Check out a previous post I wrote on microbes, We Need Bacteria, to read about the connection between microbes and food allergies.

Sources.