Mental Health and Bacteria

“You can’t focus on what’s going wrong. There’s always a way to turn things around!”
Joy, Inside Out. Walt Disney Pictures.
We’ve previously discussed the impact that bacteria could have on your immune system, especially considering they can make up 2-6 pounds of your body weight. Studies have linked gut bacteria to obesity, inflammatory bowel disease, and type 1 and 2 diabetes1. So, with gut bacteria having such extended effects on the immune system, would it be that far-fetched to believe they might influence your mood or state of mind? Anxiety disorders are estimated to affect 40 million Americans. Nearly 15 million Americans deal with depression2. This is clearly an arena where many people suffer daily. So, how did we come to link our mental state to the many millions of microbes in our digestive tracts?

Mental health is a worldwide issue. Image from Healthline.
Early on, much of the scientific evidence for causation between bacteria and mental health may have seemed rudimentary. But, rudimentary or not, interesting findings were popping up all over academia. John Cryan’s lab studied mice in a stress test, where the mice would be unable to climb out of or reach the bottom of a container of water. Eventually mice would simply float and exhibit something akin to despair. When they fed the mice a strain of bacteria (Lactobacillus rhamnosus), the mice fought on for longer periods of time, similar to mice who had been given anti-depressants3.
Mark Lyte conducted an experiment where he fed mice Campylobacter jejuni. While they showed no physical symptoms of illness, these mice displayed anxiety-like behaviors when put through an elevated maze. They refused to cross narrow bridges or unprotected ledges. Normal mice freely crossed these obstacles4. Sven Pettersson showed that germ-free mice displayed higher motor function and reduced anxiety compared to their normal counterparts5. More and more of these studies are garnering support, as the NIH recently issued four grants for $1 million each for studies linking gut bacteria to mental health.
An example of an elevated maze. Image from the Carolina Institute.
Even if these studies didn’t convince you of causation, there are additional facts to consider. Gut abnormalities have also been found in children suffering from autism and hyperactivity. It has long been known that a large share of hormones, such as serotonin, originate from your gut to help to control appetite, fullness and digestion chemicals. Coincidentally, your neurons also use chemicals like dopamine, serotonin, and gamma aminobutyric acid (GABA) to communicate and regulate mood. These factors can all cause intestinal disorders, which often coincide with anxiety and depression. Some bacteria are capable of releasing large amounts of these factors, such as L. rhamnosus, which was fed to the mice in the swimming stress test. L. rhamnosus creates lots of GABA to calm nervous activity. There’s also the possibility that bacteria might loosen the junction points in both the intestinal lining and the blood-brain barrier, potentially allowing your gut bacteria to alter one’s chemical balance with their byproducts6.
With all of the possible roles that microbes might play in psychological health, it’s no wonder many have taken to calling them “psychobiotics”. In addition, fecal transplants have been used in an attempt to rebalance the microbiome and help with a number of diseases. As evidenced above, many scientists have been looking to use bacteria to help combat disease. And, we might finally be getting some answers and mechanisms when it comes to bacteria and mental health. There’s millions of people out there dealing with anxiety, depression, or mental health problems; perhaps the millions of microbes in our guts are the key to helping us all feel better. Any thoughts on the link between gut microbiota and mental health? Let us know at
This is becoming truer and truer. Watch the full song clip from NBC’s Scrubs here.

Just when I thought I got the hang of science...Image by Greg Williams.
Contributed by Ken Lau, PhD.
  1. Diet-Induced Dysbiosis of the Intestinal Microbiota and the Effects on Immunity and Disease
  2. Anxiety and Depression Association of America
  3. Ingestion of Lactobacillus strain regulates emotional behavior and central GABA receptor expression in a mouse via the vagus nerve
  4. Campylobacter jejuni infection increases anxiety-like behavior in the hole board
  5. Normal gut microbiota modulates brain development and behavior
  6. Can the bacteria in your gut explain your mood?
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