The role of gut bacteria in the body may extend beyond the stomach and intestines all the way to the brain, a new study in mice suggests.
The results show disrupting the normal gut flora of the mice leads to changes in the animals' behavior, making them less timid and more adventurous, as well as leading to changes in their brain chemistry.
Although it's not clear if the same thing happens in humans, the findings may explain why some gastrointestinal diseases, such as irritable bowel syndrome, are often associated with disorders that can affect behavior, including depression and anxiety.
"It may be that those changes in gut bacteria not only contribute to the generation of gut symptoms, like diarrhea or pain, but may also contribute to this altered behavior that we see in those patients,” said researcher Stephen Collins, of the Farncombe Family Digestive Health Research Institute at McMaster University in Ontario, Canada.
The study is published online in the journal Gastroenterology.
Bacteria and behavior
Previous studies have suggested gut bacteria may communicate with the brain. For instance, some people with liver disease experience changes in mental abilities that improve after they are given antibiotics. Other studies have shown mice that don't have gut bacteria respond differently to stress compared with those that do.
To further investigate the link, Collins and his colleagues first gave healthy mice antibiotics to disturb their natural gut bacteria. The mice became less anxious — they were less hesitant to step off a platform and more eager to explore. When their gut bacteria was restored to normal, so was their behavior. Control mice that were given water instead of antibiotics showed no changes in behavior. Mice that didn't have any gut bacteria also showed no changes in behavior when they received antibiotics.
Disrupting the contents of the gut also appears to affect brain chemistry. Mice given antibiotics had an increased amount of a brain protein called derived neurotrophic factor, or BDNF, in their brains compared to control mice. Changes in the levels of BDNF have been previously linked to depression and anxiety.
Next, the researchers carried out some gut bacteria swapping of sorts. Different strains of mice are known to exhibit different behavior patterns. Some are more anxious while others are aggressive and hyperactive. The researchers took mice from both extremes and exchanged their gut bacteria. They saw the behavior flipped as well — the anxious mice became more active and daring and the aggressive mice became more passive.
Probiotics for the brain
The researchers suspect the bacteria are producing chemicals that can access and influence the brain, Collins said.
If gut bacteria play some role in human behavior as well, it's possible therapies that aim to restore normal gut flora, such as probiotics, may be helpful in correcting behavior and mood changes in those with gastrointestinal diseases, Collins said.
Collins and his colleagues are now studying the gut bacteria composition of patients with gastrointestinal disorders. They want to see whether the content differs among those who have symptoms of depression and anxiety compared with those who do not.
Pass it on: Changes in gut bacteria produce changes in behavior in mice.
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Rachael is a Live Science contributor, and was a former channel editor and senior writer for Live Science between 2010 and 2022. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.