May 22, 2011
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.