The bacteria in our gut may be controlling our lives more than we ever realized.
In the latest findings, published today in Nature, researchers report a link between gut bacteria and the development of multiple sclerosis in mice. Studies in mice have also examined gut bacteria in relation to obesity, depression and much more.
More human studies are emerging hinting at the role the bacteria in our guts may play well beyond helping us to digest our food.
"What has been observed in humans with regard to obesity is that there seems to be a difference in the number of kinds of bacteria in the gut," said Rob Knight of the University of Colorado, Boulder. "That number is much lower in obese people than in healthy people."
Researchers have also seen differences in bacteria between mice bred to be obese versus those of normal weight. In one experiment, researchers found that an obese mouse's gut microbes extracted more of the calories from a given parcel of food than did those of non-obese mice.
This caused the obese mice to gain more body fat than the non-obese mice did.
But even stranger, in a type of mouse with a different mutation that leads to obesity, transferring gut microbes from the obese mice into other mice led the non-obese mice to eat more.
"They're not any better at extracting energy from the food. They're just hungrier apparently," Knight said. "There are more microbial cells in your body than there are brain cells. They may be outvoting you when it comes time to order (at the restaurant)."
If gut microbes can tell mouse brains to eat more, could they have other effects on the brain? Researchers are finding that the answer is yes.
"We're now starting to see direct impacts of the gut microbial community on host behavior," Knight said.
Experiments have shown that mice with no gut microbes show differences in how much they move and in their anxiety-like behavior than mice with normal gut bacteria. Mice treated with "probiotic" Lactobacillus bacteria showed a different gene expression in the brain, reduced anxiety behavior and stress hormone levels than untreated mice.
The effects may extend to diseases that are seemingly unrelated to the digestive system. In the work published today, researchers studied mice bred to develop a disease similar to multiple sclerosis.
Those raised in an environment with no bacteria never developed symptoms. Once typical gut bacteria were introduced, the mice began to show signs of the disease.
"I think what our study really shows is the importance of the gut microbiota in the initial phase of the disease," said Gurumoorthy Krishnamoorthy of the Max Planck Institute of Immunobiology and Epigenetics in Freiburg, Germany who led the study with colleague Hartmut Wekerle.
Of course, it's impossible for humans to live a microbe-free lifestyle, but the findings suggest the microbial community may play a role in human multiple sclerosis. The team will now look for specific microbes that may be responsible for triggering symptoms.
If a healthy gut microbial community proves important for different aspects of our health, what could we do to encourage the bacteria that would prevent obesity or disease?
One option might be to take probiotics, eat a particular diet, or minimize antibiotic use, Krishnamoorthy said.
But others are investigating transferring gut bacteria from the other end-through "fecal transplants" from healthy people to those with illness. "The potential is very high, but the actual amount of research is relatively low," Knight said.
In trials of patients with Clostridium difficile infections -- a gut-wrenching intestinal problem -- more than 90 percent of patients have been cured, and their microbial communities restored to normal.
In preliminary work with huge public health implications, a study last year showed improved insulin sensitivity among patients with metabolic syndrome who received stool transfers from lean individuals.
"Then the question is which other conditions are there that could also benefit from stool transplants," Knight said.