Slimming germs

Microbes dwell all around us — and in us. This is especially true in the gut, where bacteria, fungi and other living things not only share our food but also help us stay healthy. Scientists had thought those health benefits came from microbes that fought off germs that would make us sick. But new data suggest that some of these one-celled squatters in our digestive tracts may also help animals, including people, maintain a healthy body weight.

Gut-dwelling germs actually have a lot of control over the entire body. In one new experiment, scientists extracted microbes from the poop of both thin and obese people. Then they implanted some of these microbes into the intestines of mice.

After two weeks, mice that had received germs from thin people weighed just as much as they did before — no change. But mice that had received microbes from the poop of obese people had started to blimp out. They hadn’t eaten different foods or more foods than the first group. The only difference was the germs deposited into the guts. Now the germs alone did not make someone fat or slim. What made a difference was how the particular microbes helped digest the food that was eaten.

The scientists also wanted to know: Which type of germs — those from thin people or those from obese people — would dominate if both ended up living in the same gut? To find out, they put both the trim mice and the fat mice in the same cage.

“We called it ‘the battle of the microbiota,’” Jeffrey Gordon told Science News. Gordon, a microbiologist at Washington University in St. Louis, Mo., led the new study. Microbiota refers to the microscopic organisms that live in a particular place.  

Warning: Don’t read the next part if you’re about to have lunch.

The mice in the study eat poop. Scientists describe this behavior, common among many animals, as being “coprophagic” (from the Greek words for “dung” and “to eat”). The scientists gave the mice regular food. But the mice also ate feces in their cage. In this way they ingested both types of microbes. And in less than two weeks, both types of excreted gut microbes had spread among the mice.

The trim mice stayed trim. But the mice with the obese-people microbes — and now the “trim” microbes — in their gut stopped gaining weight. In other words, the thin microbes won.

Gordon’s team published its findings Sept. 6 in the research journal Science.

“It’s like a beneficial infection,” concludes Andreas Schwiertz, who did not work on the new study. A microbiologist, he works at the University of Giessen in Germany. The finding, Schwiertz told Science News, is“very surprising.”

But there’s a twist: Lean-person microbes won out only if the mice ate a low-fat diet. When the mice ate a diet high in fat and low in fiber, like the typical unhealthy American diet, the microbial benefits diminished. Then both types of mouse gained weight, although the trim mice gained less. 

The results show that gut germs can play an important role in how the body turns food into fuel.

Having too few of the trimming germs is “not the only cause of obesity, but it is a contributor,” concludes Gordon. Other studies point to even more surprising causes of being fat. In 1997, researchers showed a virus responsible for the common cold in people could cause obesity in chickens. Moreover, the scientists told Science News at the time, the same virus seemed to also foster obesity in people. Residues from infection with the virus also showed up in many obese people (but not slim ones). In October 2010, a follow-up study showed that obese children also were more likely to have been infected with this virus. And clearly, overeating — or exercising too little — can contribute to obesity.

As it turns out, getting fat is easier than getting slim again. But the new mouse study may lead to the identification of microbes that might be added to a person’s diet to help with weight loss. The study also indicates, however, that without the right diet, a beneficial microbial mix probably won’t make a difference.

Power Words

coprophagy The eating of feces or dung.

germ Any one-celled microorganism, such as a bacterium, fungal species or virus particle. Some cause disease. Others can promote the health of higher-order organisms, including birds and mammals. The health effects of most germs, however, remain unknown.

gut Colloquial term for an organism’s stomach and/or intestines. It is where food is broken down and absorbed for use by the rest of the body.

microbe, short for microorganism. A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.

microbiology The study of microorganisms.

microbiota The microorganisms that live in a particular place or geological period.

obesity Extremely overweight. Obesity is associated with a wide range of health problems, including type 2 diabetes and high blood pressure.