Unfortunate combinations of genetics and environment are often eyed as culprits in obesity. But now researchers are considering another offender: a particular type of microbe in the gut. A study in mice has found that a tiny gut archaean, a member of a class of one-celled organisms resembling bacteria, may influence how many ingested calories turn into fat.
Hundreds of types of microbes live in the gut, where they break down many types of food. One common target is polysaccharides, found in everything from corn syrup to wheat products. Gut microbes chop polysaccharides into short-chain fatty acids that account for up to 10% of a person's daily calorie intake. Two years ago, gastroenterologist Jeffrey Gordon of Washington University in St. Louis, Missouri, and colleagues found that gut microbes can influence how many calories are transformed into fat, but they couldn't tell exactly which organisms were responsible. Recently, Gordon began to suspect archaeans. Because archaeans consume some leftovers of bacterial digestion of polysaccharides, such as hydrogen, Gordon had a hunch they might speed up digestion, possibly causing an animal to get fatter.
Gordon and his graduate student Buck Samuel turned to three common gut microbes. Two were bacteria, Desulfovibrio piger and Bacteroides thetaiotaomicron, and the third was an archaean called Methanobrevibacter smithii. The pair created mice lacking any gut microbes and then gave them various combinations of the archaeans and the two bacteria. All the mice received the same amount of food. But animals that got the archaean M. smithii and the bacterium B. thetaiotaomicron were about 54% fatter than mice given the bacteria alone. By analyzing more carefully how the microbes interacted in the lab, researchers found that the archaeans actually changed the behavior of B. thetaiotaomicron, so that the bacteria would covet the more plentiful polysaccharides in the diet known as fructans, the pair report online this week in the Proceedings of the National Academy of Sciences. "At least in our system, this helps the host get more calories from its diet," says Samuel.
Microbiologist Martin Blaser of New York University says the findings are convincing and are "beginning to establish a scientific basis" for a lingering theory that hasn't gotten much attention. "Changes in microbial composition of the human body are contributing to this phenomena of obesity," says Blaser, opening up a potentially new field of study in obesity research.