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More than 400 different kinds of bacteria live in our intestines, forming a complex, microscopic ecosystem that helps us with everything from making and absorbing vitamins to digesting food. But surprisingly little is known about how this microscopic menagerie interacts with our bodies. Recently, three researchers at Washington University School of Medicine in St. Louis found convincing evidence that some of these bacteria may actually communicate their needs to our intestinal cells, causing the cells to churn out sugars that the bacteria then eat.

Molecular biologists Per Falk, Jeffrey Gordon and graduate student Lynn Bry began their study by working with a line of mice raised for generations in a germ-free environment so their intestines wouldn’t carry any bacteria. They found that shortly after birth, the germ-free mice produced a carbohydrate that contained the sugar fucose. As the mice matured in the sterile environment, though, they stopped producing the sugar.

But when Bry added a particular member of the intestinal tract’s normal bacterial society, called Bacteroides thetaiotaomicron, which uses the fucose for food, the intestinal cells once again began manufacturing the sugar, apparently, Bry suspects, in response to an unknown chemical signal secreted by the bacteria.

To make sure she was right, Bry mutated the same strain of bacteria so that they could no longer eat fucose and then introduced the mutants to a germ-free mouse. She found that the mouse did not make the sugar, despite the presence of the mutant bacteria. Other bacteria within the mice probably make use of the fucose as well, says Gordon. Different microbes can create the beginnings of a village, which can be populated by other members of the society, and eventually build up a city.

The bacteria in human intestines are very similar to those found in mice, says Gordon, and probably also prompt intestinal cells to release various sugars. Understanding the bacteria’s needs is important, says Gordon, because benign bacteria occupy ecological niches in our intestines that would otherwise be open to invasion from disease-causing germs. Gordon also believes further research may lead to novel therapeutic measures that he calls pro-biotics. When someone is treated with antibiotics, the antibiotics disrupt the bacteria in the gut, leaving the person vulnerable to infection. Gordon speculates that once more is known about intestinal bacteria, patients may be given bacteria capsules to help repopulate their intestines with friendly microbes.

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