Interactions among the microbes in our gut could contribute to obesity, suggest findings by Buck Samuel and Jeffrey Gordon (Proc. Natl. Acad. Sci. USA 103, 10011–10016).

Credit: Justin Sonnenburg, Jaime Dant, Jeffrey Gordon; AAAS

Gordon and his colleagues had previously found that germ-free mice gained fat after they were inoculated with a suite of gut microbes. To ask why, the researchers inoculated germ-free mice with only a few species. These included two common inhabitants of the human large intestine, the polysaccharide-munching bacterium Bacteroides thetaiotaomicron (shown here) and the archaeal species Methanobrevibacter smithii.

Mice inoculated with both species did not gain weight—but they got 50 percent fatter. They also had 100–1,000-fold more microbes in their gut than mice inoculated with either microbe alone. Clearly, the microbes were somehow interacting. The researchers found that M. smithii influenced the metabolism of B. thetaiotaomicron, prompting it to consume mainly fructose-containing polysaccharides that break down into several substances, including formate—the preferred food of M. smithii.

Exactly why the mice get fatter is unclear. But the findings suggest that the collaboration between the microbes increases the efficiency of polysaccharide metabolism— increasing the yields of short-chain fatty acids such as acetate, which are used by the mouse to stimulate fat production and storage. The authors say that differences in our gut microbial communities may affect our predisposition to obesity. For some people, a diet rich in certain polysaccharides, such as fructose-containing sweeteners, could expand the waistline.