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1. Greedy guts? （The Economists Jan 4th, 2007） ALTHOUGH most people prefer not to think about it, human guts are full of bacteria. And a good thing, too. These intestinal bugs help digestion, and also stop their disease-causing counterparts from invading. In return, their human hosts provide them with a warm place to live and a share of their meals. Now it is working rather too well. A group of researchers led by Jeffrey Gordon, of the Washington University School of Medicine, in St Louis, has found that some types of microbes are a lot better than others at providing usable food to their hosts. In the past, when food was scarce, those who harboured such microbes would have been blessed. These days, paradoxically, they are cursed, for the extra food seems to contribute to obesity. Dr Gordon’s research is outlined in a paper published in this week’s Proceedings of the National Academy of Sciences (PNAS) and two others published last month in Nature. In the Nature papers, he and his team reported that obese people have a different mix of gut microbes from that found in lean people—a mix that is more efficient at unlocking energy from the food they consume. The researchers sequenced bacterial DNA from faecal samples taken from volunteers and discovered that those who were obese had a higher proportion of Firmicutes than lean people did. Bugs in the system This also turned out to be true in mice, and working with these rodents, the researchers discovered that the types of Firmicute found in obese animals are more efficient at converting complex polysaccharides into simple, usable sugars such as glucose. In effect, the Firmicutes made more energy available from the same amount of food. The researchers were even able to make mice that had been raised in a germ-free environment fatter or thinner by colonising their guts with microbes from either obese or lean mice. It sounds simple enough. Unfortunately, further probing showed that the story is a little more complicated