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AS THE world's fatties clock up the kilos, their excuses for being that way have piled up, too. Big bones, junk foods, genes or poor parenting—there are plenty of directions in which to point a chubby finger. In the past few years, a new potential culprit has emerged: gut bacteria. Human guts are full of bugs that help digestion and also stop their disease-causing counterparts from invading. In this age-old symbiosis, some bacteria are better than others at providing food to their human hosts—and also seem, by mechanisms yet unknown, to encourage those hosts' bodies to store that energy as fat and to keep the fat on.

In the past, when food was in limited supply, these bacteria would have been valuable allies. In an era of plenty, though, they are problematic. In particular, work on mice suggests obesity is associated with having a high proportion of bacteria called Firmicutes, whereas the lean favour another group, the Bacteroidetes. Such work has also raised the suggestion that transplanting “lean mouse” microbes to fat mice can make them thinner—for a while.

What is true in the mouse, though, might not be true in the man, so more research is clearly needed. But finding volunteers willing to have someone else's bacteria transplanted into their own intestines is tricky. So, in a paper in this week's Science Translational Medicine, Peter Turnbaugh of the Washington University School of Medicine, in St Louis, and his colleagues, describe a halfway house. They have created a mouse with a humanised gut flora.

They did it by taking ten-week-old mice that had been bred to be free of germs and colonising their guts by feeding them with human faeces. (Note to the squeamish: the article adds that mice are coprophagic.) After this, the mice could be tested on various kinds of diets to see how the bacteria responded. These studies show similar sorts of outcomes to those previously seen with native mouse bacteria, including the increased obesity associated with Firmicutes.

The point of it all is to allow more precise investigation of how human gut bacteria work. Such studies will also, no doubt, give rise to an industry testing new classes of “probiotics” that can be added to food in order to ameliorate its obesity-inducing effects. In other words, to sell special drinks that may make fat people less likely to pile on the pounds after eating a hamburger and fries.