Scientists have found that microbes living in the human gut may have more influence over our health than our own genes, perhaps causing psychological and physical changes like obesity. "Outnumbering our human cells by about 10 to one, the many minuscule microbes that live in and on our bodies are a big part of crucial everyday functions. The lion's share live in the intestinal tract, where they help fend off bad bacteria and aid in digesting our dinners. But as scientists use genetics to uncover what microbes are actually present and what they're doing in there, they are discovering that the bugs play an even larger role in human health than previously suspected—and perhaps at times exerting more influence than human genes themselves. One team of researchers recently completed a catalogue of some 3.3 million human gut microbe genes. Their work, led by Junjie Qin of BGI–Shenzhen (formerly the Beijing Genomics Institute) and published in the March 4 edition of Nature, adds to the expanding—but nowhere near complete—census of species that reside in the intestinal tract. Another group turned its attention to a particular host gene that seems to impact these inhabitants of the intestines. They found that in mice, a loss of one key gene led to a shift in microbiota communities and an increase in insulin resistance, obesity and other symptoms of metabolic syndrome (a cluster of these conditions). Their results were published online March 4 in Science."