The study of gut flora has received increasing attention and there has been research evidence suggesting that gut bacteria are an integral part of the body. Previous studies have also identified a correlation between host genetic variation and gut flora composition and metabolic disease and obesity. However, the relationship between human genetic variation and gut flora diversity is still assumed to be negligible. A study published Oct. 6 in the journal CELL shows that a family of gut bacteria is highly heritable and more commonly found in lean individuals. Transplanting them into the intestines of mice prevents weight gain in lab rats. In the NIH-funded study, researchers from King’s University London and Cornell University sequenced microbial genes in more than 1,000 stool samples taken from 416 pairs of twins and found that identical twins with identical genes had more similar relative levels of a particular microorganism in their gut than did heterozygous twins with only half the same genes. The results suggest that genetic factors have a greater influence on gut group species than environmental factors. This family of intestinal probiotics, whose relative abundance is greatly influenced by host genetics, was named “Christensenellaceae”. Members of this probiotic family were found to be more abundant in the gut of lean individuals than in obese people. Moreover, mice transplanted with these bacteria were less likely to gain weight, suggesting that increasing the amount of these bacteria in the intestine may help prevent or reduce obesity. The significance of this study is that 1) for the first time, it has been established that host genetic factors determine the relative abundance of intestinal bacterial species; 2) it demonstrates that multiple heritable species can coexist and co-occur; 3) the most genetically influenced Christensenellaceae probiotics are associated with host body mass index; and 4) it predicts that future “fecal transplants ” to help combat obesity and obesity-related diseases.