Disclosed herein are compositions that have substantially purified Christensenellaceae bacteria, and uses of these compositions to alter the microbiome of an individual. The addition of Christensenellaceae bacteria, such as Christensenella, to the microbiome of an individual can treat or prevent weight gain, reduce body weight, inhibit fat accumulation, reduce excess adiposity, and reduce a high body mass index (BMI), and can also treat or prevent conditions correlating with excess weight and fat and a high BMI, such as insulin sensitivity, metabolic syndrome, excess adiposity, and diabetes.