The prevalence and severity of obesity is a major cause of present-day attention on the microbiome. Pioneering work by Jeff Gordon in the mid-2000s demonstrated that transferring microbiota from obese mice to thin mice through fecal transplant caused significant weight gain, and vice versa.
We know that the microbiome breaks down much of our food into energy products that feed the body. If one consumes an obesity-inducing Western diet (including processed sugars and refined carbs) that is low in fiber and high in calories, it selects for a less diverse microbiome. This is because this processed food is readily absorbed in the small intestine and this starves the fiber munching large bowel microbes. It also can allow microbial products to seep into the body and trigger chronic inflammation which can lead to insulin resistance and other serious conditions.
Because of the complexity of the microbiome, there are currently no magic combinations of microbes you can take to help you lose weight (although there is a lot of work underway in this area). Personalized diets hold significant promise in tailoring food intake to your microbiome, and it is hoped that these can be used to help combat the obesity epidemic.