An article was published late last year by Jeff Gordon which studied the impact of human's microbiome in mice. In the paper the researchers studied pairs of human twins who were discordant in obesity, meaning one of the twins was obese and the other was not. Each twin's microbiome was transplanted into a germ-free mouse.
The results showed that the obese twin's microbiome made the mouse obese, whereas the lean twin's microbiome made the mouse lean. Then, the two mice were put together and fed a low fat diet. This caused the lean mouse's microbiome to establish itself in the obese mouse, which prevented further weight gain. When the mice were put together and fed a high fat diet, the lean mouse microbiome no longer was taken up by the obese mouse, and the lean mouse stayed lean while the obese mouse continued to gain weight.
There are a myriad of studies on the microbiome and obesity, so look for more blog posts about this subject moving forward.