In one experiment, scientists raised mice in germ-free conditions so the mice lacked intestinal microbes. the mice were fed a low-fat diet rich in the complex plant polysaccharides, such as cellulose, that are often called fiber. when the mice were 12 weeks old, the scientists transplanted the microbial community from the intestine of a single "donor" mouse into all of the germ-free mice. then they divided the mice randomly into two groups and fed each group a different diet. group 1 (the control group) continued to eat a low-fat, high-fiber diet. group 2 (the experimental group) ate a high-fat, high-sugar diet.