The researchers' colleagues divided mice into three groups: those getting gastric bypass surgery; those given sham surgery; and those given sham surgery whose diet was restricted, to match what the bypass group weighed after the operation. In this way, the scientists could separate out the effects of surgery itself on microbial communities from the effects of losing weight or consuming less food. For 23 animals overall, they analyzed fecal samples before surgery and then weekly for three months.
Although the balance of microbes changed in both the dieting animals and those given bypass surgery, the difference was more dramatic in the surgery group. Those mice displayed specific patterns in their gut, the team reports Friday in Science Translational Medicine, including a boost in three types of bacteria called Bacteroidetes, Verrucomicrobia, and Proteobacteria. All are common in the guts of healthy people.
In a sense, the postop bacterial changes are not surprising, says endocrinologist David Cummings at the University of Washington, Seattle, (although he notes it's a "herculean feat" to manage gastric bypass surgery in animals as tiny as mice). Because it bypasses part of the stomach and small intestine, the surgery alters the intestinal environment, changing elements such as pH and bile concentrations.
The big step forward, Cummings says, is what the researchers did next: They performed a series of gut microbe transplants. Animals in each of the three groups - gastric bypass, sham surgery, and restricted diets - were killed and samples of their gut microbe communities diluted. Those were infused into the stomachs of mice raised in a germ-free environment and, therefore, without a gut microbiome of their own. The animals that got microbes from the gastric bypass donors lost about 5 percent of their body weight in two weeks, even though they weren't eating any less than controls. Microbes transplanted from the dieting animals that had had sham surgery didn't lead to any notable weight loss in the recipients.