The mice who received gut bacteria from the obese people gained more weight — and experienced unhealthy metabolic changes — even though they didn't eat more than the mice who received germs from the lean twins, said study senior author Dr. Jeffrey Gordon, director of Washington University's Center of Genome Sciences and Systems Biology.
Then came what Gordon calls the battle of the microbes. Mice that harbored gut bacteria from a lean person were put in the same cages as mice that harbored the obesity-prone germs.
What happened was a surprise. Certain bacteria from the lean mice invaded the intestines of the fatter mice, and their weight and metabolism improved. But the trade was one-way — the lean mice weren't affected.
Moreover, the fatter mice got the bacterial benefit only when they were fed a low-fat, high-fiber diet.
When Ridaura substituted the higher-fat, lower-fiber diet typical of Americans, the protective bug swap didn't occur.
Why? Gordon already knew from human studies that obese people harbor less diverse gut bacteria. "It was almost as if there were potential job vacancies" in their intestines that the lean don't have, he explained.
Sure enough, a closer look at the mice that benefited from the bug swap suggests a specific type of bacteria, from a family named Bacteroidetes, moved into previously unoccupied niches in their colons — if the rodents ate right.
How might those findings translate to people? For a particularly hard-to-treat diarrheal infection, doctors sometimes transplant stool from a healthy person into the sick person's intestine. Some scientists wonder if fecal transplants from the lean to the fat might treat obesity, too.
But Gordon foresees a less invasive alternative: determining the best combinations of intestinal bacteria to match a person's diet and then growing those bugs in sterile lab dishes — like this study could — and turning them into pills. He estimates such an attempt would take at least five more years of research.