The team took gut bacteria from four sets of human twins in which one of each pair was lean and one was obese, and introduced the microbes into mice bred to be germ-free. Mice given bacteria from a lean twin stayed slim, whereas those given bacteria from an obese twin quickly gained weight, even though all the mice ate about the same amount of food.
The team wondered whether the gut microbiota of either group of mice would be influenced by mice with one type living in close quarters with animals harbouring the other type.
So the scientists took mice with the ‘lean’ microbiota and placed them in a cage with mice with the ‘obese’ type before those mice had a chance to start putting on weight.
“We knew the mice would readily exchange their microbes,” Gordon says — that is, eat each other’s faeces. Sure enough, the populations of bacteria in the obese-type mice changed to match those of their lean cage-mates, and their bodies remained lean, the team writes today in Science.
The bacterial invasion travelled only in that direction, however: the bacteria of the obese mice could not colonize the lean neighbour. This makes sense, says Gordon, who found in earlier work that the population of gut bacteria in obese people is less diverse than that in lean people, leaving unfilled niches in the microbiota. The bacteria from the lean mice seem to be able to find those vacancies, he says.