Germ-free mice, who received gut bacteria from obese humans, gained more weight and accumulated more fat than mice, who received bacteria from the guts of lean humans, according to a new study.

This finding, which demonstrates the transmission of physical and metabolic traits via communities of microbes in the gut, depends on the rodents` diet.

The researchers responsible suggest that it may represent an important step toward developing new personalized probiotic and food-based therapies for the treatment or prevention of obesity.

The new study shows directly that microbial communities from the gut can transmit lean or obese traits; it also begins to name specific players involved, along with their designated roles and how these roles are tied to the foods we consume.

Vanessa Ridaura, a graduate student at Washington University`s School of Medicine, and colleagues took samples of the microbes that were living in the guts of human fraternal and identical twins.

For each pair of twins in the study, one sibling was lean while the other was obese. The researchers then transplanted the discordant twins` gut microbiota into the guts of germ-free mice that had been raised under sterile conditions, without any microbes of their own.

According to the researchers` results, something about the microbes in the guts of Ln mice prevented the Ob mice from accumulating much fat. So, Ridaura used a combination of algorithms-some that were tried-and-true and others that she cooked up herself-to figure out which bacterial species in particular were able to invade the Ob mice.

They found that specific members of the Bacteroidetes phylum were able to enter the guts of Ob mice, settling into otherwise unoccupied niches. These bacteria then triggered changes in metabolism, among other metabolic effects, but none of the bacteria from Ob mice could invade the Ln mice to make them accumulate fat.

The study has been published in the journal Science.