'Friendly' bacteria living in our digestive systems may be helping to make us fatter, say scientists.
The human intestine is home to trillions of bacteria
Trillions of bugs live in the human gut, helping us break down food.
A team from Washington University School of Medicine claim when one type is dominant, this may impact on how many calories we extract from our diet.
Writing in the journal Nature, they described how they were able to make mice obese simply by adjusting the levels of certain types of bacteria.
They say it may be possible to treat obesity by doing the same to humans.
Obesity is recognised as a major health threat in many Western countries, including the UK, where a recent study suggested its effects consumed 9% of the total NHS budget.
While eating too much is the obvious primary cause, scientists have been looking for other factors which might pre-dispose people to gain weight.
The Washington University team, led by Dr Jeffrey Gordon, has been looking at the levels of two types of bacteria normally found in the gut - the Bacteroidetes and the Firmicutes.
They found that in obese people, the relative proportion of Bacteroidetes compared with Firmicutes was less - and a similar result was found in a strain of laboratory mice bred to be genetically obese.
Mice with no gut bacteria then had either Bacteroidetes or Firmicutes injected into their digestive systems, and while both groups then gained weight, the weight gain with Firmicutes was significantly higher.
Remarkably, when obese humans were put on a diet, the scientists found that the proportion of Firmicutes decreased, and Bacteroidetes increased, in line with their loss of body fat.
While the reason for the bacterial effect is not fully understood, it is suggested that the type of bug found to be dominant in obese people may be more efficient at 'harvesting energy' from food in their intestines.
It is also unclear how any 'imbalance' of bugs might develop in obese people
Writing in Nature, Dr Gordon's team said that if the microbial make-up of the human gut proved to be similar to those of mice, then it might produce a new 'therapeutic target' for doctors trying to control rising obesity rates.
"Manipulation of gut microbial communities could offer another approach in the treatment of obesity," they added.
Catherine Collins, chief dietician at St George's Hospital in London, said that while the study was interesting, the link between gut bacteria and obesity would be hard to prove.
"Most of the energy is extracted from food in the small intestine, where there aren't as many bacteria - most live in our guts live below this in the colon.
"My feeling is that the relationship between gut flora and obesity might be as small as the bacteria themselves."
She added: "People with weight problems should focus on eating smaller portions, and changing their diet to include more fruit and vegetables, as we know this is beneficial."