It could be that certain bacteria cause inflammation that can affect appetite as well as inflammatory bowel conditions like Crohn's disease and colitis, the researchers reported in the journal Science.
In other words, the germs make you overeat, Andrew Gewirtz of Emory University in Atlanta and colleagues reported.
Previous research has suggested that bacteria can influence how well energy is absorbed from food, but these findings demonstrate that intestinal bacteria can actually influence appetite, Gewirtz said.
The obesity epidemic is driven by people eating too much, but why are people eating more?
Gewirtz said the research suggests that bacteria may play a role -- perhaps a population of bacteria that thrive because other, competing organisms have been wiped out by antibiotics, access to clean water and other factors of modern life.
His team stumbled on the findings by accident.
We were studying mice that had colitis, Gewirtz said in a telephone interview.
The team suspected some kind of germ was responsible, so they transferred mouse embryos into surrogate mothers to prevent them from being infected by their own mothers.
Babies are colonized by bacteria and other micro-organisms soon after birth and the makeup of these colonies -- which persist for life in the skin and bowels -- are very similar to those of the mother.
The colitis was better but the baby mice became obese and developed metabolic syndrome -- a cluster of symptoms that include unhealthy cholesterol levels, too much fat around the midsection, high blood pressure and insulin resistance.
Insulin resistance means the body does not use insulin effectively to break down food and Gewirtz believes this may be the key.
The researchers remembered a recent study in which normal, slender mice became obese when fed gut bacteria from fat mice.
They worked with that team, including Ruth Ley of Cornell University in New York, to see what role the gut bacteria may be playing.
What we think is that the mice are prone to intestinal inflammation, Gewirtz said. If you have a lot of inflammatory signals about, insulin won't work properly.
Weeks of antibiotic therapy helped, and so did diets.
If we limit their food intake they are mostly OK; they certainly are no longer obese, he said. They are, however, insulin-resistant.
Gewirtz's team is now working to see if they can identify the micro-organisms involved. They are also working to see if obese people have unique patterns of gut bacteria.
Scientists know that hundreds of species of bacteria live in the gut and an average person carries about 5 pounds (2 kg) worth. On Wednesday, Chinese scientists reported in the journal Nature that they found 1,000 different species in human intestines.
So could you treat obesity by taking an antibiotic to wipe out the offending germs that are making people overeat?
It is very hard to replace the bacteria that you have, Gewirtz said. Studies already show it is difficult to treat conditions like Crohn's disease, even with months of antibiotics.