Bacteria that live in the gut have been used to reverse obesity and Type-2 diabetes in animal studies.
Research, published in Proceedings of the National Academy of Sciences, showed that a broth containing a single species of bacteria could dramatically alter the health of obese mice.
It is thought to change the gut lining and the way food is absorbed.
Similar tests now need to be take place in people to see if the same bacteria can be used to shed the pounds.
The human body is teeming with bacteria - the tiny organisms outnumber human cells in the body 10 to one.
And there is growing evidence that this collection of bacteria or "microbiome" affects health.
Studies have shown differences between the types and numbers of bacteria in the guts of lean and obese people.
Meanwhile gastric bypass operations have been shown to change the balance of bacteria in the gut.
Researchers at the Catholic University of Louvain, in Belgium, worked with a single species of bacteria Akkermansia muciniphila. It normally makes up 3-5% of gut bacteria, but its levels fall in obesity.
Mice on a high fat diet - which led them to put on two to three times more fat than normal, lean, mice - were fed the bacteria.
The mice remained bigger than their lean cousins, but had lost around half of their extra weight despite no other changes to their diet.
They also had lower levels of insulin resistance, a key symptom of Type-2 diabetes.
Prof Patrice Cani, from the Catholic University of Louvain, told the BBC: "Of course it is an improvement, we did not completely reverse the obesity, but it is a very strong decrease in the fat mass.
"It is the first demonstration that there is a direct link between one specific species and improving metabolism."
Adding the bacteria increased the thickness of the gut's mucus barrier, which stops some material passing from the gut to the blood. It also changed the chemical signals coming from the digestive system - which led to changes in the way fat was processed elsewhere in the body.
Similar results were achieved by adding a type of fibre to diets which led to an increase in the levels of Akkermansia muciniphila.
Prof Cani said it was "surprising" that just one species, out of the thousands in the gut, could have such an effect.
He said this was a "first step" towards "eventually using these bacteria as prevention or treatment of obesity and Type-2 diabetes" and that some form of bacteria-based therapy would be used "in the near future".
Prof Colin Hill, a microbiologist at University College Cork, said: "It's a very exciting study, we've had lots linking bacteria and weight gain but this is the first time an intervention seems to work.
"I don't think it's feasible that you can eat cream cakes and chips and sausages all day long and then eat bacteria to reverse all that."
He said it was more likely that the research would lead to understanding of exactly what happens in the gut, which could lead to tailored dietary advice for people trying to lose weight.