Bacteria in your guts are not just simple guests eating on your leftovers.

Beside digestive conditions, these bugs induce more severe problems, like obesity and now they were found to influence how fat is digested and deposited in the liver.

An imbalanced gut flora can affect this metabolic path and trigger diseases. Our gut has up to 100 trillion bacteria, about 10% of the number of the human body's cells.

In 2006, one team revealed that obese people carry a somewhat different bacterial species load than normal weighed people. Other approaches showed that gut microbes can induce insulin resistance, determining type 2 diabetes and fatty liver disease, a severe condition.

To see how microbes impair metabolism, a team at the Nestlé Research Center in Lausanne, Switzerland, led by biochemist Jeremy Nicholson of Imperial College London studied the health of seven mice whose gut flora was substituted with that of a 3-week old human infant. These results were compared with those achieved from mice with normal gut flora. After four weeks, the team determined levels of 12 bile acids in each mouse's urine, blood, liver and small intestine.

The liver synthesizes about 6 types of bile acids, which are released into the small intestine to help digest fats. From the gut, the bile acids reach again the liver and control cholesterol metabolism and other endocrine processes.

"But different gut microbes can modify the structure of bile acids in different ways, adding or snipping off various parts of the molecules, which would presumably alter their fat-dissolving abilities," said Nicholson.

17 variants of bile acid were detected in the gut, assigned into two chemical classes.

The mice with the human bacterial flora had more of one class of bile acid and less of the other, translated into significantly different metabolites levels in the urine, blood and liver compared to mice type bacterial flora.

"The results are the first to provide a detailed analysis how gut bugs shift the balance of metabolites present," said medical microbiologist David Relman of Stanford University.

"They also mean that altering gut microflora has far-reaching physiological consequences for the host animal."

Mice with the human bacteria also presented more LDL (bad cholesterol), in the liver and less glutathione, a natural antioxidant that impedes tissue damage.

"Major changes to gut flora like this could predispose the body toward disease." said Nicholson.