Germs in the gut may help drive appetite, says new research into the link between obesity and bacteria.

Previous studies have shown that overweight people and normal-weight people harbor different types and amounts of microbes that naturally live in the intestine. To determine why, scientists are peering into mice.

Emory University researchers noticed that mice with an altered immune system were fatter than regular mice, and had a collection of disorders - high blood pressure, and cholesterol and insulin problems - called metabolic syndrome, often a precursor of heart disease and diabetes.

Everyone is born with a sterile digestive tract that within days is flooded with bacteria from first foods and the environment. Altered immunity in these mice meant somewhat different bacteria grew in their intestines than in normal rodents - driving bigger appetites, metabolic syndrome and a low-grade inflammation believed key to obesity's illnesses, Emory associate pathology professor Andrew Gewirtz reported Thursday in the journal Science.

To prove it, Gewirtz transferred bacteria from the fat mice directly into the germ-free intestines of normal newborn mice - and those mice began eating more and developed inflammation and insulin problems.

Restrict access to food and the altered mice don't gain weight, but still experienced the other symptoms, he noted.

"People are getting obese because they're eating more, but it suggests the reason they're eating more may not simply be that calories are cheap and available," Gewirtz said. "The reason they're eating more may be an increased appetite resulting from changes in intestinal bacteria."

His next step is to study how gut bacteria changes in people having weight-loss surgery.