Babies with high numbers of bifidobacteria and low numbers of Staphylococcus aureus may be protected from excess weight gain, according to a team of researchers from the University of Turku in Finland.

Their study was published in the March issue of The American Journal of Clinical Nutrition.

The researchers suggested their findings may help explain why breast-fed babies are at lower risk for later obesity, since bifidobacteria are prevalent in the guts of breast-fed babies.

Other studies repeatedly have found that being breast-fed is associated with a reduced risk of excess weight or obesity in childhood, with the risk lowered from 13 percent to 22 percent.

In the new study, researchers evaluated children who had been part of a long-term study to evaluate the effect of probiotics on allergic disease. Probiotics are potentially beneficial bacteria found in foods such as yogurt and in dietary supplements.

The children had been evaluated at birth, five more times before age 2, and then again at ages 4 and 7. The researchers in the original study had also tested for intestinal microbes in fecal samples collected at 6 months and 12 months.

For this latest study, the Finnish researchers selected 49 participants from the larger study -- 25 of them were overweight or obese at age 7 years, and 24 were normal weight at the same age.

When they looked at the fecal samples, the average bacterial counts of bifidobacteria when taken at 6 months and 12 months were twice as high in those who were a healthy weight as in those who got heavy.

Those who stayed at a healthy weight also had lower fecal S. aureus levels at 6 months and 12 months than did those who got heavy.

The S. aureus may trigger low-grade inflammation, the authors speculated, and that may also contribute to developing obesity.

In other research, gut bacteria in adults have been found to be altered in obese adults who lost weight. Someday, the Finnish researchers speculated, tinkering with gut flora may help prevent or treat obesity.

The latest study doesn't pinpoint exactly why intestinal bacteria are linked with the development of obesity, said Connie Diekman, director of university nutrition at Washington University in St. Louis and president of the American Dietetic Association.

"The exact role that bacteria in the intestine play in development of obesity is still the subject of much research," she said, "but the benefits of breast-feeding are clear. Breast-feeding provides not only the proper nutrition for your infant, but it provides benefits that may impact long-term health and weight issues as well."

However, she added that, "while breast-feeding may play a role in the weight of children, so many other factors influence weight that parents shouldn't ignore good role modeling of healthy food choices, proper portions and regular physical activity. Healthy weight is a combination of factors, and no single issue will be the cause of weight gain or the magic answer to weight loss."

Another expert who has studied how obesity changes microbes in the gut calls the new study unique, because it collected information over several years and could look for differences in gut microflora. "The finding, that the lean children harbored higher levels of bifidobacteria at younger ages, is very intriguing," says Ruth Ley, a research assistant professor at Washington University School of Medicine in St. Louis.

Still, she says, research on the role of gut bacteria in regulating body weight is in the very early stages.