MONDAY, June 21 (HealthDay News) -- Babies who are born vaginally pick up different bacteria than those who are delivered by cesarean section, potentially affecting how their immune systems develop, a new study suggests.

The findings could provide more insight into why babies born through cesarean sections appear to be more at risk of allergies and asthma, researchers say. The bacteria they're exposed to at birth may help explain the relationship, since coming into contact with germs seems to help babies build defenses against them.

"We want to understand what the differences are and how they are important for the baby's health," said study author Maria G. Dominguez-Bello, an associate professor in the department of biology at the University of Puerto Rico.

The research is preliminary, she said, but it could help determine whether babies will benefit by being exposed to germs at birth that they otherwise wouldn't encounter.

While germs may sound like a bad thing, they're often beneficial to the body.

"We are all colonized -- our skin, mouth, intestines, vagina, ears -- by bacteria that have evolved with man," Dominguez-Bello said. "We are just now starting to unveil what these bacteria are, what they do, why they are important for organs to function." Colonization means the organism is present but not causing infection

In the new study, researchers aimed to determine what types of germs colonize the bodies of babies as they're born. (The womb itself is free of germs, Dominguez-Bello noted.)

The researchers tested bacteria from the skin and mouths of 10 babies within a day after their birth. They also tested bacteria from their mothers.

Those who were born vaginally clearly picked up bacteria from their mothers since the germs matched. Those germs, which were linked to vaginal infections, gum disease and the digestion of milk, appear to have been acquired as they passed through the birth canal.

"It's very clear that the moms give the bacteria to the newborn babies. The babies are like magnets," Dominguez-Bello said.

The babies born by C-section harbored germs linked to skin infections, acne, diphtheria and food poisoning.

One theory is that it takes a little longer for babies born via cesarean section to come into contact with germs they need to survive.

"In order to be healthy adults, they'll be have to be colonized and end up having 10 times as many bacteria as their own cells," she said.

Dr. Athos Bousvaros, an associate professor at Harvard Medical School, said the study appears to be valid, although it only looked at a small number of babies over a short period of time.

"We don't know if this different colonization will persist over time," he said. Other factors -- antibiotics, genetics and breast-feeding -- may also play a role in how germs colonize babies, he pointed out.

"Pregnant moms should not be overly concerned about having a C-section based on this research," Bousvaros said. "The research does not associate this difference in colonization with the babies developing illness -- babies have been born by C-section for decades, and are generally quite healthy. As always, however, C-section should only be done if deemed medically necessary."

The study findings are published in the online edition of the Proceedings of the National Academy of Sciences for the week of June 21 to 25.

More information

The U.S. National Library of Medicine has more on C-section births.