Our microbes, ourselves: how bacterial communities in the body influence human health.

In the womb, a fetus enjoys the protection of a sterile environment. Only when the mother's amniotic sac ruptures before delivery does her baby face microbes for the first time. As he's squeezed through the birth canal, he picks up millions of bacteria from his mother. Most of the microbes are friendly and quickly take up residence on the baby's skin and in his gastrointestinal tract.

The bacteria not only persist but also form complex communities throughout the newborn's body that will aid in his general well-being throughout life. The body's microbes play a critical role in digesting food, metabolizing drugs, and maintaining overall health.

In fact, in every person's body, there are 10 times as many microbial cells as there are human cells. "The microbial part of ourselves is highly evolved," says Jeffrey Gordon, a microbiologist at Washington University in St. Louis. "These organisms have learned to adapt to life with us."

It's no wonder then that this vast microbiota has captured the attention of researchers working to understand not just health, but also diseases, particularly those lacking clear diagnoses or effective treatments. With new laboratory techniques, these researchers have begun to survey the microbial communities in the body. Several groups already report that disruptions in these communities are related to conditions including obesity, inflammatory bowel disease, vaginal infections, and gum disease.

Scientists have long recognized that the body's microbiota matters. In the 19th century, Louis Pasteur declared that normal microbes are important in human health and that their disruption can lead to disease. Until recently, however, scientists studying human-microbial populations had been hampered because the majority of such microbes can't be cultured in the lab. Now, researchers can extract DNA from a sample and rapidly identify thousands of bacterial species at once without having to grow each bug in a dish.

New studies are also showing that microbes within a community work together to influence health, a finding that may have a large impact on conventional views of disease. Instead of an illness being caused by the presence or absence of a single pathogen, "the real pathogenic agent is the collective," says David…