They unraveled DNA in an attempt to explain what humans need to be healthy.
Staphylococcus epidermidis bacteria, left, lives on the skin, providing a first line of defense from harmful bacteria, while Enterococcus faecalis is among the good bacteria in the human gut.
They live on your skin, up your nose, in your gut -- enough bacteria, fungi and other microbes that collected together could weigh, amazingly, a few pounds.
Now scientists for the first time have mapped just which critters normally live in or on us and where, calculating that healthy people can share their bodies with more than 10,000 species of microbes. Many of these bacteria work to keep humans healthy, and results reported Wednesday from the government's Human Microbiome Project define what's normal in this mysterious netherworld.
It turns out that nearly everybody harbors low levels of some harmful types of bacteria, pathogens that are known for causing infections. But when a person is healthy -- like the 242 U.S. adult participants -- those bugs quietly coexist with benign or helpful microbes, perhaps kept in check by them.
'A whole new way'
The next step is to explore what doctors really want to know: Why do the bad bugs harm some people and not others? What changes a person's microbial zoo that puts them at risk for diseases ranging from infections to irritable bowel syndrome to psoriasis?
"This is a whole new way of looking at human biology and human disease, and it's awe-inspiring," said Dr. Phillip Tarr of Washington University at St. Louis, one of the lead researchers in the $173 million project, funded by the National Institutes of Health.
'It takes a village'
About 200 scientists from nearly 80 research institutions worked together for five years on this first-ever census to begin answering what microbes live in healthy people, where and what they do by unraveling the DNA of these microbes. The results were published Wednesday in 16 reports in the journals Nature and the Public Library of Science.
"It takes a village to finish science these days," said Harvard computational biologist Curtis Huttenhower, lead co-author of a study in Nature.
Not only do good bacteria help keep people healthy, but they also are thought to help explain why individuals react differently to various drugs and why some are susceptible to certain infectious diseases while others are impervious.
The microbiome starts to grow at birth, said Lita Proctor, program director for the Human Microbiome Project. As babies pass through the birth canal, they pick up bacteria from the mother. "Babies are microbe magnets," Proctor said.
Over the next two to three years, the babies' microbiomes mature and grow while their immune systems develop, learning not to attack the bacteria. Babies born by Caesarean section, Proctor added, start with different microbiomes, but it is not yet known whether they remain different.
In adults, the body carries 2 to 6 pounds of bacteria, even though these cells are minuscule -- one-tenth to one-hundredth the size of a human cell.
In another surprise, there isn't one core set of bacteria that perform certain functions. A wide variety can do the same jobs, they found. That's fortunate considering people carry a customized set of microbes, one that varies depending on where you live, your diet and a host of other factors. Your microbial zoos also can change, such as when taking antibiotics. Dr. Bruce Birren of the Broad Institute of MIT and Harvard, said it may be that our lifestyle and environment "induces each of us to have arrived at a solution that works for us."
The Washington Post and New York Times contributed to this report.