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Scientists unveiled the results of an unusual census today. It's the first catalog of the bacteria, viruses and other microorganisms that populate every nook and cranny of the human body.

As NPR's Rob Stein reports, researchers hope the advance marks an important step towards understanding how microbes help make humans human.

ROB STEIN, BYLINE: The human body contains about 100 trillion cells, but only maybe one in 10 of those cells is actually human. The rest are from bacteria, viruses and other microorganisms.

LITA PROCTOR: The human we see in the mirror is made up of more microbes than human.

STEIN: That's Lita Proctor of the National Institutes of Health. She's in charge of a massive project. Hundreds of scientists are exploring this invisible world, what scientists call the human microbiome.

PROCTOR: The definition of a human microbiome is all the microbial microbes that live in and on our bodies, but also all the genes, all the metabolic capabilities they bring to supporting human health.

STEIN: These microbes aren't just along for the ride. They're there for a reason. We have a symbiotic relationship with them. We give them a place to live and they help keep us alive.

PROCTOR: They belong in and on our bodies. They help digest our food and they provide many kinds of protective mechanisms.

STEIN: They teach our immune systems how to recognize dangerous invaders. Martin Blaser at New York University says they even produce helpful compounds that reduce inflammation and fight off other bugs that could make us sick.

MARTIN BLASER: These microbes are part of our evolution. As far as we can tell, they are very important in human health and probably very important in human disease, as well.

STEIN: These bugs generally don't make us sick, but when we disrupt the delicate ecosystems that they carefully construct in different parts of our bodies, scientists think that can make us sick. Again, Lita Proctor.

PROCTOR: There can be a disturbance in the immune system. There can be some kind of imbalance and then you get a microorganism which, under normal circumstances, lives in a benign way and can become a disease-bearing organism.

STEIN: Things like taking too many antibiotics or obsession with cleanliness, even maybe delivering babies by C-sections. So the idea behind the Microbiome Project was to get the first map of what a normal, healthy microbiome looks like.

More than 200 scientists spent five years analyzing samples from more than 200 healthy adults. The samples came from 18 different places on their bodies, their mouths, noses, guts, behind each ear, inside each elbow.

PROCTOR: This is the only study to date anywhere in the world where people's microbiomes across the human body were sampled and analyzed. Here was an effort to really investigate the full landscape, if you will, of the human microbiome across the body.

STEIN: The first wave of results are being published in 16 papers and four scientific journals this week. Among the results, scientists identified some 10,000 species of microbes, including many never seen before.

George Weinstock is working on the project at Washington University in St. Louis.

GEORGE WEINSTOCK: This is like going into unchartered territory, going into a forest and finding a new species of butterfly or a new type of mammal or something like that, a new kind of bird.

STEIN: Those 10,000 species have more than eight million genes, which is more than 300 times the number of human genes and scientists found some very interesting things when they compared microbiomes.

PROCTOR: People were very different from each other, but skin was more like skin and gut was more like gut, so the composition of microbes and the kinds of genes they have are very much habitat-specific.

STEIN: Now that scientists have an idea of what a healthy microbiome looks like, George Weinstock says they can start to explore this super organism, this complex mishmash of human and microbial cells.

WEINSTOCK: How do they talk to our human cells and how do our human cells talk back to them? Because it's really a concert that they're playing together and that's what makes us who we are.

STEIN: Scientists have already discovered some intriguing clues. For example, the microbes in a pregnant woman's birth canal start to change just before she gives birth. Scientists think that's so their babies are born with just the right microbiomes they'll need to live long, healthy lives.

Rob Stein, NPR News.

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