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Biologists have taken a census of a very large population: the bacteria that live on our skin. Consider this: A human body contains 10 times as many bacterial cells as it does human cells. And as NPR's Richard Harris reports, it turns out the diversity of life on our skin is remarkable.

RICHARD HARRIS: When you think of an ecosystem, you probably think of the savannah, or the ocean or the rich of life of a tropical rain forest. Well, Julia Segre, at the National Human Genome Research Institute, has been exploring an ecosystem much closer to home: our skin.

Dr. JULIA SEGRE (National Human Genome Research Institute): We think of the skin as a desert, but within the desert of our dry skin, there are these streams - and those are the creases of our bodies. And then there are the oases, so places that are very moist and rich. That would be something like the underarm or the bellybutton.

HARRIS: Segre and her colleagues sampled the bacterial wildlife from 20 different spots on the bodies of 10 volunteers. They found, like on our planet, there are astonishing differences from one place to the next.

Dr. SEGRE: Of the human skin, we think of the inside of the nose as the rain forest.

HARRIS: There, the researchers discovered a huge wealth of biological diversity. Oily parts of the skin, like next to the nose or behind the ear, are a host to a whole different ecosystem of microorganisms. And even dry spots turn out to harbor different mixes of organisms.

Segre and her colleagues report the results in the current issue of Science magazine. She says this is fascinating stuff, but she says it's also potentially useful.

Dr. SEGRE: We had always wondered why it is that skin diseases strike in one place or another. Eczema is always inside the elbow. Psoriasis is on the outside of the elbow. Those human cells are the same.

HARRIS: It turns out the skin's the same, but the microbes that live inside the elbow and outside the elbow are quite different. Now, psoriasis and eczema are apparently not caused by bacteria, but they could be a reaction triggered by a change in the ecosystems of the germs on our skin.

Dr. SEGRE: So when we think about what promotes health and also what causes disease, we have to consider that it could be the bacteria and the fungi and the other microorganisms that live together with us, that they could be out of balance.

HARRIS: Now, considering that the field of microbiology is more than 100 years old, you'd think scientists would've discovered all this long ago. The problem is, 99 percent of skin germs don't grow in the laboratory, so scientists couldn't identify them. Now they can by fingerprinting their DNA, and that's opened up a whole new world. Martin Blaser at New York University sees enormous opportunity in this field.

Dr. MARTIN BLASER (New York University): The first step is to do the kind of census that is done here. And then a further step is to begin to understand what are the interactions that our microbes are having with our own cells, first in health and then ultimately in disease.

HARRIS: And the microbes are not just interacting with us, but also with one other.

Dr. BLASER: If this is like other ecosystems, the microbes are both competing with one another and cooperating.

HARRIS: And skin is just the start. The National Institutes of Health is now embarking on a follow-on to the Human Genome Project called the Human Microbiome Project. Julie Segre says we won't really understand human biology without a deeper appreciation for our fellow travelers.

Dr. SEGRE: The human genome is really an amalgamation of the human cells and the bacterial cells. And it's time for us to turn attention to the other organisms that live together with our human cells.

HARRIS: In fact, given the enormous variety of bacteria that Segre has cataloged with this latest census, it's likely that the bacteria in total have far more genes than we do. It'll be a challenge to decipher them all, but that's the goal.

Richard Harris, NPR News.

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