The human body contains 10 times as many bacterial cells as it does human cells. Biologists have now taken a census of the bacteria that live on our skin, and it turns out that the diversity of life there is quite remarkable. The bacteria between our eyebrows are different from those on the elbow or in some other nook or cranny.
The bacteria are part of genuine ecosystems — akin to life on the savannah, the ocean or the rich life of a tropical rain forest.
"We think of the skin as a desert," says Julie Segre, who studies skin, the body's largest organ, at the National Human Genome Research Institute. "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."
Segre and her colleagues sampled the bacterial wildlife from 20 different spots on the bodies of 10 healthy volunteers. They purposely chose spots on the body associated with different dermatological conditions to better understand how the balance of bacteria affects our health. Segre and her team found that, like on our planet, there are astonishing differences from one place to the next.
"We think of the inside of the nose as the rain forest," she says, because it contains the greatest wealth of biological diversity.
The nose is also where bacteria stay that often recolonize other parts of the body, she says. Just like the tropics, which served as biological refuges during the ice ages.
Oily parts of the skin, like next to the nose or behind the ear, are host to a whole different ecosystem of microorganisms. And even dry spots turn out to harbor different mixes of organisms. Segre and her colleagues' work is reported in the current issue of Science magazine. Not only are these findings new, but Segre says they're also potentially useful.
A Balance Of Bacteria
"We had always wondered why it is that skin diseases strike in one place or another," Segre says. "Eczema is always inside the elbow. Psoriasis is on the outside of the elbow. Those human cells are the same."
Despite their proximity, the microbes that live inside the elbow and outside the elbow are quite different. Psoriasis and eczema don't appear to be caused by bacteria, but they could be a reaction, triggered by a change in the ecosystems of germs on our skin.
"So when we think about what promotes health and 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," Segre says.
New Technology Allows Better Analysis
Considering that the field of microbiology is more than 100 years old, one might think scientists would have discovered all this long ago. But the problem has been that 99 percent of skin bacteria don't grow in the laboratory, so scientists couldn't identify them. Now, they can fingerprint bacteria's DNA and identify specific species. And that's opened up a whole new world.
Martin Blaser at New York University sees enormous opportunity in this field.
"The first step is to do the kind of census that is done here," he says. "And then a further step is to begin to understand what are the interactions that microbes are having with our own cells — first in health, and ultimately in disease."
The microbes are not just interacting with us, but also with one other.
"If this is like other ecosystems, the microbes are both competing with one another and cooperating," Blaser says.
How Bacteria Ecosystems Affect Us
And skin is just the start. The National Institutes of Health is now embarking on a follow-up to the human genome project, called the Human Microbiome Project. Segre says we won't really understand human biology without a deeper appreciation for our fellow travelers.
"The human genome is really an amalgamation of the human cells and the bacterial cells," she says, "and it's time for us to turn attention to the other organisms that live together with our human cells."
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 will be a challenge to decipher them all, but that's the goal.