Your skin is crawling with hundreds of kinds of bacteria, NIH researchers find.

There are up to 100 times more kinds of bacteria thriving in "vibrant communities" in healthy skin than previously known, report NIH researcher Elizabeth A. Grice, PhD, and colleagues at the National Human Genome Research Institute.

Those bugs aren't diseases -- they are part of us, says study leader Julia A. Segre, PhD.

"We have to recognize that we are an aggregation, a mixture, of both human cells and microorganisms -- bacteria that live on our bodies and coexist with us," Segre said in a podcast from the journal Science, which published the study in its May 29 issue.

Human DNA is only a small fraction of the genetic material in our bodies. The Segre team's work is part of a huge NIH-funded effort -- the Human Microbiome Project -- to find out exactly what these non-human parts of us are and how they contribute to health and disease.

Segre and colleagues collected bacteria from 20 sites on the bodies of 10 healthy volunteers. These sites ranged from the webs of the toes to the navel to the fold between the eyes.

"The skin is like a desert. It is mostly dry, but there are areas of your skin that are streams: These moist creases behind your ear and under your neck, for example, and in these moist areas there is a great density of bacteria living there," Segre said. "There are also these oases, like in your nose or in your belly button, that have a huge diversity of bacteria. This may begin to explain why, while our skin is primarily dry, there is a huge diversity of bacteria that can grow on our skin."

But like geographical deserts, the dry areas of our skin -- such as the inside of the forearm, the palm, and the buttock -- also teem with life. As deserts have fewer species than rainforests, so the dry areas of the skin have less diverse flora and fauna than moist skin.

Segre's team found that different body sites have different mixtures of bacteria, and that different people tend to have the same kinds of bacteria in the same body sites.

"We found the bacteria in my underarm are more similar to the bacteria in your underarm, than my underarm bacteria are to my forearm bacteria," Segre said.

That offers a clue to disease, because different skin diseases tend to appear in specific places on the body.

"We are using these findings to start to explore how the microbiome contributes to disease such as eczema or MRSA [methicillin-resistant staph] infection. We know there is a contribution, but we think there may be even a greater bacterial contribution than we had previously appreciated,"

That contribution is not one-sided. We tend to think of bacteria as germs that cause disease -- but the new findings suggest that a healthy crop of normal bacteria prevents disease.

"For example, 1.5% of Americans have MRSA in their nose -- but they don't show any signs of infection," Segre said. "Maybe it is that the other bacteria are keeping the MRSA in check and not letting it grow and create an infection. Or maybe it is because the MRSA is changing between when it's up in someone's nose and when it causes an infection."

In future studies, the researchers will compare the microorganisms living on healthy people to those living on people with diseases.

"We want to see if there is a shift between what we find in normal individuals and what we find in someone with a skin disease," Segre said.

"Our results underscore that skin is home to vibrant communities of microbial life, which may significantly influence our health," Grice says in a news release.