Close Look at Human Arm Finds Host of Microbes

Previous studies of microbes on human skin have been limited to examining those that can be grown in laboratory dishes. But scientists have long suspected that only provided insights into a small fraction of the creatures present, because many organisms cannot be easily grown in the lab.

Blaser's team swabbed an area of skin about the size of silver dollar on the right and left forearms of three healthy men and three healthy women. They then used sophisticated molecular techniques to amplify and analyze fragments of bacterial DNA captured by the swabs.

The analysis revealed 182 species, the researchers reported. Of those, 30 had never been seen. They identified an additional 65 species when they sampled four of the volunteers eight to 10 months later, including 14 new species.

"We found a lot of diversity -- both in terms of distant relatives but also cousins. And not just first cousins, but second, third and fourth cousins," Blaser said.

On average, each person's skin harbored about 50 species, but only four of them were found on all six people, suggesting that the mix of bacteria varies significantly from person to person. But those four species accounted for more than half of all the DNA sequences found, indicating that a relatively few species tend to dominate. And when the researchers analyzed the bacteria using a broader classification, phylum, they found three phyla on all six subjects that accounted for 95 percent of all present species.

"It appears that there is a conserved infrastructure or scaffolding of organisms that's common in human skin, and then a lot of transient or uncommon organisms that are person-specific," Blaser said.

"It's like New York City. About 8 million people live here, but there are about 40 million tourists in a year. So at any one time the residents outnumber the tourists, but in aggregate the tourists outnumber the residents," he said.

In fact, when the researchers sampled four of the six volunteers a second time, they found many of the species detected earlier were gone.

"This indicates that there's a lot of tourism going on," Blaser said. "There's a lot of transiency."

Blaser's team did not examine what any of the organisms were doing.

"This is how you start this kind of exploration -- first you ask the question: What's there?" Blaser said. "It's like when people first went to Africa or Antarctica to start to fill in the map."

Scientists assume that most of the organisms have a symbiotic relationship with their human hosts and play some type of beneficial role. But the next step will be to try to characterize their functions.

"We're interested in understanding how we interact with these organisms and how they are communicating with human cells and vice versa," Blaser said.

Some of the organisms may also play a role in diseases such as eczema and psoriasis.

"These are chronic inflammatory diseases of the skin of unknown cause. If these microorganisms have something to do with skin disease, knowing what's there may help us diagnose or perhaps treat these diseases," he said.

Blaser noted that human skin probably has myriad distinct ecosystems, noting that a similar study looking at fungi produced similar results but also found tremendous diversity in different parts of the body.

"If we looked at the armpit or the scalp, who knows what we'd find?" Blaser said. "It's like we're at the zoo, but so far we've just looked in the primate house. We haven't even gotten to the cat house or the elephant house."