What you don't see with the naked eye is a world of microscopic organisms jostling each other for territory - and depending on your point of view, the realization can be infinitely fascinating or horribly icky.
It turns out that skin, the largest organ in the human body, is swarming with scores of different species of bacteria - 182 by the count of researchers in a study published Monday-and eight per cent have never been identified before.
"The skin is home to a virtual zoo of bacteria," said co-author Dr. Martin Blaser, head of the department of medicine and a professor of microbiology at the New York University School of Medicine.
Blaser hypothesized that normal flora (bacteria, etc.) may play a role in human metabolism, a function that's been refined over millions of years of evolution.
"And in the 20th century, they have started to change as a result of things that we're doing," Blaser said in an interview, pointing to widespread antibiotic use, access to clean water, having smaller families and eating commercially prepared foods.
"Ultimately what we want to do is compare disease and health," said Blaser, suggesting that keeping bacterial populations stable may be part of staying healthy.
"And it may be that what we're doing with soaps and antibiotics and other things is that we're changing that balance."
To conduct the study, published in the online edition of the Proceedings of the National Academy of Sciences, the researchers took swabs from both inner forearms of six healthy subjects, three men and three women. The area was chosen because it is easily accessible and not exposed to repeated washing as hands are.
"We didn't tell them to be particularly clean, we just made sure they didn't take antibiotics up to one month prior to the test," Dr. Zhan Gao, a senior research scientist in Blaser's lab, explained. She chose three men and three women to have a balance of sexes and set up a clean room so samples wouldn't be contaminated.
The scientists then identified the different species of bacteria by extracting genetic material and analyzing sequences of DNA.
"There are organisms that we know and there are organisms that have not been identified before," Blaser explained. "And we also showed that in an individual person, I think they had between 30 and 60 species (each) in total."
The six subjects differed markedly in the overall composition of bacterial populations on their skin, and only four bacteria were common to all: Propionibacterium acnes, Corynebacterium tuberculostearicum, Streptococcus mitis, and Finegoldia AB109769 (for those who want to know).
"This is a surprise," said Gao. "But many things affecting the skin affect bacteria, such as the weather, exposure to light and cosmetics use."
Roughly half of the bacteria represented the genera Staphylococcus, Streptococcus Propionibacteria and Corynebacteria, which have long been considered more or less permanent residents on human skin.
Yet three species were found only on the men, suggesting males and females may harbour different bacteria.
"Our data suggest that there's a large population of transient organisms that are coming and going that are present in smaller numbers," said Blaser.
So why are the researchers looking beyond the merely skin-deep?
For one thing, they want to figure out what bacteria populations are present on normal, healthy skin, so they can compare them to those colonizing skin with such inflammatory diseases as eczema and psoriasis.
"I think that this is a kind of exploration," said Blaser. "When people went to Africa or Antarctica or wherever and they put features on the map, you'd say, 'Well what are the implications?' It's discovery. We've discovered what's there. And now people can begin to apply this knowledge for particular problems."
Blaser, whose research team has previously analyzed fungi on skin and bacteria in the stomach and esophagus, said bacteria have been living in animals for about a billion years and in humans since we first emerged on the planet.
"And most of the time these bacteria are good for us. They've co-evolved with us. They are part of our physiology. But whenever you carry anything there's a cost. Sometimes those good bacteria can kill us," he said, noting that flora that keep the intestines healthy can prove fatal if leaked into surrounding tissue through injury.
"From our study, it's our guess that the organisms we identified are mostly friendly."
