Here's a finding that'll make your skin crawl: A healthy human epidermis is colonized by roughly 1,000 species of bacteria.
Furthermore, the microorganisms have evolved to exploit the unique attributes of those body parts they call home, according to a study to be published today in the journal Science.
Some thrive in the desert of the forearm. Others are happiest in the tropical rain forest of the armpit.
The study, conducted by a team of researchers from the National Institutes of Health, reflects a growing realization that bacteria have colonized us inside and out -- and that their presence is not only harmless but also probably essential to the proper functioning of the body.
One striking example of that fact: Mice bred to be entirely germ-free have smaller hearts and are unable to digest food properly.
"We live in a microbial world, and these things are not all out to get us," said Noah Fierer, a microbial ecologist at the University of Colorado at Boulder, who has analyzed bacteria that live on hands.
"You don't want to live in a sterile world," said Fierer, who wasn't involved with the new report. "You probably can't live in a sterile world."
The results reported today will lay some groundwork for the Human Microbiome Project, a $115-million NIH venture aimed at cataloging the bacteria and other organisms that inhabit the skin, gut, nose, mouth and vagina.
Among the more than 19 square feet of skin on a typical adult, the NIH team focused on 20 specific areas, ranging from the oily patch between the eyebrows to the moist spaces between the toes.
Senior author Julia Segre and her colleagues used Q-Tip-style swabs to gather bacterial samples from 10 racially diverse volunteers, half men and half women. They collected 112,283 organisms altogether.
The specimens were classified according to a gene known as 16S rRNA, which is easy to identify and gives each bacterial species a unique signature. More than half belonged to one of three big groups that made them a cousin either of the bacterium that causes acne; one that causes diphtheria; or Staphylococcus aureus, the culprit behind many dangerous antibiotic-resistant infections.
Moist areas -- such as the belly button and the inner bend of the elbow -- have up to 10 times as many bacteria per square inch compared with dry areas, like the inside of the mid-forearm, the scientists found.
But the forearm turned out to have the greatest diversity of bacterial species, with a median of 44 among the 10 human volunteers.
The least diverse site sampled was the oily area behind the ear, with a median of 15, according to the study.
The study deliberately focused on regions associated with diseases such as eczema and psoriasis in the hope that the discoveries will help scientists understand those disorders better.
"We don't really know what causes skin diseases," said Segre, a senior investigator at the National Human Genome Research Institute. Perhaps, she suggested, an outbreak ensues when a dominant species of bacteria gets kicked out by a rival species.
Also a mystery is what all these microbes use for food.
"They obviously have to be eating something," Fierer said. "Probably some of them are eating dead skin cells or oils that come from your skin. Who knows?"
Roughly 100 billion individual bacteria live on skin, and when you add all their genes together they dwarf the 20,000 contained in the human genome, researchers said.
The microbes are probably doing something useful, said Dr. Martin Blaser, a microbiologist and infectious disease specialist at New York University Langone Medical Center, who in his studies has identified 183 kinds of bacteria on human arms.
After all, the locations of bacterial species is relatively consistent from person to person, perhaps implying some function that confers a benefit to the host.
"I can't prove it, but I think the idea that they are just hanging out is completely incorrect," Blaser said.
Segre agreed that bacteria have been getting a bad rap.
"We have to lose this language of warfare," she said. "Our goal is to keep the bacterial ecosystem in balance and move away from the concept that all bacteria are bad."