Where can you find your skin's most diverse community of bacteria? Not in a sweaty armpit or linty belly button. According to a new survey of the bacterial ecosystem that covers us, the diversity hot spot of the body's exterior is the forearm. And the surprises don't end there.
Microbes that live in and on our bodies outnumber our own cells 10 to one, but researchers have only recently begun to catalog the residents on our skin. Traditionally, scientists identified human skin bacteria by swabbing volunteers and culturing the samples, but those results skewed toward microbes that grow well in the lab. Thanks to ever-evolving gene-sequencing technology, scientists can now use microbial RNA to identify organisms. With these techniques, researchers have found an unexpectedly wide variety of bacteria on human skin (Science, 23 May 2008, p. 1001). But no one had ever systematically compared bacterial colonies from different areas on the human body.
To do so, scientists from the National Human Genome Research Institute in Bethesda, Maryland, recruited 10 volunteers and asked them to wash with mild soap for 1 week. Then, after 24 hours without bathing, the volunteers arrived at the lab, where researchers swabbed and scraped their skin in 20 places--everywhere from the nostril to the navel to that bane of low-rise jeans aficionados, the gluteal crease. The team analyzed ribosomal RNA from the samples and classified the microbes based on their genomes.
The researchers found about 1000 species total, which were fairly consistent from person to person; it turns out we all have similar tenants in our noses and on our backs. The number suggests that our skin is as variegated as our guts, which house anywhere from 500 to 1000 bacterial species. The team also found vast differences across the skin, according to the study published in tomorrow's issue of Science. Contrary to what acne-prone teenagers might expect, oily areas such as the forehead and scalp are actually less diverse than dry areas such as the forearm (though one is enough for grief: Propionibacterium acnes thrives in oily spots). The most barren region was behind the ear, with a median diversity of 15 species. In comparison, the forearm teemed with a median 44 species. A follow-up with five of the volunteers months later found that bacterial makeup changed little over time.
Why some neighborhoods are more varied than others is unknown. It could be because of skin properties such as hair or oil, exposure to bacteria, or some combination. As for the forearm, geneticist and co-author Julia Segre speculates that exposed arms make a good landing pad for bacteria. Contrasted with how we clean our hands, we rarely lather up our forearms. Whatever the reason, the research shows that location matters. "This paper really highlights that the skin is an ecosystem and that the bacteria that live on our skin are not homogenous," says Segre.
The research "could contribute to explaining why certain skin diseases appear at certain sites of the body and not others," says dermatologist Richard Gallo of the University of California, San Diego. "It's a straightforward description of something that needed to be described." The next step, Segre says, is to investigate the relationship between microbial ecosystems and diseases such as eczema and psoriasis.