How Does Your Garden Grow—On Your Skin?
Posted: under Health, Medicine, and Healthcare.
One thousand species of bacteria live on your skin. But species diversity is greatest—probably not where you would guess—on your forearm. In contrast, diversity is the least behind your ear. In numbers of species, skin flora are comparable to flora in your gut, which also hosts 500-1000 species of these microscopic plants. Moreover, the bacterial cells your body hosts outnumber your own cells 10 to 1.
This information comes from researchers at NIH working on the Human Microbiome Project, an effort to discover the microbial communities living in different parts of the body in health and disease. Their article appeared this week in Science.
The investigators took skin scrapings from 10 volunteers, who had been instructed to wash with mild soap for a week and not at all for one day prior to the sampling. Scrapings were done at 20 skin sites, distributed among locations that were oily like the ear, moist like the armpit, and dry like the forearm. Bacterial species were identified by sequencing ribosomal RNA.
The largest number of species (44 on average) resides on the forearm, with only 19 species on average found behind the ear. The researchers also found that different individuals harbored the same species in the same sites on their bodies, and more variation occurred between different sites on the same individual than between same sites on different individuals.
Why does the forearm host the most? I suppose that we stick are arms into all sorts of places, but generally we only wash our hands up to the wrist. Think about that the next time you feel an urge to wipe your nose on your forearm. On the other hand the least diverse location is behind the ear, and when was the last time you poked your ear into some unusual place?
One bacterial species of great concern is methycillin-resistant Staphylococcus aureus. It turns out that this species is found most often inside the nose and in the crease of skin outside the nose. So wash your nose.
An aspect of the study worth noting is that the researchers sampled volunteers who had refrained from antibacterial washes for a week. I wonder what they would have found if they had sampled people who had washed with Dial or any of the antibacterial soaps that are commonly in use. It also makes me wonder how our modern cleanliness fetish (I plead guilty!) may be affecting our health. One of the theories regarding the increase of autoimmune diseases (asthma, arthritis, Crohn’s, and others) is that we may be living lives too clean.
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May 30 2009