Armpits Are "Rain Forests" for Bacteria, Skin Map Shows

Like a dune-rippled desert, the skin of a human foot (shown in an undated picture) is just one of the body's "habitats." Each skin region boasts a unique and surprisingly diverse array of microbes, according to a "topographic" skin map released in May 2009.


Long considered a source of odor and embarrassment, the humble armpit may be coming up in the world.

On the microbial level, a person's underarms are akin to lush rain forests brimming with diversity—and that's a good thing—according to a new "topographic map" of human skin.

Most of our skin is like an arid desert, said study co-author Julia Segre, of the National Human Genome Research Institute in Bethesda, Maryland.

"But as you walk through this desert you encounter an oasis, which is the inside of your nose," she said. "You encounter a stream, which is a moist crease. [These] areas are like habitats rich in diversity."

And like the "friendly" bugs in the human digestive system, these native bacteria of the epidermis promote skin health and could even help scientists find new ways to treat skin diseases.

Well Adapted

The study, published this week in the journal Science, has revealed that human skin hosts a much more diverse set of bacteria than previously thought.

Samples of skin bacteria grown in the lab had suggested that a single genus, Staphylococcus, was dominant on human skin.

But by looking at the microbes' genes, Segre's team found at least 18 different phyla of bacteria dwelling in 20 different skin habitats. (In biology, a phylum is a group of animals that are similar enough that they likely share a common origin.)

What's more, these microbes are adapted to their habitats rather than to individual humans, Segre said.

"The bacteria in my underarm are more similar to those in your underarm than they are to those on my forearm," she said.

The map presents a new way of looking at various skin conditions, the study authors note.

For example, researchers could compare the map of the body's "normal" bacteria with one accompanying a wound or a disease such as eczema. This could reveal how these ailments—and our treatments of them—act on good and bad skin bacteria.

No Cause for Alarm

Germophobes needn't freak out. Serge stresses that many of the microbes are "healthy bacteria" that keep our largest organ in good condition.

For example, germs that live in naturally oily regions, such as the outside of the nose, feed on the skin's lipids and produce natural moisturizers to prevent skin from becoming chapped.

"People are eating probiotic yogurts to promote [beneficial] bacteria growth in the gut, but we want to sterilize the skin," Serge noted.

"We should think about proper sanitation with the skin, but not sterilization. There are good bacteria that really promote healthy skin."