Bacterial 'Zoo' Thrives on Human Skin

Feb. 5, 2007 — No human walks alone, according to a recent finding that skin harbors at least 182 species of bacteria, many of which were previously unknown.

Researchers liken human skin — the body's largest organ — to one of the world's last unexplored domains. The study, published in the Proceedings of the National Academy of Sciences, represents the first molecular exploration of this largely uncharted realm, according to the authors.

"It is becoming increasingly clear that the human body is the home to thousands of species of microbes, many times for years, decades, or for our entire lifespan," coauthor Martin Blaser told Discovery News.

Blaser, chair of the Department of Medicine at the New York University School of Medicine, and colleagues swabbed the forearms of six healthy men and women.

Although different parts of the body, including moist areas, may support even more bacteria, the scientists chose the forearm because it is not washed as frequently as the hands, is easily accessible and is not, as Blaser put it, "too personal."

The researchers extracted DNA from the samples and matched it to databases of information about the DNA of different strains of bacteria.

Just as each person is unique, so too is his or her bacteria, the researchers found. More than 71 percent of all species identified were unique to just one of the six test subjects. Three species were only found on the men's skin, suggesting that some bacteria exclusively live on men or women.

The majority of species, however, fell into four groups: Propionibacteria, Corynebacteria, Staphylococcus and Streptococcus. While related bacteria are not always associated with disease, some in these groups can cause ailments from acne to pneumonia.

The good news is that skin bacteria appear to be mostly on our side.

"They almost certainly help us defend against nastier bacteria, such as pathogens, that might invade," explained Blaser, who added they also might help to keep the skin "well oiled and toned."

Some bacteria stay put for a long time while others are more transient, coming and going.

Blaser thinks most species are acquired from other people — usually one's mother — but also from other family members, friends, partners and even pets.

"These types of surveys, whether conducted in the gut, mouth, or skin, are revealing a heretofore unappreciated level of biodiversity associated with our bodies," said Jeffrey Gordon, director of the Center for Genome Sciences at Washington University's School of Medicine.

He added, "An inspiring or sobering thought: As adults we have 10 times more microbial cells than human cells, and perhaps 100 times more genes embedded in the genomes of our microbial partners than in our own human genome."

Blaser and his team next plan to study people who suffer from skin diseases, such as psoriasis or eczema, to see if they possess different microbes than those found on people with healthy skin.