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Unexpected Microbe Diversity on Human Skin

A new study of the skin's microbiome—all of the DNA, or genomes, of all of the microbes that inhabit human skin—reveals that our skin is home to a much wider array of bacteria than previously thought. The accomplishment provides a new foundation for developing strategies to treat and prevent difficult skin diseases such as eczema, psoriasis, acne and antibiotic-resistant infections.

The skin is one of the body's first lines of defense against illness and injury. Its health depends upon the delicate balance between our own cells and the millions of bacteria and other one-celled microbes that live on its surface. To better understand this balance, NIH researchers drew on the power of modern DNA sequencing technology and computational analysis.

Scientists at NIH's National Human Genome Research Institute (NHGRI), National Cancer Institute (NCI) and the NIH Clinical Center took skin samples from 20 sites on the bodies of 10 healthy volunteers. They extracted DNA from each sample and sequenced a type of gene that is specific to bacteria, the 16S ribosomal RNA gene.

The researchers reported in the journal Science on May 28, 2009, that they identified, classified and compared more than 112,000 bacterial gene sequences. Previous methods, which involved growing microbial samples from human skin in the laboratory, had uncovered a far less diverse collection.

The greatest influence on bacterial populations appears to be body location. For example, the bacteria that live under your arms likely are more similar to those under another person's arm than they are to the bacteria on your own forearm. There was also considerable variation in the number of bacteria species at different sites, with the most diversity on the forearm (44 species on average) and the least diversity behind the ear (19 species on average).

In general, dry and moist skin had a broader variety of microbes than oily skin. Dry areas include the inside surface of the mid-forearm, the palm of the hand and the buttock. Moist areas include inside the nose, armpit and side of the groin. Oily sites include beside the nose, inside the ear and the upper chest and back.

To look for changes over time, the researchers sampled some volunteers twice, with the samples taken about 4 to 6 months apart. Most of the resampled volunteers were more like themselves over time than they were like other volunteers. However, the stability of the microbial community was dependent on the site surveyed. The greatest stability was found in samples from inside the ear and nose. The least stability was found in samples from behind the knee.

"Our results underscore that skin is home to vibrant communities of microbial life, which may significantly influence our health," says Dr. Elizabeth Grice, a postdoctoral fellow at NHGRI and the study's first author.

This study is among early research laying the groundwork for NIH's Human Microbiome Project, which aims to characterize human microbial communities and their role in health and disease. Other efforts are sampling the microbiomes of the nose, digestive tract, mouth and vagina.