6 February 2007

Skin Samples Rife With Unknown Bacteria

by Kate Melville

Scientists from the NYU School of Medicine say that the skin, the largest organ in our body, is a kind of zoo of bacterial inhabitants, with nearly ten percent being previously unknown species. Surprisingly, the NYU study, published in the Proceedings of the National Academy of Sciences, is the first to identify the composition of bacterial populations on the skin using a method that involved extracting 16S ribosomal DNA from the samples. "This is essentially the first molecular study of the skin," says NYU's Dr. Martin J. Blaser. The skin has been, he adds, effectively terra incognita, until now.

The three year study, part of an emerging effort to study human microbial ecology, found that some of the bacteria on the skin appear to be more or less permanent residents while others are transient. "Many of the bacteria of the human body are still unknown," says Dr. Blaser. "Ultimately what we want to do is compare disease and health. Keeping the bacterial populations in our body stable may be part of staying healthy."

For their bacterial audit, the researchers took swabs from the inner right and left forearms of six individuals picking the region halfway between the wrist and the elbow for its convenience. Because they also wanted to study change over time, they took swabs from four of the individuals 8 to10 months after the first test. Roughly half (54 percent) of the bacteria identified in the samples represented the genera Propionibacteria, Corynebacteria, Staphylococcus and Streptococcus, which have long been considered more or less permanent residents in human skin. The researchers found evidence for 182 species of bacteria in the skin samples and eight percent were unknown species that had never before been described.

The subjects differed markedly in the overall composition of the bacterial populations on their skin. They only had four species of bacteria in common: Propionibacterium acnes, Corynebacterium tuberculostearicum, Streptococcus mitis, and Finegoldia AB109769. "This is a surprise," said co-researcher Dr. Zhan Gao. "But many things affecting the skin affect bacteria, such as the weather, exposure to light, and cosmetics use." Almost three-quarters of the total number of bacterial species were unique to individual subjects, suggesting that the skin surface is highly diversified in terms of the bacteria it harbors. Three bacterial species were only found in the male subjects: Propionibacterium granulosum, Corynebacterium singulare, and Corynebacterium appendixes. While the sample is too small to draw conclusions, the scientists believe that women and men may harbor some different bacterial species on their skin. In each individual, the bacterial populations varied over time while revealing a core set of bacteria for each individual.

The team used a powerful molecular method that involved extracting a subunit of genetic material called 16S ribosomal DNA from the samples. "It is kind of a common currency, it's a conserved gene," explained Dr. Blaser. The researchers then bred colonies of each single species of bacteria present in the skin samples. Then they picked out the species-specific genetic regions in the bacteria. After sequencing those regions, the researchers consulted 16S rDNA databases to determine the bacterial species present in each sample. Many bacteria in the database only exist as sequences and have nether been named or extensively studied. Those are termed SLOTUs, or species-level taxonomic units. All up, the team found a total of 182 species - or SLOTUs - and 91 genera of bacteria in the skin samples.

Next up, the research team wants to look at diseased skin. "We plan to ask the question: Are the microbes in diseased skin, in certain diseases like psoriasis or eczema, different than the microbes in normal skin?" said Dr. Blaser.

Source: New York University Medical Center and School of Medicine