Researchers have pointed out that the skin, the largest organ in our body, is a kind of zoo for almost 182 species of bacteria.
The study revealed that eight percent were unknown species that had never before been described, in the first study to identify the composition of bacterial populations on the skin using a powerful molecular method.
"Not only were the bacteria more diverse than previously estimated, but some of them had not been found before. The skin is home to a virtual zoo of bacteria," said Martin J Blaser, MD, Frederick King Professor and Chair of the Department of Medicine and Professor of Microbiology at NYU School of Medicine, one of the authors of the study.
The researchers analysed the bacteria on the forearms of six healthy subjects; three men and three women. "This is essentially the first molecular study of the skin," said Dr Blaser.
The skin has been, terra incognita, an unknown world that he and his colleagues have set out to understand much like explorers, according to Blaser
"There are probably fewer than ten labs in the US looking at this question. It is very intensive work," he said.
Zhan Gao, MD, senior research scientist in Dr Blaser's lab, led the research, which took more than three years to complete. Some of the bacteria on the skin appear to be more or less permanent residents; others are transient, according to the study.
This research is part of an emerging effort to study human microbial ecology. Dr Blaser's laboratory has previously examined the bacterial population in the stomach and the esophagus.
"Many of the bacteria of the human body are still unknown. We all live with bacteria all our lives and occasionally we smile, so they are not that bad for us," said Dr Gao.
Individuals harbour quite different bacteria on their skin. The most numerous cells in our body are microbial -- they outnumber our cells 10 to 1. The body has microbes native to the body, including the skin, and these populations change according to how we live, according to Zhan Gao.
"Ultimately what we want to do is compare disease and health," said Dr Blaser.
Keeping bacterial populations in our body stable may be part of staying healthy, he says.
In the new study, 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.
"It is not where they wash their hands...And they don't have to undress," he said.
The researchers wanted to be able to compare two similar parts of the body. 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, or 54.4 per cent, 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 six individuals 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. But many things affecting the skin affect bacteria, such as the weather, exposure to light, and cosmetics use," said Dr Gao.
Almost three-quarters, or 71.4 per cent, 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 harbours, according to the study.
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 predominant bacteria don't change much. But the more transient bacteria did change over time," said Dr Gao.
"What that suggests. is that there is a scaffold of bacteria present in everybody's skin. Some stay and others come and go," Dr Blaser added..
The team found a total of 182 species or SLOTUs and 91 genera of bacteria in the skin samples. When compared with earlier studies, the researchers found that these three phyla are also dominant in the esophagus and the stomach.
In terms of bacterial species, however, the insides of the body, for example the stomach, and the exterior of the body, the skin, show vast differences in bacterial populations. Skin condition can change markedly due to a variety of factors such as climate, diet, personal hygiene, and disease. But skin is never devoid of bacteria, particularly its more permanent residents. In healthy individuals these bacteria are not pathogens.
"Without good bacteria, the body could not survive," said Dr Gao.
The next step for the research team is 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," asks Dr Blaser.
This study is published February 5, 2007, in the online edition of the Proceedings of the National Academy of Sciences.
