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By Bruce Agnew
Special to NHGRI
The billions of microbes that reside on human skin - probably more than 10 billion microbial cells on each of us – may be fellow travelers but they aren’t free riders. Instead, the skin microbiota plays an intricate role in the human immune system, actually directing many immune functions and helping to defend its host against invading bacterial pathogens.
In a review article in the Nov. 21, 2014, issue of Science, Julie Segre, Ph.D., head of the Microbial Genomics Section of the National Human Genome Research Institute (NHGRI), and Yasmine Belkaid, Ph.D., head of the Mucosal Immunology Section of the National Institute of Allergy and Infectious Diseases (NIAID), sum up what researchers have learned so far about the advantages, and some of the downsides, of this intimate partnership.
The underlying advantage, of course, is that the microorganisms that live on human skin-called commensals – have evolved to defend their dwelling place – us.
For example, the skin bacterium Staphylococcus epidermidis produces a secretion that reduces inflammation and speeds wound healing by binding to an immune-system receptor. S. epidermidis also inhibits tissue colonization by the highly pathogenic S. aureus, the source of many hospital-acquired infections that in its methicillin-resistant form is known as the “superbug” MRSA.
Other skin microorganisms perform a wide variety of functions that help out their host. Among these functions are overall immune system screening, tissue repair, wound healing, inflammation control, production of defensive anti-microbial peptides (AMPs), acceleration of the complement arm of the immune system, regulation of interleukin-1 (IL-1), and modulation of T cells, key actors in directing immune system response. In fact, the richest concentrations of immune cells in the body are at sites populated by commensal cells, and the skin is “one of the largest reservoirs of memory T cells in the body,” Drs. Belkaid and Segre report.
Most people don’t realize the skin microbiota’s contribution to their well-being. “Humans want to eat yogurt because they have the concept that the microbes in their gut are providing a benefit,” said Dr. Segre. “Whereas on their hands, all people want to do is use hand sanitizer and sterilize themselves. They really do not perceive that there is a benefit to the microbes that reside on their skin.”
A potential downside, of course, is that any disturbance of the fine balance of the skin microbes-called dysbiosis-can be costly. If the microbiota is sick, the human host will probably get sick, too.
More: Genome Research
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