In the study, researchers introduced a parasite (Leishmania major) that causes skin infections to several groups of mice. One group was bred to have none of the normally present microbe colonies on their skin. A second group had typically present microbe communities, called "commensal bacteria," on their skin.
Mice with no skin microbes couldn't mount an effective defense against the parasite. In simple terms, they had far more bugs in their ears than mice with the normal skin microbial communities.
After the researchers introduced common bacteria found on the skin -- Staphylococcus epidermidis -- to the bacteria-free mice, their immune systems became much more effective in fending off the parasite.
The results suggest that skin bacteria, like bacteria found on other parts of the body, are important for a healthy immune system, the researchers said.
"The skin bacteria are really critical for controlling immune cells in the skin. They educate immune cells, tell them what to do," explained study author Shruti Naik, a doctoral candidate at University of Pennsylvania and research fellow at the U.S. National Institute of Allergy and Infectious Diseases. "The pathogen is stealthy, like a burglar. It doesn't want the immune system to detect it. The commensals stimulate the immune system, acting as an alarm saying, 'There is a bug here. You need to fight and ward off this bug.'"
The study is published in the July 26 issue of Science.
A growing body of research suggests that the microbes living in various places on human bodies -- nasal passages, mouth, gastrointestinal tract, urogenital tract and skin -- play an important, though not fully understood, role in human health. So far, microbes of the gut have been the best studied, Naik said.
"It's becoming more and more clear that bacteria that live with you are really important for human health. A lot of the focus of research is in understanding the importance of bacteria in the gut," Naik noted. "But there
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