Human skin encounters countless germs every day, and researchers expected to find that the colonies of bacteria, viruses and fungi in skin fluctuated over time. Instead, they found the germs stay fairly constant.
However, skin hosts micro-environments, which can either attract or repel germs. "We describe the difference between the sweaty armpit and the smooth forearm as being like a rain forest and a desert," said study co-author Julie Segre.
An analysis of skin samples finds feet, in particular, seem to change the most over time on the germ front, said Segre, a senior investigator with the U.S. National Human Genome Research Institute.
The findings aren't likely to affect the ongoing debate about whether we're keeping ourselves too clean. These germs can live deep in the skin, allowing them to remain unaffected by hand-washing, said Segre.
Still, the research does offer insight into the "baseline" of your skin, she said, allowing researchers to better understand how things go out of whack. Bacteria, viruses and fungi cause conditions such as acne, athlete's foot and warts.
The skin's "microbiome" -- containing bacteria, fungi and viruses -- is thought to be important to human health. Segre said it can help the body resist nasty germ invaders and maintain the barrier between the skin and inner organs.
The new study aimed to discover how stable these skin germs are over time. This can help researchers understand what happens when skin disease develops, Segre said.
For the study, Segre and colleagues analyzed 17 skin sites of 12 healthy volunteers three times over two years.
The researchers found that skin germs as a whole remained fairly steady, although individuals have their own "microbial fingerprints."
"One person had a higher amount of fungi on their skin, another person had a lot of bacterial viruses on the side of their nose," Segre said. She thought these collections of germs might be temporary, but "when we examined the person's skin community a year later, it was still true."
Germs on the feet were the most variable of all, but it's not clear why. One possibility, Segre said, is that the feet encounter a lot of temperature differences.
Dr. Stanley Spinola, a scientist who praised the research, said the variation seen in feet may have something to do with moist areas between the toes or differences in footwear -- from sneakers to leather shoes to flip-flops or none at all.
How is this research useful?
"The study shows over a long period of time, our skin microbiome stays pretty stable although we encounter different environments," said Spinola, who is chair of microbiology and immunology at Indiana University School of Medicine.
This is helpful because it gives researchers insight into the normal variation, allowing scientists to better study how disease causes differences, he said.
Elizabeth Grice, an assistant professor of dermatology at the University of Pennsylvania, put it this way: "The fact that these [germ] populations are so stable suggests that they are important."
Some researchers, she said, had assumed the collections of germs might not be that vital. They figured the germs would change a lot because of encounters with the outside world, said Grice, who wasn't involved in the study.
Dr. Tiffany Scharschmidt, an assistant professor of dermatology at the University of California, San Francisco, said the research highlights the importance of the germs, even though scientists aren't quite sure what they do. She did not work on the study.
So should you be wary of overusing germ-killing hand sanitizers and wipes, which could potentially destroy useful germs?
Scharschmidt said it's fine to wash your hands and use these products in health care settings and other places with a high risk of germ transmission. But "we do need to consider potential deleterious effects of this 'war' against all bacteria," she said.
The study findings were published May 5 in the journal Cell.
For more about healthy skin, try the U.S. National Institute of Arthritis and Musculoskeletal and Skin Diseases.
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