Bug 'zoo' living on your skin
Researchers on a safari for microbes have found that human skin is populated by a veritable menagerie of bacteria: 182 species.
Some apparently live there permanently and others just drop by for a visit.
There's no need for alarm, says US microbiologist Professor Martin Blaser of the New York University School of Medicine.
The bacteria have been with us for quite a while and some are helpful.
In research published online today in the Proceedings of the National Academy of Sciences, Blaser and his colleagues took swabs from the forearms of six healthy people to study the bacterial populations in human skin, our largest organ.
"We identify about 182 species," Blaser says. "And based on those numbers, we estimate there are probably at least 250 species in the skin."
"In comparison," Blaser adds, "a good zoo might have 100 species or 200 species. So we already know that there are as many different species in our skin, just on the forearm, as there are in a good zoo."
While some bacteria cause disease, others reside normally in our bodies, for example in the digestive tract, performing useful chores.
"Without good bacteria, the body could not survive," says Dr Zhan Gao, a scientist in Blaser's lab involved in the study.
The researchers note that microbes in the body actually outnumber human cells 10-to-1.
"Our microbes are actually, in essence, a part of our body," Blaser says.
"We think that many of the normal organisms are protecting the skin. So that's why I don't think it's a great idea to keep washing all the time because we're basically washing off one of our defence layers," he says.
It has long been known that bacteria reside in the skin, but Blaser and his colleagues used a sophisticated molecular technique based on DNA to conduct a rigorous census.
The inhabitants proved to be more diverse than had been thought, with about 8% of the species previously unknown.
Some bacteria seemed to be permanent residents of the skin. Four genera (Staphylococcus, Streptococus, Propionibacteria and Corynebacteria) accounting for a bit more than half the population. Others were more transient.
In each person, the population of bacteria changed over time although a core set existed for each.
The volunteers included three men and three women, and the findings suggested the two sexes may carry different bacteria.
The researchers have already studied bacteria in the stomach and oesophagus and found that the insides of the body and the skin have major differences in bacterial populations.
"Microbes have been living in animals probably for a billion years. And the microbes that we have in our body are not accidental. They have evolved with us," Blaser says.