Your body is home to 10 times as many microbes as its own cells. But they can be quite picky about where they will settle – and what other bacteria they'll share fences with.
That's the conclusion of the most comprehensive map ever of the microbial communities flourishing in the human body.
In addition, some unlikely-sounding parts of the body – such as the back of the knee and the index finger – have far more microbial diversity than the gut.
Elizabeth Costello at the University of Colorado in Boulder and her colleagues swabbed 18 sites on eight humans four times over the course of four months. The team wanted to find out what shapes bacterial communities across the body. Previously, it was unclear how important the specific location on the body was for encouraging bacteria compared with, for instance, variation between people or the progression of time.
"We didn't know whether it was because of differences in skin chemistry or whether it was simply because those microbes had been in the right place at the right time," says Rob Knight, a co-author at the university.
To investigate, the team disinfected "plots" on the foreheads and left forearms of volunteers, wiping out the indigenous microbial communities. The plots were then inoculated with communities transplanted from a different part of the body.
They found that microbial communities transplanted from the tongue to the forearm thrived, whereas those shifted from the tongue to the forehead faltered and developed to resemble the community they had replaced.
This suggests that skin chemistry exerts a much stronger control over community growth on oily skin, like that of the forehead, than it does on dry parts like the forearm.
Oily skin tends to have less diverse communities and resists to foreign microbes more than other areas, says Knight. "Message: if you're a microbe hoping to invade a new bit of skin, the forehead is not a great place to start."
The team found that the microbes lurking on the forearm, palm, index finger, back of the knee and sole of the foot were often more diverse than those in the gut, "traditionally considered to be very diverse", says David Relman, who researches human microbial ecology at Stanford University in California but was not involved in the research.
The similarity of certain microbial communities between individuals was also striking, says Relman. "This study suggests that your mouth is more like my mouth than it is like your armpit, in terms of its microbial composition."
Still, unusual similarities within the body did emerge. The composition of microbial communities that exist in our navels is the same as those on the backs of our knees. Our armpits and the soles of feet also share a similar biota, composed mainly of Staphylococcus.
Some 22 phyla of microbes call the human home, although just four – Actinobacteria, Firmicutes, Proteobacteria and Bacteroidetes – dominate.
The study has implications for the prevention and treatment of disease, says Knight. For example, it suggests that the flow of microbes around the bodies of people with HIV may be more complex than previously thought.
It also raises the possibility of engineering microbial communities to provide treatments to parts of the body that are easy for them to invade.
Journal reference: Science, DOI: 10.1126/science.1177486
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