The back of your knee probably has more microbes than your mouth or your gut--that's just one of the somewhat disturbing revelations from a study published today online in Science. Researchers from the University of Colorado, Boulder have developed the most complete map yet of the microbes that dwell on and in us. "The highest diversity skin sites were the forearms, palm, index finger, back of the knee and sole of the foot. The armpits and soles of the feet showed some similarities, perhaps because they are from dark and moist environments," said Noah Fierer, one of the study's authors, in a statement.

Scientists are mapping our microbial inhabitants in order to better understand their role in human health and disease. As I noted in a previous feature:

Each of us contains roughly 10 times as many microbial cells as human ones. And while some microbes make us sick, many play vital roles in our physiology. They give us the ability to digest foods whose nutrients would otherwise be lost to us, and they make essential vitamins and amino acids our bodies can't. And yet, because the vast majority of these microbes die when extracted from their native habitat, they have been impossible to study and have remained a mystery...

New ultrafast DNA-sequencing technologies allow scientists to study the genetic makeup of entire microbial communities, each of which may contain hundreds or thousands of different species. For the first time, microbiologists can compare genetic snapshots of all the microbes inhabiting people who differ by age, origin, and health status. By analyzing the functions of those microbes' genes, they can figure out the main roles the organisms play in our bodies.

The new study, which analyzed 27 sites on the body of nine different volunteers, found that microbial diversity varies highly, both between individuals and from place to place in the same person. According to a release from the University of Colorado, Boulder:

The study showed humans carry "personalized" communities of bacteria around that vary widely from our foreheads and feet to our noses and navels, said CU-Boulder's Rob Knight, senior author on the paper. "This is the most complete view we have yet of the microbial side of ourselves, one that our group and others will be adding to over the coming years," said Knight, an assistant professor in CU-Boulder's chemistry and biochemistry department. "The goal is to find out what is normal for a healthy person, which will provide a baseline for further studies to look at people with diseased states. One of the biggest surprises was how much variation there was from person to person in a healthy group of subjects."

"We have an immense number of questions to answer," said Fierer, an assistant professor in CU-Boulder's ecology and evolutionary biology department who was a co-author on the study. "Why do healthy people have such different microbial communities? Do we each have distinct microbial signatures at birth, or do they evolve as we age? And how much do they matter? We just don't know yet."