The armpit showed less variation in bacteria than many areas
Scientists have developed an atlas of the bacteria that live in different regions of the human body.
Some of the microbes help keep us healthy by playing a key role in physiological functions.
The University of Colorado at Boulder team found unexpectedly wide variations in bacterial communities from person to person.
The researchers hope their work, published in Science Express, will eventually aid clinical research.
They say that it might one day be possible to identify sites on the human body where transplants of specific microbes could benefit health.
The study was based on an intensive analysis of the bacteria found at 27 separate sites on the bodies of nine healthy volunteers.
Not only did the bacterial communities vary from person to person, they also varied considerably from one site on the body to another, and from test to test - but some patterns did emerge.
What is healthy?
Lead researcher Dr Rob Knight said: "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.
BODY SITES ANALYSED
Back of the knee
Soles of the feet
"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."
There are an estimated 100 trillion microbes living on or inside the human body.
They are thought to play a key role in many physiological functions, including the development of the immune system, digestion of key foods and helping to deter potentially disease-causing pathogens.
The researchers took four samples from each volunteer over a three-month period - usually one to two hours after they had showered.
They used the latest gene sequencing and computer techniques to draw up a profile of the microbes found at each specific site.
Most sites showed big variations in the bacteria they harboured from test to test even within the same individual.
However, there was less variation in the bacteria found in the armpits and soles of the feet - possibly because they provide a dark, moist environment.
The least variation of all was found in the mouth cavity.
Skin sites in the head area, including the forehead, nose, ear and hair, were dominated by one specific type of bacterium.
Sites on the trunk and legs were dominated by a different group.
Researcher Dr Noah Fierer said: "We have an immense number of questions to answer.
"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?"
The researchers disinfected the forearms and foreheads of some volunteers, and "inoculated" both sides with bacterial communities from the tongue.
The tongue bacteria lasted longer on the forearms than foreheads.
Dr Elizabeth Costello, who also worked on the study, said: "It may be that drier areas of the skin like forearms make generally more hospitable landing pads for bacteria."
A previous study by the same examined the bacteria on 102 human hands.
In total, they identified more than 4,200 species of bacteria, but only about five were shared by all 51 participants.
Dr Knight said understanding the variation in human microbial communities held promise for future clinical research.
"If we can better understand this variation, we may be able to begin searching for genetic biomarkers for disease," he said.
"Because our human genomes vary so little but our repertoire of microbial genes vary so much, it makes sense to look for variations that correlate with disease at specific locations."