The first detailed survey of the microbial "zoo" living on the human body has found far more species than expected, with a significant proportion being new to science.
A molecular technique has been developed by scientists that can identify each individual bacteria living on the surface of the skin.
They found that on average people have 182 species of bacteria living at any one time on their forearms, and about 8 per cent of these have never been formally described by scientists.
Martin Blaser, a microbiologist at New York University School of Medicine, said it was the first time the human skin had been analysed with such a powerful forensic technique.
"This is essentially the first molecular study of the skin, which is home to a virtual zoo of bacteria," Professor Blaser said. "There are probably fewer than 10 labs in the United States looking at this question. It is very intensive work."
The study, published in the journal Proceedings of the National Academy of Sciences, involved taking swabs from the lower forearms of six men and women. The lower forearm was chosen because it is an area of skin that is usually not washed very frequently and yet is easily accessible without having to remove clothing.
About half of the bacteria identified by the molecular analysis belonged to one of four established bacterial groups known to live on human skin - the propionibacteria, corynebacteria, staphylococcus and streptococcus. Almost three quarters of the total number of species were found to be unique to each individual, showing that the range of bacteria differs substantially between people.
The findings also suggested there may be differences between men and women in terms of the colonies of the bacteria.
The scientists also found the variety of bacteria on any one individual changed over the several months in which the swabs were taken. The predominant species did not change much but the transient bacteria did vary over time.
"What that suggests is that there is a scaffold of bacteria present in everybody's skin. Some stay and others come and go," Professor Blaser said.
The study was part of a larger effort to examine the entire range of human microbial ecology. It is estimated that the body's microbial cells outnumber human cells by 10 to one.
Professor Blaser said: "Many of the bacteria of the human body are still unknown. We all live with bacteria all our lives and occasionally we smile, so they're not that bad for us."