Wash your hands, folks, especially women.

A new study found that women have a greater variety of bacteria on their hands than men do. And everybody has more types of bacteria than the researchers expected to find.

"One thing that really is astonishing is the variability between individuals, and also between hands on the same individual," said Rob Knight, a University of Colorado biochemistry assistant professor and a co-author of the paper.

"The sheer number of bacteria species detected on the hands of the study participants was a big surprise, and so was the greater diversity of bacteria we found on the hands of women," added lead researcher Noah Fierer, an assistant professor in Colorado's department of ecology and evolutionary biology.

The researchers aren't sure why women harboured a greater variety of bacteria than men, but Fierer suggested it may have to so with the acidity of the skin. Knight said men generally have more acidic skin than women.

Other possibilities are differences in sweat and oil gland production between men and women, the frequency of moisturizer or cosmetics applications, skin thickness or hormone production, he said.

Women also may have more bacteria living under the surface of the skin where they are not accessible to washing, Knight added.

Asked if guys should worry about holding hands with girls, Knight said: "I guess it depends on which girl."

Left, right hands share few bacterial types

He stressed that "the vast majority of the bacteria we have on our body are either harmless or beneficial … the pathogens are a small minority."

The researchers took samples from the palms of 51 college students — that's 102 hands — and tested the samples using a new, highly detailed system for detecting bacteria DNA.

They identified 4,742 species of bacteria overall, only five of which were on every hand, they report on Monday's online edition of Proceedings of the National Academy of Sciences.

The average hand harboured 150 species of bacteria.

Not only did individuals have few types of bacteria in common, the left and right hands of the same individual shared only about 17 per cent of the same bacteria types, the researchers found.

Handwashing did not eliminate bacteria

The differences between dominant and non-dominant hands were probably due to environmental conditions like oil production, salinity, moisture or variable environmental surfaces touched by either hand of an individual, Fierer said.

Knight said the researchers hope to repeat the experiment in other countries where different hands are assigned specific tasks.

While the researchers stressed the importance of regular handwashing, they also noted that washing did not eliminate bacteria.

"Either the bacterial colonies rapidly re-establish after handwashing, or washing [as practised by the students included in this study] does not remove the majority of bacteria taxa found on the skin surface," the researchers said in their report.

While the tests could determine how many different types of bacteria were present, they could not count the total amount of bacteria on each hand.

The research was funded primarily by the National Institutes of Health and the National Science Foundation.