For the gender that considers itself the more fastidious (and has the studies to back up the claim), we women may be chagrined to learn that we harbor more varieties of germs on our hands than men do.
In fact, we all -- male and female -- have whole worlds on our hands, and they’re more diverse than anyone suspected.
In a study published online Monday in the Proceedings of the National Academies of Science, researchers at the University of Colorado at Boulder swabbed 102 human palms and found more than 4,700 species of bacteria.
Species varied from person to person; just five were shared among all 51 of the study’s student volunteers. They even differed from hand to hand. An individual’s right had different species than the left.
It’s well-established that hands are hotbeds for bacteria. That’s why everyone from mothers on up to the national Centers for Disease Control and Prevention admonish us to wash them.
What set this study apart is that it looked not at abundance but at diversity.
It did so by extracting DNA from samples gathered on cotton swabs rather than using the standard method of trying to culture the samples in petri dishes to see what kinds of bacteria would grow. According to Noah Fierer, assistant professor of ecology and evolutionary biology and the paper’s lead author, the older technique tends to greatly underestimate the number of species.
"Bacteria are tough to identify," he said. "Most of them can’t be grown in the lab. The best way we have of identifying them is to look for their DNA."
Fierer doesn’t know why women have a greater variety of bacteria than men do. It could be that men’s more acidic skin discourages some species, or that sweat, hormones or women’s greater use of hand creams play a role.
"The findings don’t necessarily mean that women have more germs than men, just more variety," he said, rather gallantly, in a phone interview.
Not all bacteria found on hands are harmful. Most are probably neutral, Fierer said, and some may protect the skin from pathogenic varieties. But until scientists better understand what’s normal across individuals, it will be difficult to determine which species contribute to or protect from diseases.
Washing hands, by the way, reduces the abundance but not the variety of microbes. A diverse community re-establishes itself within hours, the study found.
This doesn’t mean that applying soap and water is futile.
"We’re not saying at all that washing hands is not a good idea," Fierer said. "We know that it reduces abundance and has a large effect on pathogens."
The study was funded by the National Institutes of Health and the National Science Foundation. (For a video interview with the researchers, click here.)
-- Mary Engel
Photo: Human palms are home to many species of bacteria. Here, hands reach for the ball during an Olympic volleyball game in August.
Credit: Robert Gauthier / Los Angeles Times