Assistant Professor Noah Fierer, the lead author of the study, says that the new findings have implications for better understanding human bacteria, and should help establish a "healthy baseline" to detect microbial community differences on individuals that are associated with a wide variety of human diseases.
Reporting their study in the online edition of the Proceedings of the National Academy of Sciences, the researchers revealed that they used powerful gene sequencing techniques for their research, which revealed that a typical hand in had roughly 150 different species of bacteria living on it.
The researchers detected and identified more than 4,700 different bacteria species across 102 human hands in the study, but only five species were shared among all 51 participants.
"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," said Fierer.
He further said that the diversity of bacteria on individual hands was not significantly affected by regular hand washing.
Fierer said that the research team's observations suggested that the standard skin culturing of human skin bacteria, a technique used by many labs, dramatically underestimated the full extent of microbial diversity.
The researcher said that skin pH might be behind the higher bacterial diversity on women's hands, since men generally have more acidic skin, and other research has shown microbes are less diverse in more acidic environments.
He added that the difference could also be due to differences in sweat and oil gland production between men and women, the frequency of moisturizer or cosmetics applications, skin thickness or hormone production.
The study also revealed that the right and left palms of the same individual shared an average of only 17 percent of the same bacteria types.
The participants, who were CU undergraduates, shared an average of only 13 percent of bacteria species with each other.
Fierer said that while the composition of bacterial communities on dominant and non-dominant hands of subjects was significantly different, diversity levels were similar.
He said that the differences found between dominant and non-dominant hands were likely due to environmental conditions like oil production, salinity, moisture or variable environmental surfaces touched by either hand of an individual.
The researchers also observed that some groups of bacteria were less abundant following hand washing, while others were more abundant.
Biochemistry Assistant Professor Rob Knight stressed that regular hand washing with anti-bacterial soap was beneficial.
"The vast majority of bacteria are non-pathogenic, and some bacteria even protect against the spread of pathogens. From a public health standpoint, regular hand washing has a very positive effect," Knight said.
In their study report, the researchers wrote: "Although hand washing altered community composition, overall levels of bacterial diversity were unrelated to the time since the last hand washing. Either the bacterial colonies rapidly re-establish after hand washing, or washing, as practiced by the students included in this study, does not remove the majority of bacteria taxa found on the skin surface."
Knight revealed that the research team used the metagenomic survey to simultaneously analyze all of the bacteria on a given palm surface.
According to him, the procedure involved isolating and amplifying tiny bits of microbial DNA, then building complementary DNA strands with a high-powered sequencing machine that allowed the team to identify different families, genera and species of bacteria from the sample.
He said that the richness of bacteria types on the palm was three times higher than that found on the forearm and elbow.
Fierer added that the total diversity of hand bacteria appeared to match or exceed levels of bacteria colonizing other parts of the body, including the esophagus, the mouth and lower intestine.
"I view humans as 'continents' of microscopic ecological zones with the kind of diversity comparable to deep oceans or tropical jungles. Today we have the ability to answer large-scale questions about these complex microbial communities and their implications for human health that we weren't even asking six months or a year ago," Fierer said.