NEW YORK (GenomeWeb News) – Humans, particularly women, have far more bacterial diversity on the palms of their hands than previously realized, new research suggests.
In a paper scheduled to appear online this week in the Proceedings of the National Academy of Sciences, University of Colorado researchers used barcoding and pyrosequencing to assess bacterial communities on the right and left palms of more than 50 volunteers, identifying nearly 5,000 bacterial species. Their results suggest that bacterial species composition on the hands varies widely from person to person and even from hand to hand in the same individual, with women having significantly more diverse bacterial communities than men.
“The sheer number of bacteria 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,” lead author Noah Fierer, an ecology and evolutionary biology researcher at the University of Colorado, said in a statement.
By understanding which bacterial species are normally on and in the human body, researchers hope to gain insights into changes in the microbial community that are associated with human disease. For example, efforts such as the Human Microbiome Project and the International Human Microbiome Consortium are attempting to catalogue the diverse microbial communities associated with the human body.
Although the current study was not done as part of the Human Microbiome Project, Fierer told GenomeWeb Daily News that he believes the work has implications for this effort as well. For example, because there seems to be so much variation in the bacterial communities on the skin of healthy individuals, he noted, it may be difficult to establish a baseline and determine which changes are associated with disease.
For this study, Fierer and his colleagues used a sterile swab to sample the bacterial communities on the right and left hands of each of the 51 undergraduate volunteers. They then used the Roche 454 sequencing platform to sequence 16S rRNA genes, which allowed them to look at the bacterial diversity and communities in sample.
From the 332,000 informative sequences they obtained, the researchers found barcodes representing 4,742 different bacterial phylotypes. On average, each palm was home to more than 150 unique bacterial species. Of these, just five were found on the hands of all the participants.
Based on these results, the bacterial diversity on the hands appears to be about three times as diverse as that found in studies of forearm or elbow skin and more closely resembles the level of bacterial richness in the mouth, esophagus, or lower intestine.
Overall, species from the Propionibacterium, Streptococcus, Staphylococcus, Corynebacterium, and Lactobacillus genera were the most prevalent on human hands. But the species composition and diversity varied widely from one individual to the next and even within the same individual.
For example, the researchers found that the bacterial diversity was about 40 percent higher on women’s hands than on men’s hands, Fierer said. The researchers speculated that these may be due to differences in skin environment between men and women caused by factors such as sweat and oil gland production, hormone production, skin thickness, or differences in skin acidity.
In the same individual, just 17 percent of the species were shared between the dominant and non-dominant hand, on average. Meanwhile, an average of 13 percent of bacterial phylotypes were shared between different individuals. The team also saw differences in bacterial species composition on individuals’ palms depending on their handedness and time since hand washing.
When they followed up in a smaller study of four men and four women, the researchers again found sex differences between the bacterial communities. In addition, that experiment suggested that bacterial species composition changed immediately following hand washing, with overall species diversity increasing — potentially due to the removal of the dominant species, Fierer said. The original species composition seems to become re-established just two to four hours after hand washing, he added.
At present, Fierer and his team are doing experiments to try to determine whether the high levels of diversity are a characteristic of microbial communities on or in all parts of the body or whether microbial communities in other parts of the body are more consistent from one individual to the next.