Women's hands home to more bacteria

Women's hands home to more bacteria

Human being have far more different kinds of bacteria on the palms of their hands than previously thought and women's hands are home to a more diverse range than men's, said US researchers this week.

The study was the work of lead author Noah Fierer, Assistant Professor in the ecology and evolutionary biology department at the University of Colorado at Boulder (CU-Boulder) and colleagues, and is to be published this week in the Proceedings of the National Academy of Sciences, PNAS.

Fierer and colleagues said their findings increase our understanding about human bacteria and what constitutes a "healthy baseline" so that differences from this can be used to track disease.

For the study, Fierer and colleagues used powerful gene-sequencing methods to establish that the typical human hand was home to about 150 different species of bacteria although the number of different species they found on a total of 102 different hands came to more than 4,700 of which only 5 were common to all 51 participants, who were all CU undergraduates.

The method they used allowed them to analyze all the bacteria on the surface of a given palm. They isolated and amplified small pieces of bacterial DNA, then built complementary DNA strands with a high-powered sequencing machine so they could identify families, genera and species of bacteria.

Fierer said they were surprised by the "sheer number of bacteria species detected on the hands of the study participants" and by the "greater diversity of bacteria we found on the hands of women".

There were other surprising results. For example, the left and right hands on the same person differed significantly too. On average the right and left palms of the same person only shared about 17 per cent of the same species of bacteria.

In fact, the total diversity of bacteria present on human hands appears to be comparable to or even exceeds that known to exist in in other parts of the body, including the esophagus, the mouth and lower intestine said Fierer.

The investigators found 332,000 gene sequences altogether, some 100 times more than the number discovered by other studies of skin bacteria.

Fierer said the study also confirmed that the standard method of culturing used by many labs to find bacteria on human skin dramatically understimates the full range of microbial diversity.

Skin pH differences between men and women's skin may explain why women's hands carry a wider range of bacteria said Fierer: men's skin tends to be more acidic and other studies have shown that bacteria don't survive so well in acidic environments.

Other reasons could be gender differences in skin thickness, hormones, sweat and oil gland secretion, and use of skin products like moisturizers, they said.

The differences between right and left palms could be because of dominant and non-dominant hand effects which affect oil production, salinity and moisture on the palms, as well as the effect of touching different surfaces with different hands, said the researchers.

The study also showed that the range of species was not affected by regular hand washing.

Fierer and colleagues found that as expected, levels of some bacteria went down after hand washing, but unexpectedly, others went up. They stressed the importance of washing with anti-bacterial soap.

"The vast majority of bacteria are non-pathogenic, and some bacteria even protect against the spread of pathogens," said co-author Rob Knight, who is Assistant Professor of chemistry and biochemistry at CU-Boulder.

"From a public health standpoint, regular hand washing has a very positive effect," he added.

Fierer and colleagues wrote that although hand washing changed the bacteria composition, the overall diversity of bacteria found on the participants' hands was not related to how long their hands went unwashed. They speculated that:

"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."

The researchers also found that compared to that found on nearby regions of the body like the forearm and elbow, the palm of the hand was home to three times the range of bacteria.

Fierer said he saw human bodies as "continents of microscopic ecological zones" harbouring diverse ecosystems 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," he added.

The other authors of the paper were Micah Hamady of CU-Boulder's computer science department and Christian Lauber of CU-Boulder's Cooperative Institute for Research in Environmental Sciences.

Click here to see video of Fierer talking about the study (CU-Boulder).

Look out for Fierer et al's article this week in PNAS early release.

Sources: University of Colorado at Boulder.

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