Ladies, your hands are a zoo. Sampling the bacterial DNA on human skin has revealed that while women’s hands get washed more often than men’s, they teem with a more diverse selection of germs.

What’s more, the average person’s hands probably carry at least 3000 different bacteria belonging to more than 100 species. This startling cornucopia may make it possible to tell which objects have been touched by someone, just by looking at the bacteria left behind.

Noah Fierer and colleagues at the University of Colorado at Boulder in the US swabbed the palms of 51 undergraduates coming out of an exam. They then used the PCR technique to amplify the bacterial DNA present, and sequenced the genetic material using a high-throughput method called pyrosequencing.

Previous surveys of skin bacteria involve growing the result of such swabs in culture, but many species were missed as not all the bacteria sampled grew. The new method caught 332,000 genetically distinct bacteria belonging to 4742 different species – a hundredfold increase on previous studies. Each student carried on average 3200 bacteria from 150 species on their hands.

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Germ fingerprints

Everyone’s bacterial “fingerprint” was unique. All students carried known skin denizens such as Streptococcus, Staphylococcus and Lactobacillus, but 45% of the species were rare. Only five species were found on all the students’ hands, while any two hands – even belonging to the same person – had only 13% of their bacterial species in common. Certain bacteria were more frequent on the dominant hand (right for a right-hander), others on the non-dominant hand.

Unexpectedly, women had different bacteria from men and significantly more kinds, even though women reported washing hands more often; bacterial diversity needs time to recover when hands are washed. In a follow-up experiment, the team tracked eight people after they washed their hands. Some bacteria preferred clean hands, other types showed up much later, but men always had fewer types of bacteria on their hands.

“We were pretty surprised to see such clear differences between men and women,” says Fierer. “We don’t know the causes.” Differences in sweat and sebum production, hormones and even the use of cosmetics might be involved, but it could simply be that men’s skin is more acid – acid environments tend to have less microbial diversity.

This unexpected wealth of germs could pose problems for the Human Microbiome Project, launched last year with the aim of determining how our microbial fellow-travellers contribute to disease. “It will be difficult to associate changes in skin bacterial communities with skin disease,” says Fierer, “because the background variability among healthy individuals is so large.”

On the other hand, he says, this may make it possible to identify objects touched by a certain individual just by looking at the selection of bacteria they leave on it.

Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0807920105)