Scientific Method / Science & Exploration

Belly Button Biodiversity finds no two navels are quite the same

And locates extremophiles among those who don't bathe.

A new citizen science project has provided a unique glimpse into the biodiversity present on the human body through a bit of navel gazing. Or, rather, a bit of navel sequencing. The project got a set of volunteers to run a cotton swab through their belly buttons, and then looked at all the bacterial species growing there. The results suggest that the belly button is a diverse environment, and the communities living there respond to the habits of their host. Notably, a few species that were discovered for the first time on human skin were found in an individual who hadn't bathed in a few years.

The results are the product of the Belly Button Biodiversity project, which has two goals. One is scientific, in that it's trying to sample the biodiversity of a unique habitat on the human skin. The human body houses more bacterial cells than it has human cells, and we've only recently developed the technology to get a more complete picture of the organisms living there. Given that the ecosystems in places like the skin and gut influence human health, a better understanding seems long overdue.

But the project is also an example of what's been termed citizen science, where the public itself participates in producing scientific results. In this case, the public got to swab itself, see what grew out of the cultures, and continued to be informed of further results as the work progressed. For the data in the new paper, they obtained samples from groups already interested in science: people who visited a science museum during Darwin Day festivities, and science communicators who attended the ScienceOnline meeting last year. (I was one of the attendees; although I did not have my navel sampled, I know several people who did.) The engagement with the public didn't end there: "Citizens participated in this study not only in sampling but also in hypothesis generation (via twitter and online comments) and data visualization and were provided with images of bacterial cultures of their samples."

To get a full sampling of the bacteria present, instead of just those that grew on culture dishes, some of the swabs were used to prepare DNA for sequencing. After adding a short DNA sequence (called a barcode) to identify the donor, the DNA from 60 individuals was amplified by PCR, with the PCR being designed to pick up ribosomal RNA genes. The results were sequenced in a single run of 454 sequencing machine.

The results were analyzed in terms of "phylotypes," or groups of identical sequences. These probably represent either a single species or a group of closely related ones. (As the authors note, "bacterial species definitions are vague," anyway.) In all the belly buttons combined, there were a total of 2,368 phylotypes. It wasn't really the total that was interesting, though; rather, the distribution of the phylotypes was. Of that total, nearly 2,200 were present on less than 10 percent of the people sampled, and most of these showed up on only a single individual. In other words, in terms of species, most belly buttons looked quite different from each other. And there was no sign that diversity was tailing off, either, which suggests we'll keep finding more species as we sample more people.

Further testifying to the diversity, there was no phylotype that showed up on everyone sampled. In fact, only eight phylotypes were found in more than two-thirds of the people sampled. These, however, tended to be the most common bacteria in the navel, and they tended to be closely related from an evolutionary perspective. So, there do seem to be at least some commonalities between people.

One of the more amusing discoveries was that, for the first time, they found a few examples of Archaea living on human skin. These organisms are called extremophiles, since they tend to live in very harsh environments. And, well, these seemed to fit the bill: "Two of these three phylotypes were from an individual who self-reported not having showered or bathed for several years." As best as I could tell, he must have attended the Darwin Day celebration.

The results suggest that we've barely scratched the surface (or poked the belly button?) of the communities living in this environment. A bit more work will likely discover additional phylotypes, and allow the authors to test various models of ecological behavior. And it just might get a few more people engaged with science in the process.

PLoS one, 2012. DOI: 10.1371/journal.pone.0047712  (About DOIs).

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