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Feet come first when it comes to body parts with most fungi

The human skin's surface is home to the "skin microbiome," an ecosystem where fungus, bacteria and viruses coexist. Sometimes, things within the microbiome can go wrong: Fungal skin infections affect about 29 million people in the United States.

Research on the topic is lacking because growing fungi in the laboratory is a slow, difficult process. Thanks to genome mapping, researchers are now able to take a closer look at our skin and find out exactly where the different fungi grow.

Researchers from the National Human Genome Research Institute (NHGRI) and the National Cancer Institute (NCI) examined 14 skin areas on 10 healthy adults to map the fungi on them using DNA sequencing. What they found were a whole lot of fungi. The study was published online in Nature on May 22.

"Applying DNA sequencing to a study of the skin's fungi is the natural progression in understanding microbial life that co-exists on our bodies," co-author and NHGRI scientific director Dr. Daniel Kastner said in a press release. "Along with recent genome sequencing to define bacterial diversity, this analysis of fungal diversity provides a more complete human microbiome picture."

The sequencing revealed 5 million markers for fungi, representing more than 80 different types of fungus, or genera. Traditional culturing methods have found only 18 types of fungus on the skin, the researchers pointed out.

Fungi from two phyla, Ascomycetes and Basidiomycetes, were found at all 14 skin sites. Researchers discovered that fungus from the genus Malassezia is mostly found on the head and trunk of the body, and was found at 11 out of the 14 places tested on the body.

"DNA sequence-based methods of identification enabled us to differentiate among species of fungi and to conclude that the diversity of fungi is highly dependent on the body site rather than the person who is sampled," co-author Dr. Heidi Kong, a dermatologist and an investigator in the dermatology branch of NCI's Center for Cancer Research, said in a press release. "Our study focused on areas of the skin where we commonly find skin diseases that have been associated with fungi."

Our feet -- which include our toes, toe webs, nails and heels -- had some the most diverse kinds of fungi. Heels alone had 80 different genera types, toes had 60 and toe webs had 40 different types of fungi. In twenty percent of the subjects, researchers saw that they had heel and toe web scaling or toenail changes that could be fungal infections. However, while people with possible heel infections had similar fungal communities at that site of the foot, people with possible toenail infections had diverse fungal communities.

In comparison, hands, which are beacons for bacteria, barely had any fungi. The inside of the bend of the arm, inside of the forearm and inside of the palm only had 18 to 32 different genera of fungi. Head and trunk body sites -- including the back, back of the neck, inside the ears, behind the ears, and between the eyebrows -- just had two to 10 different genera types.

"The bottom line is your feet are teeming with fungal diversity, so wear your flip flops in locker rooms if you don't want to mix your foot fungi with someone else's fungi," co-senior author Julie Segre, senior investigator for the NHGRI Genetics and Molecular Biology Branch, said in a press release.

Jack Gilbert, an environmental microbiologist at Argonne National Laboratory in Illinois who was not involved in the study, pointed out that the fungal mapping only involved 10 people, so it was hard to make generalizations for everybody. Still, he was impressed with the results.

"The work is literally pioneering when we think about microbial dynamics: No one's looked at it before," Gilbert told National Geographic.

"(This is a) vital first step for understanding how those pathogens and the rest of organisms may be helping to protect us from other organisms and contributing to disease," he added.