Foot fungi a thriving, diverse community

For house-hunting fungi, feet are prime real estate.

More than 80 different types of fungi make human feet home, researchers report May 22 in Nature. The tiny organisms stake claims all over a person’s skin, but only the feet carry such a diverse group of settlers, says study coauthor Julie Segre, a geneticist at the National Human Genome Research Institute in Bethesda, Md.

The study is the first census of skin-dwelling fungi. By helping to identify differences between healthy and unhealthy fungi, it could one day lead to targeted treatments for athlete’s foot or toenail infections.

“This study will get people’s attention,” says physician and microbiologist Martin Blaser of the New York University School of Medicine. Researchers have generally focused on our bacterial companions, not our fungal ones, he says.

Unlike bacteria, fungi are hard to grow in the lab. Culturing fungi from an infected toenail can take weeks, says study coauthor Heidi Kong, a dermatologist at the National Cancer Institute in Bethesda, Md. Instead of tallying up different skin microbes by trying to grow them in the lab, Kong, Segre and their colleagues went hunting for fungal DNA.

The team swabbed 13 skin sites (plus toenails) on 10 healthy volunteers, and analyzed the DNA in each sample to figure out which types of fungi lived where. Using molecular tags that stick only to fungal DNA, the researchers could separate their quarry from the genetic material of other organisms such as bacteria, humans and viruses.

Across all areas but the feet, one fungal genus, Malassezia, tended to dominate — with different species prevailing in spots such as the forehead, the crease behind the ear or the forearm. The feet, however, housed bustling beds of fungal diversity.

Researchers found about 40 different types of fungi packed on the toenails, 60 wedged between the toes and 80 nestled at the bottom of the heel.

 Heels and toes may host such hearty fungal gardens because feet are cooler than other body parts, and fungi don’t like it hot (SN: 1/1/11, p. 15). Feet also rub against fungi-friendly surfaces such as sweaty socks and locker room floors.

When people hear about the team’s results, “they say, ‘Eww, I need to take a shower,’” says Kong. “But in fact, these organisms may actually be helping protect us.”

Healthy fungal carpets probably prevent dangerous fungi from sticking to the feet, Segre says.