There's a fungus among us—a hundred different species in fact—and nearly all take up residence on our feet, according to a study that appears in the journal Nature this week.
Only a few fungi species were found on other body parts known to house fungi—such as behind the ears and on palms—according to the most thorough analysis to date of our fungal "landscape." (Also see "Armpits Are 'Rain Forests' for Bacteria, Skin Map Shows.")
The minute bioforms that live with us are the subject of much recent study. But most research has focused on easily studied bacteria instead of fungi, whose DNA was harder to sequence until recently, explained study leader Julie Segre, senior investigator at the National Human Genome Research Institute, which is part of the National Institutes of Health in Bethesda, Maryland.
"Our human cells live together with bacteria and fungi; in that way I consider the human as an ecosystem," said Segre, who likens herself to an explorer charting exotic lands.
Just "as you think about exploring new terrains, we are exploring behind the ear, the toenails—we're going into these unknowns."
Mapping the body's fungi will help scientists figure out how to keep fungal communities—which are mostly friendly—from becoming harmful, for example, by turning into an infection. It's unknown what triggers this switch.
Fungal infections affect up to 29 million people in the U.S.—mostly in the form of athlete's foot, toenail issues, and jock itch, she said. Foot health—especially in the elderly—is vital to overall health.
Fungal Foot Fetish
For the study, Segre and colleague Heidi Kong recruited ten healthy people from the Washington, D.C., area and cotton-swabbed 11 fungal body-and-arm hot spots: the back, palm, elbow pit, ear canal, the space between the eyebrows, the crease between the thigh and the groin, the upper part of the sternum, nostril, back of the head, forearm, and behind the ear. (Explore a human-body interactive.)
The team also sampled three parts of the volunteers' feet: the heel, toenail, and webbing between the toes.
The team then put the swabbed samples directly into an advanced DNA sequencer—a technology of the Human Genome Project—to find out which species were living in which area. (The toenails were clipped to obtain the DNA.)
They discovered that a few species in the genus Malassezia dominated the core body, but that the feet host between 80 and 100 species, depending on the place on the foot—the heel, for instance, was more thickly populated than the toenails.
Why do fungi have a foot fetish? As our lowest extremities, feet are cooler compared with the rest of our body and thus more attractive to cold-loving fungi, noted Segre. The team also compared fungal communities in volunteers with toenail infections and discovered that each infection had a different makeup of fungi species.
This may explain why treating foot fungal infections is so challenging—there's no one-size-fits-all treatment, she said. The study could help scientists identify different types of toenail infections, which could lead to more effective antifungal treatments.
In the Garden of Microbes
But killing off all fungal hitchhikers is not always the best course of action. Segre stressed that fungi have the important job of preventing disease-causing microbes from sticking to our skin. (Read "Fungi Need Some Love, Too.")
"It's an ironic feature of our culture that people want to populate their guts with beneficial microbes," she said. Yet "everyone wants to sanitize their exterior and has this ... gross feeling about bacteria and fungi that live on our skin."
Microbes break down oils and provide natural moisturizing. When you put on lotion, she said, you're actually "fertilizing your microbial garden. They need those oils to grow."
Jack Gilbert, an environmental microbiologist at Argonne National Laboratory in Illinois, noted that the study is a preliminary investigation with only ten adult subjects, so it is too early to make "sweeping statements." But "the work is literally pioneering when we think about microbial dynamics: No one's looked at it before," said Gilbert, who was not involved in the study.
This is a "vital first step for understanding how those pathogens and the rest of [our cohabitating] organisms may be helping to protect us from other organisms and contributing to disease." (Learn more about human diseases.)
Meanwhile, there are measures you can take to prevent fungal infections. For those who shower in moist, fungi-rich locker rooms, Segre has this advice: Unless you want to swap species with your fellow gym-goers, "you probably should be in flip-flops."