Your skin is patchwork of ecosystems—some moist and humid, others hot and dry—that is as diverse as any continent.
Perhaps not surprisingly, skin, our interface with the world, supports the body’s most diverse population of bacteria. There are at least 1,000 different species found on skin, along with dozens of fungi and other microbes. Most of these bacteria aren’t harmful, and many in fact serve protective functions. These species live among the dead skin cells that make up the outer layer of our skin, defending us from disease while they guard their own turf against other microbes.
One example of this relationship is a strain of the bacterium Bacillus subtilis, which is sometimes found on the skin, which produces bacitracin—a common ingredient in many over-the-counter antibiotic ointments. B. subtilis also releases toxic chemicals to kill fungus, possibly including Trichophyton interdigitale and other species that cause athlete’s foot.