Here’s a finding that will make your skin crawl: A healthy human epidermis is colonised by roughly 1,000 species of bacteria.

Furthermore, the microorganisms have evolved to exploit the unique attributes of the body parts they call home, according to a study to be published in the journal Science.

Some thrive in the desert of the forearm; others are happiest in the tropical rain forest of the armpit.

The study, conducted by a team of researchers from the National Institutes of Health, reflects a growing realisation that bacteria have colonised us inside and out — and that their presence is not only harmless but also probably essential to the proper functioning of the body.

One striking example of that fact: Mice bred to be entirely germ-free have smaller hearts and are unable to digest food properly.

“We live in a microbial world and these things are not all out to get us,” said Noah Fierer, a microbial ecologist at the University of Colorado at Boulder. Fierer, who has analysed bacteria that live on hands, was not involved with the new report.

“You don’t want to live in a sterile world,” he said. “You probably can’t live in a sterile world.”

The results reported Friday will lay some groundwork for a $115 million NIH venture dubbed the Human Microbiome Project, which is aimed at cataloging the bacteria and other organisms that inhabit the skin, gut, nose, mouth and vagina.

Of the more than 19 square feet of skin on a typical adult, the NIH team focused on 20 specific areas, ranging from the oily patch between the eyebrows to the moist space between the toes.

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