Human sweat packs a germ-killing punch

People working up a lather at the gym may be doing more than shedding a few pounds. They could be protecting their skin from infection.

In an upcoming Nature Immunology, German researchers report that human sweat contains a novel microbe-killing molecule, which they’ve dubbed dermicidin. Further study of this small protein, or peptide, may lead to new ways of defeating disease-causing germs, the scientists suggest. The peptide may even explain how sweat glands originally arose in animals, adds another biologist.

The discovery of dermicidin was serendipitous. While studying skin cancer, Claus Garbe of the University of Tübingen in Germany and his colleagues discovered a human gene that’s active in sweat glands but not in any other tissue they tested.

Intrigued, they conducted more experiments and learned that the gene encodes a protein that cells in sweat glands secrete. An unknown enzyme then chops the protein into a smaller form, which sweat then carries to the skin’s surface. Garbe’s team confirmed this peptide’s presence in the sweat of four people.

At that point, the investigators suspected that their mysterious peptide might be a microbe killer, even though its structure differs considerably from defensins and cathelicidins, the two known classes of antimicrobial peptides. They verified the suspicion by demonstrating in test-tube experiments that dermicidin can kill four different kinds of bacteria and one fungal species.

The investigators also showed that the peptide retains its killing capability in solutions whose acidity and salt content resemble sweat; other antimicrobial peptides don’t function well or at all in such conditions.

A major remaining question is how dermicidin destroys microbes, says Garbe’s colleague Birgit Schittek. Other germ-killing peptides, which have positive charges, are drawn to and punch holes through the negatively charged membranes of microbes. Dermicidin, however, has a negative charge.

The German investigators plan to explore the dermicidin vulnerability of other microbes, especially bacteria resistant to traditional antibiotics. They’ll also examine whether people differ in the amount of dermicidin they produce in their sweat. Some with frequent skin infections may have problems making the peptide, Schittek speculates.

Researchers are already working to incorporate other antimicrobial peptides in topical gels. Too little is now known about dermicidin to predict whether it can be turned into a therapeutic agent, Schittek cautions.

Michael Zasloff of the University of Pennsylvania School of Medicine in Philadelphia, who discovered one of the first defensins, suggests that dermicidin may have permitted the evolution of sweat glands. Without such a protective element, he notes, a sweat gland is a “warm, wet, delightful cave that a microorganism should feel very happy to find a home in.”

Given their findings with defensins and cathelicidins, biologists suspect that peptides related to dermicidin exist. “If it were a member of larger family, that would fit in with the theme of these antimicrobial peptides,” says Charles Bevins of the Cleveland Clinic Foundation.

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