Not all germs are bad for you. From probiotics to healthy gut bacteria, we now know that being exposed to a wholesome microbiota can actually be a good thing. And you can add a new microscopic friend to the list: the common human skin bacteria, Staphylococcus epidermidis.

Researchers were studying the antimicrobial powers of Staphylococcal bacteria — the most numerous of the many types of bacteria that normally live on human skin — when they uncovered something unexpected. Mice that were treated with a particular strain of the bug ended up being remarkably resistant to skin cancers, reports Science News.

Further investigation pinpointed this anticancer superpower to a compound produced by the bacteria known as 6-N-hydroxyaminopurine, or 6-HAP for short. Researchers noticed that the compound had a familiar structure; it looked like one of the building blocks of DNA.

“Because of that structure, we wondered if it interfered with DNA synthesis," said Richard Gallo of the University of California, San Diego, one of the researchers on the study.

Sure enough, that hunch was spot on. 6-HAP was found to block the enzyme that builds DNA chains and prevented the chains from growing. While that might sound like a dangerous trait, the compound does not operate on normal skin cells. Instead, it acts on cancerous cells and prevents their runaway growth.

The findings highlight “the potential of the microbiome to influence human disease,” explained Lindsay Kalan, a biochemist at the University of Wisconsin–Madison.

Gallo suspects that we may have evolved a mutually beneficial relationship with this bacteria over time: “Perhaps we evolved to provide a safe haven for these organisms because they also benefit us when they’re doing this,” he theorized.

The compound still worked when isolated from the bacteria, both when injected and when applied topically, which means that it could lead to future treatments, and not just for skin cancers. Researchers found that it was also effective in reducing growth rates of lymphoma cells, so this natural defense might have a much wider application than first meets the eye. Of course, further research will be needed to analyze exactly how S. epidermidis makes 6-HAP, and also whether there is a toxic level of the stuff. Only about 20 percent of the population has this bacteria on their skin, so we'll want to make sure it's safe for everyone.

It's a reminder that only a very small percentage of the organisms teeming all around us are actually bad for us. At the very least, this study might make you want to think twice about using anti-bacterial soap or body wash.

Bryan Nelson ( @@brynelson ) writes about everything from environmental problems here on Earth to big questions in space.

Bacteria on your skin are fighting off cancer
Mice treated with a particular strain of Staphylococcus epidermidis were remarkably resistant to skin cancers.