If you were plagued by pimples in your teen years, you may have had bacteria to blame - but not all of them. Researchers have found that not all strains of the bacteria commonly associated with acne are created equal: Some may cause problem skin, but one appears to protect the skin and keep it healthy. The discovery may help dermatologists develop new, strain-specific treatments for acne.

Although acne is practically a rite of passage - more than 80 percent of Americans suffer from the skin condition, which can cause pimples, cysts and red, inflamed skin at some point in their lives - it's not entirely understood. Past studies have pointed to Propionibacterium acnes, a bacterium that lives in the skin's follicles and pores, as a potential culprit, but that work had not precisely revealed its role.

Molecular biologist Huiying Li of UCLA's David Geffen School of Medicine and colleagues decided to take a closer look at the microbe.

Armed with over-the-counter pore-cleansing strips, they sampled bacteria from the noses of 101 people, 49 of whom had acne and 52 of whom had clear skin. Then they examined the bacterial DNA, looking for patterns or variations in the microbes' genes that would help them identify specific strains of bacteria.

Whether they had clear or pimply skin, all the study participants sported similar abundances of P. acnes living in their pores, but not all of the strains were the same.

The researchers found a number of different strains of the

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microbe, including 66 that had never been identified before. When they sequenced the genomes of each strain, they discovered that two of the strains, RT4 and RT5, were found predominantly in people with acne and that one strain, RT6, was found almost exclusively in people with clear skin. Because this "good" strain contains genes known to fight off bacterial viruses and other potentially harmful microbes, the researchers suspect that it may actively ward off the strains that are associated with disease, thereby keeping skin healthy.

"Just [as] good strains of bacteria in yogurt, for example, are good for the gut, these good strains of P. acnes could be good for the skin," says Li, whose team reported the findings in the Journal of Investigative Dermatology.

Acne is often treated with antibiotics or other antimicrobial drugs. The team suggests that further studies of strain differences could lead to probiotic treatments that would boost or supply beneficial microbes. Lotions or medications that target bad strains of bacteria or foster good ones could offer a gentler and more effective way to ease problem skin, Li says.

"This is a great study: It was very carefully done, it addressed an important organism in the human microbiome, and it produced some very interesting results," says Martin Blaser, a physician and microbiologist at the New York University School of Medicine.

He notes, though, that the work has some limitations: It doesn't prove that the bad strains of P. acnes are actually causing acne, and it doesn't explain why some people carry certain P. acnes strains and others don't. "But they found some strong associations," he says, "and this is a good beginning."