Key to MRSA Virulence in Community Discovered

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"That [sudden increase] was alarming, and nobody knew why it was happening," said Moran, who co-authored a 2006 study showing that community-associated MRSA accounts for almost 60 percent of skin infections that require a visit to the emergency room. "Something about these strains made them very well-suited to spread throughout the population."

These peptides could explain that virulence, at least in part, said Philip Tierno, director of clinical microbiology & immunology at New York University Medical Center and author of The Secret Life of Germs: Observations and Lessons From a Microbe Hunter."

"Virulence, it seems, is caused by these peptides, which can kill phagocytic cells [neutrophils], which come to your defense when staph is invading your body," he explained.

Staphylococci, Tierno noted, induce pus formation by recruiting and activating white blood cells. "That very induction of phagocytes [neutrophils] is key to your successful eradication of the organisms in the body," he said. However, "Staph has a defense. These peptides that can kill these phagocytic cells, thereby rendering you defenseless."

The genes encoding these toxins are found in the genomes of all sequenced MRSA strains, but community-associated MRSA strains produced the toxins at higher levels than the hospital strains, which typically cannot infect healthy individuals. Thus, they may explain the enhanced virulence of the community-associated strains.

The bacteria would fly under the immune system's radar, so to speak, by not expressing the peptides until the bacteria were either present in very large numbers, or perhaps after being engulfed by neutrophils and enclosed in a small space.

In either case, the mechanism would detect that situation and begin production of the peptides in earnest to fight back against the immune system.

"From the bacterial point of view, the most important thing is to get rid of the neutrophil," Otto said.

According to Moran, these findings suggest new drug possibilities -- antibodies that can remove the peptides from circulation, for instance.