From: Reuters
Published November 11, 2007 10:06 PM

Drug-resistant bacteria found to trick immune system

WASHINGTON (Reuters) - Drug-resistant bacteria called methicillin-resistant Staphylococcus aureus, or MRSA, may be able to first lure and then destroy immune system cells when they are the most vulnerable, researchers said on Sunday.

The study may help explain why MRSA spread outside of hospitals are harder to fight and seem to be spreading more easily.

But the findings may also lead to new and better antibiotics to fight the bacteria, the researchers reported in the journal Nature Medicine.

"This elegant work helps reveal the complex strategy that S. aureus has developed to evade our normal immune defenses," Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said in a statement.

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"Understanding what makes the infections caused by these new strains so severe and developing new drugs to treat them are urgent public health priorities."

The NIAID's Michael Otto and colleagues found that some strains on MRSA secrete a compound called phenol-soluble modulin or PSM. It attracts immune system cells called neutrophils, the researchers found, and then blows them up in a process called lysis.

S. aureus is common and usually only causes pimples or boils, although infections can spread to surrounding tissue.

MRSA is treatable only with a few antibiotics. It is common in hospitals, where it can killed weakened patients.

"Recently there has been an alarming epidemic caused by community-associated (CA)-MRSA strains, which can cause severe infections that can result in necrotizing fasciitis or even death in otherwise healthy adults outside of healthcare settings," Otto's team wrote in their report.

Necrotizing fasciitis is the so-called flesh-eating disease that can destroy healthy tissue and even kill patients.

"In the United States, CA-MRSA is now the cause of the majority of infections that result in trips to the emergency room. It is unclear what makes CA-MRSA strains more successful in causing human disease compared with their hospital-associated counterparts," they added.

"Here we describe a class of secreted staphylococcal peptides that have a remarkable ability to recruit, activate and subsequently lyse human neutrophils, thus eliminating the main cellular defense against S. aureus infection."

Neutrophils are key immune cells involved in clearing bacterial infections, so destroying them would allow the bacteria to thrive almost unmolested.

"We're not saying the PSM-alpha gene cluster is the only element contributing to the virulence and survival of CA-MRSA, but it is a major factor," Otto said in a statement.

In October researchers reported that MRSA was far more common than had been known, with 94,000 hospitalizations and almost 19,000 deaths in 2005 in the United States.

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