The antibiotic-resistant staph infection called MRSA that has caused thousands of infections in Canadian hospitals seems to attack the body's defences by causing immune system cells to explode, new research has found.
The study may help explain why MRSA spreads outside of hospitals easily and may also lead to new and better treatments.
Thousands of people in Canada fall ill every year with MRSA, methicillin-resistant Staphylococcus aureus. Canadian surveillance has shown that MRSA rates continue to climb in Canadian hospitals and as of 2005, were 10 times higher than they were when surveillance started in 1995.
The infection long has been associated with hospitals and long-term health-care facilities, such as nursing homes, where it can attack people with reduced immune systems.
But many recent cases have involved aggressive community-associated strains (CA-MRSA), which can cause severe infections such as flesh-eating disease and even death in otherwise healthy people outside of health-care settings.
Earlier this year, a commentary written by infectious disease experts and published in the Canadian Medical Association Journal noted that clones of CA-MRSA, are "spreading with alarming rapidity": "CA-MRSA is an 'old foe with new fangs': a pathogen combining virulence, resistance and an ability to disseminate at large," the authors wrote.
While only 14 per cent of serious MRSA infections are the community-associated kind, doctors have been puzzled about what makes the strains more successful in causing disease compared to their hospital-associated counterparts.
Dr. Michael Otto of the National Institute of Allergy and Infectious Diseases in the U.S. led a team of researchers to find out.
Using mice and human blood, the team found that CA-MRSA strains secrete a compound called phenol-soluble modulin or PSM. PSM seems to cause inflammation, which attracts immune system cells called neutrophils. The PSM then blows the neutrophils up in a process called lysis, thereby wiping out a main defence against infection and allowing the bacteria to thrive almost unfettered.
Both hospital-associated and community-associated MRSA contained genes for the creation of PSM. But their production was much higher in the CA-MRSA, the researchers said.
The findings appear in the online edition of the journal Nature Medicine.
"This elegant work helps reveal the complex strategy that S. aureus has developed to evade our normal immune defences," Dr. Anthony S. Fauci, NIAID director, said in a statement.
"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 added. But he said the finds could lead to findings on how to activate the immune system to defeat them.
"Understanding what makes the infections caused by these new strains so severe and developing new drugs to treat them are urgent public health priorities."
