One Strain Behind Epidemic of Staph Infections

Government study found most MRSA infections in community linked to single evolving bacterium

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MONDAY, Jan. 21, 2008 (HealthDay News) -- A single strain of an evolving bacterium has been responsible for most of the community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) infections that have spread rapidly in the United States during the past five years, a new government study finds.

Typically, CA-MRSA causes boils, but it can lead to life-threatening conditions that are difficult to treat, according to the study.

The National Institute of Allergy and Infectious Diseases (NIAID) researchers said their findings resolve debate about the molecular evolution of CA-MRSA in the United States and rule out the possibility that multiple strains of USA300 emerged randomly with similar characteristics.

This single strain of USA300, which has spread with "extraordinary transmissibility" in the past five years, was identified by analyzing the genomes of USA300 collected from 10 patients infected in different parts of the United States between 2002 and 2005. Eight of the 10 samples had almost identical genomes, indicating they were from a common strain. The remaining two were related to the other eight, but more distantly.

The researchers also found that two of the eight almost identical USA300 samples caused far fewer deaths in laboratory mice than the other samples. This appears to support an emerging belief that tiny genetic changes among evolving strains have a major impact on disease severity and the potential for development of drug resistance.

The study was published in this week's online issue of the Proceedings of the National Academy of Sciences.

"The USA300 group of strains appears to have extraordinary transmissibility and fitness," research leader Frank R. DeLeo said in a prepared statement. "We anticipate that new USA300 derivatives will emerge within the next several years and that these strains will have a wide range of disease-causing potential."

It's hoped this research will lead to new test that can quickly identify specific MRSA strains.

A second study, led by the same NIAID scientists, discovered new information about how MRSA bacteria -- including the USA300 group -- avoid destruction by the human immune system's white blood cells. The study found that MRSA senses danger and turns the tables, killing the white blood cells.

That study was recently published online in The Journal of Immunology.

"Scientists are pressing ahead quickly to learn more about how some MRSA strains evade the immune system and spread quickly. The information presented in these two studies adds important new insights into that expanding knowledge base," NIAID Director Dr. Anthony S. Fauci said in a prepared statement.

More information

The U.S. Centers for Disease Control and Prevention has more about MRSA.

SOURCE: National Institute of Allergy and Infectious Diseases, news release, Jan. 21, 2008

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