One Strain Behind Epidemic of Staph ...( Government study found most MRSA infe...),Health

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

MONDAY, Jan. 21 (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
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