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article imageMRSA dominates with the help of skin bacteria

By Tim Sandle     Mar 5, 2014 in Science
There could be an explanation for how the most common strain of methicillin-resistant Staphylococcus aureus (MRSA) rapidly rose to prominence. MRSA is a common cause of hospital acquired infections.
MRSA is a strain of staphylococcus bacteria that is resistant to methicillin and certain first-line antibiotics called beta-lactams. The bacteria can cause serious infections of the skin, blood, lungs and bones. Infections with drug-resistant strains, like MRSA, can be particularly difficult to treat.
The appearance of a certain strain of MRSA — called USA300 — as a major pathogen at the turn of the century has puzzled scientists. That might be about to change according to new research published in the journal mBio®. The research suggests that the MRSA strain recently acquired a number of genes from common skin bacteria that allow it to grow and thrive on the skin where other strains of of bacteria cannot.
Since it was first identified in the late 1990s the USA300 strain of MRSA has undergone an extremely rapid expansion across the U.S. It is now the predominant cause of community-acquired MRSA skin and soft tissue infections and has been implicated in MRSA outbreaks among professional football teams. The strain is genetically distinguished from other strains by a cluster of genes known as the arginine catabolic mobile element (ACME).
Using phylogenetic analysis researchers have shown that the segments of ACME were assembled in a generally harmless bacterium called Staphylococcus epidermidis (which typically found on human skin). These segments then appear to have horizontally transferred to the common ancestor of USA 300 strains. This is thought to have been a very recent event that coincided with the emergence and spread of this strain.
The findings suggest that these properties gave USA 300 a major selective advantage during skin infection and colonization, contributing to its unfortunate evolutionary success as a major hospital contaminant.
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