Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) infections are caused primarily by a single strain USA300 of an evolving bacterium that has spread with "extraordinary transmissibility" throughout the United States during the past five years, according to a new study led by National Institutes of Health (NIH) scientists.
CA-MRSA, an emerging public health concern, typically causes readily treatable soft-tissue infections such as boils, but also can lead to life-threatening conditions that are difficult to treat.
The study, from the National Institute of Allergy and Infectious Diseases (NIAID) of NIH, resolves debate about the molecular evolution of CA-MRSA in the United States. The findings rule out the previously held possibility that multiple strains of USA300, the most troublesome type of CA-MRSA in the United States, emerged randomly with similar characteristics. The study also offers a hypothesis for the origin of previous S. aureus outbreaks, such as those caused by penicillin-resistant strains in the 1950s and 1960s.
A second study led by the same NIAID scientists takes the issue of the evolution of MRSA a step further, revealing new information about how MRSA bacteria in general, including the USA300 group, elude the human immune system.
The first study, which appears online this week in the Proceedings of the National Academy of Sciences, found that the USA300 group of CA-MRSA strains, collectively called the epidemic strain, comprises nearly identical clones that have emerged from a single bacterial strain. It is the first time scientists have used comparative genome sequencing to reveal the origins of epidemic CA-MRSA. Frank R. DeLeo, Ph.D., at NIAID's Rocky Mountain Laboratories (RML) in Hamilton, Mont., led the research.
“Scientists are pressing ahead quickly to learn more about how some MRSA strains evade the immune system and spread rapidly,” says NIAID Director Anthony S. Fauci, M.D. “The information presented in these two studies adds important new insights to that expanding knowledge base.”
To understand how CA-MRSA is evolving in complexity and spreading geographically, Dr. DeLeo's group sequenced the genomes of 10 patient samples of the USA300 bacterium recovered from individuals treated at different U.S. locations between 2002 and 2005. They then compared these genomes to each other and to a baseline USA300 strain used in earlier studies. Eight of the 10 USA300 patient samples were found to have nearly indistinguishable genomes, indicating they originated from a common strain. The remaining two bacteria were related to the other eight, but more distantly.
Interestingly, of the eight nearly indistinguishable USA300 patient samples, two caused far fewer deaths in laboratory mice than the others, highlighting an emerging view that tiny genetic changes among evolving strains can profoundly affect disease severity and the potential for drug resistance to develop.