Community Associated MRSA Evolved from a Single Strain

Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) is a recent public health concern that typically causes treatable skin infections such as boils, but also can lead to life-threatening conditions that are more difficult to treat. A study from the National Institute of Allergy and Infectious Diseases (NIAID) of NIH, reports that infections are caused mostly by a single strain, known as USA300, of an evolving bacterium. It has spread with “extraordinary transmissibility” throughout the United States during the past five years, according to the study [1], published online in the Proceedings of the National Academy of Sciences.

Another recent study [2] from the same research team examines the issue of the evolution of MRSA further, revealing new information about how MRSA bacteria in general, including the USA300 group, elude the human immune system. These studies are giving scientists a clearer idea of how resilient such bacteria can be.

“The USA300 group of strains appears to have extraordinary transmissibility and fitness,” says Dr. Frank R. DeLeo PhD, and lead researcher. “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.” Ultimately, Dr. DeLeo and his researchers hope that their work will lead to the development of new diagnostic tests that can quickly identify specific strains of MRSA. This is the first time scientists have used comparative genome sequencing to reveal the origins of epidemic CA-MRSA.

Small Genetic Changes Equal Big Changes in Diseases

To figure out how CA-MRSA is evolving spreading around the world, Dr. DeLeos group sequenced the genomes of 10 patient samples of USA300 recovered from patients treated at various U.S. locations between 2002 and 2005. Then they compared the genomes to each other, as well as to a baseline USA300 strains 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. The fact that out of the eight nearly indistinguishable USA300 patient samples, two caused far fewer deaths in laboratory mice than the others, highlights an emerging view that tiny genetic changes among evolving strains can profoundly affect disease severity and the potential for drug resistance to develop.

The neutrophil study gives scientists new details concerning the mechanisms MRSA uses to prevent its destruction by neutrophils, human white blood cells that ingest and destroy microbes. Neutrophils react within an hour of tissue injury and are the textbook sign of acute inflammation [3]. When exposed to hydrogen peroxide, hypochlorous acid (the active ingredient in household bleach), or antimicrobial proteins—all killer chemicals released by neutrophils—MRSA senses danger, escapes harm and turns the tables on the white blood cells, destroying them. Work is continuing in Dr. DeLeos lab to understand how the bacterium senses and survives attacks by neutrophils.


1. A Kennedy et al. Epidemic community-associated methicillin-resistant Staphylococcus aureus: Recent clonal expansion and diversification. Proceedings of the National Academy of Sciences DOI:10.1073/PNAS.0710217105 (2008)

2. A Palazzolo-Ballance et al. Neutrophil microbicides induce a pathogen survival response in community-associated methicillin-resistant Staphylococcus aureus. The Journal of Immunology (2008) 180:500-509.

3. Cohen, Stephen. Burns, Richard C. Pathways of the Pulp, 8th Edition. St. Louis: Mosby, Inc. 2002. page 465.

Image: S. aureus bacteria escaping destruction by human white blood cells. Rocky Mountain Laboratories, National Institute of Allergy and Infectious Diseases (NIAID)