HealthDay (1/22, Preidt) reports that, according to a study appearing online in the Proceedings of the National Academy of Sciences, a "single strain of an evolving bacterium has been responsible for most of the community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) infections." Researchers as the National Institute of Allergy and Infectious Diseases "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." After "analyzing the genomes of USA300 collected from 10 patients infected in different parts of the United States between 2002 and 2005," the team found that "[e]ight of the 10 samples had almost identical genomes, indicating they were from a common strain." Moreover, "two of the eight almost identical USA300 samples caused far fewer deaths in laboratory mice than the other samples," which may lend support to "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."