The lingering houseguest is a comedy staple—as in “The Thing That Wouldn’t Leave,” an old SNL skit—but when the visitor is MRSA, it’s no laughing matter. MRSA, or methicillin-resistant Staphylococcus aureus, is resistant to almost all antibiotics related to penicillin. Of particular concern is the virulent and easily transmissible USA300 strain, which has been responsible for most community-associated infections since the 1990s. USA300 most commonly causes skin and soft tissue infections (SSTIs). It can also result in necrotizing pneumonia and endocarditis.

According to a new study, USA300 can settle into households, stay for years, and spread from person to person. Also, when USA300 turns homebody, it can evolve so that it becomes genetically unique to a particular abode.

These results appeared March 10 in the journal mBio, in an article entitled, “Transmission and Microevolution of USA300 MRSA in U.S. Households: Evidence from Whole-Genome Sequencing.”

“We performed whole-genome sequencing of 146 USA300 MRSA isolates from SSTIs and colonization cultures obtained from an investigation conducted from 2008 to 2010 in Chicago and Los Angeles households that included an index case with an S. aureus SSTI,” wrote the authors. “Identifying unique single nucleotide polymorphisms and analyzing whole-genome phylogeny, we characterized isolates to understand transmission dynamics, genetic relatedness, and microevolution of USA300 MRSA within the households.

The study’s authors, scientists from the University of Chicago and Emory University, also compared genetic information from the reexamined MRSA samples with previously published genome sequences of 35 USA300 MRSA isolates from San Diego and 277 USA300 MRSA isolates from New York City, as well as with the completed genomes of the bacteria USA300 TCH1516 and FPR3757. They created an evolutionary tree to show the relationships among the bacterial strains.

The researchers found that isolates within households clustered into closely related groups, suggesting a single common USA300 ancestral strain was introduced to and transmitted within each household. Researchers also determined from a technique called Bayesian evolutionary reconstruction that USA300 MRSA persisted within households from 2.3 to 8.3 years before their samples were collected, and that in the course of a year, USA300 strains had a 1 in a million chance of having a random genetic change, estimating the speed of evolution in these strains. Researchers also found evidence that USA300 clones, when persisting in households, continued to acquire extraneous DNA.

“We found that USA300 MRSA strains within households were more similar to each other than those from different households,” noted senior study author Michael Z. David, M.D., Ph.D., an assistant professor of medicine at the University of Chicago. Although MRSA is introduced into households rarely, he said, once it gets in, “it can hang out there for years, ping-ponging around from person to person. Our findings strongly suggest that unique USA300 MRSA isolates are transmitted within households that contain an individual with a skin infection.”

The authors of the mBio article also indicated that that fluoroquinolone-resistant USA300 clones emerged around 1995 and were more widespread in Los Angeles and New York City than in Chicago. “Our findings strongly suggest that unique USA300 MRSA isolates are transmitted within households that contain an individual with an SSTI,” they concluded. “Decolonization of household members may be a critical component of prevention programs to control USA300 MRSA spread in the United States.”








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