An international team led by investigators at the Massachusetts Eye and Ear Infirmary reports in the American Society for Microbiology journal mBio this week on its comparative genomic analysis of vancomycin-resistant Staphylococcus aureus strains. The researchers specifically focused on the strains' positions within the clade most commonly associated with hospital-acquired methicillin-resistant S. aureus infection in the US — clonal cluster 5, or CC5.
The team produced draft genome sequences for all available VRSA strains, each of which was thought to represent a new acquisition of the vancomycin-resistant transposon Tn1546 from enterococcal donors. In their analysis, the researchers found that each strain acquired Tn1546 independently and that they last shared a common ancestor more than 50 years ago, "well before the occurrence of vancomycin resistance in this species," as they write in mBio. The team also found that VRSA and CC5 strains share "a constellation of traits that appears to be optimized for proliferation in precisely the types of polymicrobic infection where transfer could occur."
In a statement, lead author Veronica Kos from Massachusetts Eye and Ear says that while "this group of MRSA has properties that appear to make it more social, so they can live with other bacteria like Enterococcus," and thus allow it to more readily take up new resistances, "some of these properties weaken the strain's ability to colonize, and may be limiting their spread."