Vancomycin-resistant S. Aureus (VRSA)
High-level vancomycin resistance in S. aureus has been rarely reported. In vitro and in vivo experiments reported in 1992 demonstrated that vancomycin resistance genes from Enterococcus faecalis could be transferred by horizontal gene transfer to S. aureus, conferring high-level vancomycin resistance to S. aureus. Until 2002 such a genetic transfer was not reported for wild S. aureus strains. In 2002, a VRSA strain was isolated from the catheter tip of a diabetic, renal dialysis patient in Michigan. The isolate contained the mecA gene for methicillin resistance. Vancomycin MICs of the VRSA isolate were consistent with the VanA phenotype of Enterococcus species, and the presence of the vanA gene was confirmed by polymerase chain reaction. The DNA sequence of the VRSA vanA gene was identical to that of a vancomycin-resistant strain of Enterococcus faecalis recovered from the same catheter tip. The vanA gene was later found to be encoded within a transposon located on a plasmid carried by the VRSA isolate. This transposon, Tn1546, confers vanA-type vancomycin resistance in enterococci.
From 2002 to 2010, ten additional VRSA isolates were reported, eight from the United States, one from Iran, and one from India.
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