Until recently, the glycopeptides, notably vancomycin, were the mainstay of treatment of infections caused by MRSA; however overuse of vancomycin has led to the emergence of vancomycin-intermediate S. aureus (VISA) and vancomycin-resistant S. aureus (VRSA) strains.
The first reports of S. aureus
strains with reduced susceptibility (MIC 4-8 µg/mL) came from Japan
in 1997. S. aureus
strains with reduced susceptibility have since been reported worldwide.
In 2002, the first strain of VRSA was isolated in the United States in Michigan
. As of 2010, four VRSA isolates have been identified in the US. In three of the four cases, a strain of vancomycin-resistant Enterococcus
(VRE) was also isolated from the same patient and it is believed that transfer of the vanA
gene (like mec
A for methicillin, van
A codes for resistance to vancomycin) could have occurred in this setting. The emergence of these strains is alarming because they demonstrate complete vancomycin resistance.