Until recently, the glycopeptides, notably vancomycin, were the mainstay of treatment for 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. In 2010, four VRSA isolates had 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 mecA for methicillin, vanA codes for resistance to vancomycin) could have occurred in this setting. As of 2019, 52 VRSA strains have been isolated worldwide. In 2021, the Centers for Disease Control and Prevention (CDC) confirmed the 16th case of VRSA in the United States. Because of the exchange of genetic material (vanA gene) from vancomycin-resistant enterococci (VRE) to methicillin-resistant Staphylococcus aureus (MRSA), allowing for the emergence of VRSA, CDC requests that clinical laboratories save these patient isolates for confirmation by public health partners. The emergence of these strains is alarming because they demonstrate complete vancomycin resistance.
