What are the mechanisms of resistance for VRSA and VISA?
All VRSA isolates to date contained the vanA vancomycin resistance gene. The vanA gene is usually found in enterococci and typically confers high-level vancomycin resistance (MICs= 512-1024µg/ml) to these organisms. Most VRSA-positive patients had a history of infections caused by both vancomycin-resistant enterococci (VRE) containing vanA and MRSA. It is likely that the vanA determinant was transferred via plasmids or transposons from the VRE to the MRSA strain, resulting in the VRSA. The mechanism of reduced vancomycin susceptibility in VISA strains is not fully understood. VISA cells have thicker cell walls that contain many cell wall subunits capable of binding vancomycin extracellularly and changes in several metabolic pathways.. Vancomycin must reach the cell membrane and bind to the growing cell wall complex to inhibit cell growth.
