The present invention relates to the identification of a specific cysteine (Cys) residue in MgrA of Staphylococcus aureus and homologs in other bacteria that acts as a redox switch to regulate virulence of S. aureus and expression of bacterial genes that confer resistance to antibiotics. In addition, MgrA is found to be a key virulence determinant for S. aureus. Oxidative regulation of other Cys containing proteins in various bacteria has been observed, leading to the ability to modulate virulence and resistance of these organisms.