Covalently coupling toluidine blue O–tiopronin to a gold nanoparticle forms an enhanced, exceptionally potent antimicrobial agent when activated by white light or 632 nm laser light. Aqueous solutions of tiopronin–gold nanoparticles had no antimicrobial effect and, when added to solutions of toluidine blue O, did not enhance the antimicrobial effect of the latter. The minimum bactericidal concentration of the covalently coupled toluidine blue O–tiopronin gold conjugate for Staphylococcus aureus was at least four times lower than that of free toluidine blue O.