Gold Nanoparticles Enhance the Toluidine Blue-Induced Lethal Photosensitisation of Staphylococcus aureus

Lethal photosensitisation of microbes forms the basis of a new approach to the prevention and treatment of infectious diseases. In this study we report on the ability of gold nanoparticles to enhance the antimicrobial effectiveness of the photosensitiser toluidine blue O (TBO) when illuminated with white light. A suspension of Staphylococcus aureus (approximately 107-108 cfu/ml) was exposed to white light in the presence of either TBO or a combination of TBO and gold nanoparticles and the number of survivors determined. The presence of either 2 nm or 15 nm nanogold particles resulted in an approximately one log10 enhancement of the kills achieved using TBO alone. Such an enhancement, if operative in vivo, would improve the efficacy of antimicrobial photodynamic therapy. Mechanistic studies showed that the 15 nm gold nanoparticles enhance the light capturing ability of the TBO. Furthermore the excited TBO molecules relax through a non-radiative process, but not through the production of singlet oxygen.