Biogenic gold nanoparticles enhance methylene blue-induced phototoxic effect on Staphylococcus epidermidis

There is considerable current interest in photodynamic inactivation (PDI) as potential antimicrobial therapy. This study reports successful implementation of PDI of Staphylococcus epidermidis using methylene blue (MB) in combination with biogenic gold nanoparticles (GNP). Monodispersed colloidal GNP were synthesized by reduction of Au+3 in the presence of cell-free filtrate of Trichoderma koningii and were characterized by a number of techniques including UV–Vis and fluorescence spectroscopy, transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FTIR) to be 12 ± 3 nm spherical gold particles coated with proteins. Studies on the role of the cell-free filtrate proteins in the synthesis of the GNP indicate that the process is nonenzymatic but involves interactions of various amino acids with gold ions. A Xe lamp (550–780 nm) or a He–Ne laser (632 nm) was used as light sources to study the effect of MB alone, the GNP alone, and the MB–GNP mixture on the viability of bacterial cells. Lethal photosensitization of S. epidermidis with the MB–GNP mixture was achieved after 5 and 10 min exposure to laser or Xe lamp, respectively. It has been found that the MB–GNP mixture exhibits a significant antibacterial activity already in the absence of any light source and gives an enhanced antimicrobial response when using either a laser or a Xe lamp source for photosensitization.