The effect of antimicrobial photodynamic therapy (aPDT) on Gram-positive bacterium Staphylocccus aureus was studied. Methylene blue (MB) at non-toxic concentration of 31.25µg/ml was used as a photosensitizer. LEDs diodes were used as a light source to study the effect of methylene blue alone and the MB-gold nanoparticle mixture on the viability of S. aureus cells. Biogenic gold nanoparticles (biolAuNPs, 10ppm) and chemically synthesized gold nanoparticles (chemAuNPs, 3ppm) were tested as enhancement agents. In the presence of MB alone as a photosensitizer, the killing effect was about 92% after 30min of irradiation. The aPDT therapy was enhanced by addition of biolAuNPs and chemAuNPs and killing rate of S. aureus was 95-96% after 30min of irradiation. The probable mechanism of enhancement of MB-mediated photodynamic bactericidal efficacy against S. aureus in the presence of gold nanoparticles is discussed leading to the conclusion that colloidal gold increases the accumulation of MB in bacterial cells.