Killing of cutaneous microbial species by photodynamic therapy

Background Photodynamic therapy (PDT) utilizes photosensitizers and light, Whereas PDT use in cancer treatment has been widely accepted, antimicrobial PDT (APDT) is still in its early stages of development, Objectives To study microbial killing in vitro using APDT. Methods We used a combination of methylene blue and visible light, and a range of microbial species representative of those encountered on the skin in health and disease, Using standard light intensity conditions (slide projector, 25 cm distance from target, 42 mW cm(-2)) and methylene blue dye at 100 mug mL(-1), kill rates and subsequent D-values were determined against Straphylococcus aureus. S. epidermidis, Streptococcus pyogenes, Corynebacterium minutissimum, Propionibacterium acnes and Candida albicans. Results D-values for these species were 72, 66, 48, 120, 30 and 660 s, respectively. The effects of light intensity on the killing of S, epidermidis showed the kill rate to be proportional to the light intensity. A high rate of cell kill was also obtained using natural sunlight, Conclusions Overall, these results indicate that APDT of the skin may represent a useful alternative to conventional antimicrobial treatment.