Effects of Combined 405-nm and 880-nm Light on Staphylococcus aureus and Pseudomonas aeruginosa in Vitro

Objective: The aim of this study was to determine the effect of a combination of 405-nm blue light and 880-nm infrared light on Staphylococcus aureus and Pseudomonas aeruginosa in vitro. Background Data: Reports indicate that certain wavelengths and treatment parameters of light promote the growth of bacteria, but our earlier study indicates that light at specific wavelengths and intensities are bactericidal for specific organisms(1). Methods: Two common aerobes, Staphylococcus aureus and Pseudomonas aeruginosa were tested because of their frequent isolation from skin infections and wounds. Each organism was treated simultaneously with a combination of 405-nm and 880-nm light emitted by a cluster of Super Luminous Diodes (SLDs). Doses of 1, 3, 5, 10, and 20 Jcm⁻² were used. Colony counts were performed and compared to untreated controls using Student t tests and one-way ANOVA with Tukey and Scheffe post hoc analyses. Results: The results revealed significant dose-dependent bactericidal effects of the combined blue and infrared light on Staphylococcus aureus(F _4,94 = 5.38, p = 0.001) and Pseudomonas aeruginosa (F _4,95 = 21.35, p < 0.001). With P. aeruginosa, the treatment reduced the number of bacteria colonies at all doses, achieving statistical significance at 1, 3, and 20 J cm⁻² doses and reducing bacterial colony by as much as 93.8%; the most effective dose being 20 J cm⁻². Irradiation of S. aureus resulted in statistically significant decreases in bacterial colonies at all dose levels; the most decrease, 72%, was also achieved with 20 Jcm⁻². Conclusion: Appropriate doses of combined 405-nm and 880-nm phototherapy can kill Staphylococcus aureus and Pseudomonas aeruginosa in vitro, suggesting that a similar effect may be produced in clinical cases of bacterial infection.