Bactericidal Effect of 0.95-mW Helium-Neon and 5-mW Indium-Gallium-Aluminum-Phosphate Laser Irradiation at Exposure Times of 30, 60, and 120 Seconds on Photosensitized Staphylococcus aureus and Pseudomonas aeruginosa In Vitro

Studies have demonstrated a bactericidal effect of laser irradiation when lasers with power outputs of (6 mW are directed toward pathogenic or opportunistic bacteria previously treated with a photosensitizing agent. The purpose of this study was to determine the bactericidal capabilities of irradiation from lasers with power outputs of less than 6 mW on photosensitized microorganisms. Two bacteria that commonly infect skin lesions, Staphylococcus aureus and Pseudomonas aeruginosa, were used. The 2 lasers used, the 0.95-mW helium -neon laser and the 5-mW indium-gallium-aluminum-phosphate laser, emit light at a wavelength close to the absorption maxima of the sensitizing agent chosen, toluidine blue O. This agent was used because of its proven effectiveness in sensitizing bacteria. For each bacterial strain, toluidine blue O was added to a 108 cells/mL solution until a 0.01% weight/volume ratio was obtained. These mixtures were spread on agar-coated petri dishes, which were then exposed to 1 of the 2 lasers for 30, 60, and 120 seconds. The cultures were then grown overnight and examined for one or more visible zones of inhibition. The areas surrounding the irradiated zone provided a control for the effects of toluidine blue O alone. To determine the effects of laser irradiation without prior toluidine blue O sensitization, separate plates were established using unsensitized bacteria. Although inconsistencies between plates were noted, both lasers produced at least one zone of inhibition in both bacterial species at all 3 time periods. The 5-mW laser, however, produced a greater number of these zones. Laser-induced microbial killing of photosensitized organisms could have clinical applications in the treatment of infected skin lesions, pending in vivo studies.