In vitro bactericidal activity of 465–470 nm blue light phototherapy and aminolevulinic acid on Staphylococcus pseudintermedius

Background: Staphylococcus pseudintermedius is the principal pathogen causing bacterial skin infections in dogs. Photodynamic therapy (PDT) involving the combination of light and a topical photosensitizer is used to treat human skin infections. Although the antimicrobial effects of PDT have been demonstrated using in vivo and in vitro studies in humans, its effects on dogs and their pathogens are unclear. Objectives: The aim of this study was to demonstrate the in vitro efficacy of PDT over a 465–470 nm spectrum to kill S. pseudintermedius using δ-aminolevulinic acid (ALA) as the photosensitizer. Methods: Six S. pseudintermedius isolates from canine skin were exposed to blue light-emitting diodes (LEDs) at 465–470 nm, with or without ALA. The light doses were 18.4, 36.8 and 55.2 J/cm². The number of colony-forming units and optical densities of broth cultures were measured and then compared with Dunnett's test. Bacterial viability was monitored using fluorescence microscopy and the fluorescence intensity values were compared with a paired Student's t-test. Results: Blue light inhibited the growth of S. pseudintermedius; the effect significantly increased with the addition of ALA as a photosensitizer and with increasing light doses. Live/dead staining confirmed that PDT reduced bacterial viability and exerted an antibacterial effect. Conclusion and clinical importance: Blue light has a strong antibacterial effect on S. pseudintermedius in a light dose-dependent manner. ALA alone did not exhibit bactericidal action, but its combination with blue light increased the effect of PDT compared to blue light alone.