Photodynamic inhibition of Trichophyton rubrum: in vitro activity and the role of oxidative and nitrosative bursts in fungal death.

Objectives: Antimicrobial photodynamic inhibition (aPI) is based on the use of a light source and a photosensitizer to kill pathogens. Little is known about aPI of dermatophytic fungi and its mechanism of action. We aimed to evaluate aPI of Trichophyton rubrum.

Methods: We performed tests using toluidine blue (TBO) as a photosensitizer and a 630 nm light-emitting diode (LED) as a source of light to target 12 T. rubrum isolates. Susceptibility testing with cyclopiroxolamine, time-kill curves and quantification of reactive oxygen species (ROS), peroxynitrite (ONOO·) and nitric oxide (NO·) were performed.

Results: The optimal conditions for in vitro aPI were 10 mg/L for TBO and 48 J/cm(2) for LED; these conditions were fungicidal or inhibited >98% of fungal growth depending on the strain tested. LED or TBO treatment alone did not inhibit growth. The MICs of cyclopiroxolamine were 2.0 mg/L for 90% of the strains. Analysis of time-kill curves revealed that pathogen death occurred 24 h post-treatment. Quantification of ROS, ONOO· and NO· revealed improvement after aPI.

Conclusions: Photodynamic inhibition was more efficient in promoting cell death than the antifungal cyclopiroxolamine against T. rubrum. ROS, ONOO· and NO· were important in the fungicidal activity of aPI. A suggested mechanism for this activity is that TBO is excited by LED light (630 nm), reacts with biomolecules and increases the availability of transition electrons and substrates for nitric oxide synthase, thereby increasing the oxidative and nitrosative bursts in the fungal cell.