Optimizing of photodynamic parameters: determining the key factors for effective inactivation of Streptococcus mutans biofilms with phycocyanin.

For higher photodynamic efficiency, much more knowledge about the importance of the multiple parameters is required. In this in vitro study, optimized parameters (dose, light intensity, and irradiation time) were defined for the effect of phycocyanin (PC) concentrations (sub-minimum inhibitory concentrations (sub-MIC) ) activated with a 635 nm diode laser to reduce the Streptococcus mutans biofilms. Two power densities, 0.2 and 1 W/cm, were compared at different irradiation times, resulting in light energy doses of 12-60 J/cm. The light source was a diode laser (wavelength = 635 nm) and a set output power of 100 and 500 mW. After the treatments, the biofilms were analyzed based on the log colony forming units/milliliter of S. mutans. A significant biofilm reduction occurred at all evaluated fluences and concentrations except the 1/8 MIC concentration of PC compared to the control. An increase in PC concentration and power density together was associated with a greater biofilm reduction. Antimicrobial photodynamic therapy (aPDT) was more efficient against S. mutans biofilm cells at a ½ MIC concentration of PC, a power density of 1 W/cm and 60 J/cm energy dose (2 log reduction). The energy dose should not be evaluated in isolation to compare the photoinactivation effects on S. mutans biofilms. In addition, the appropriate duration of light exposure and photosensitization concentration should not be disregarded. These characteristics will contribute to the success of aPDT in the future.