Comparative effect of photodynamic therapy on separated or mixed cultures of Streptococcus mutans and Streptococcus sanguinis

Antimicrobial photodynamic therapy (aPDT) has shown to exert a bactericidal effect against Streptococcus sanguinis and Streptococcus mutans. However, this efficacy has been reported for either type of bacteria separately. Bacterial suspensions of both strains, separately or together, were treated with concentrations of methylene blue (MB) and rose bengal (RB). Suspensions were irradiated with a light–emitting diode lamp (λ center at 625 nm for MB and λ center at 515 nm for RB) using a fluence of 18 J/cm². RB-aPDT at concentrations of 0.16–0.62 and 0.16–0.31 μg/mL, and MB-aPDT at concentrations of 0.62–1.25 and 0.31–1.25 μg/mL inhibited the growth of S. mutans and S. sanguinis respectively by 6 log₁₀. In suspensions of both strains together, the same 6 log₁₀ reduction in bacterial growth was achieved using the same concentrations of each photosensiziser. In conclusion, RB-aPDT and MB-aPDT appear to exert the same bactericidal effect against suspensions of S. sanguinis and S. mutans either for single strain treatment or for samples constituted by both bacteria mixed together. RB shows to be slightly more efficient than MB.