Improvement of Antibacterial Efficacy Through Synergistic Effect in Photodynamic Therapy Based on Thiazinium Chromophores Against Planktonic and Biofilm-Associated Periodontopathogens

Objective: Aim of the study was to improve the antibacterial efficacy of toluidine blue (TBO)/methylene blue (MB)-mediated photodynamic systems with light-emitting diode (LED) or laser irradiation administered to planktonic and biofilm-associated periodontopathogens. Background data: Antibacterial photodynamic therapy (PDT) is a common, noninvasive adjunctive clinical method to inactivate microorganisms. So far, the disadvantage of this method has been its limited effectiveness in eliminating pathogens. Methods: An anaerobic cocktail consisting of six representative periodontal pathogens was prepared as initial culture for planktonic samples and biofilms grown on human tooth slides. Both types of microbial samples were exposed to three commercial photodynamic systems (PDT1: TBO, 630 nm LED, PDT2: TBO, 635 nm laser, PDT3: MB, 665 nm laser) in conventional and a new modified approach (PDTplus) based on the use of an oxygen supplement (photosensitizer+hydrogen peroxide). The microbial viability was characterized by bacterial growth [colony forming units (CFU)], total bacterial cell counts, and microbial vitality. Statistical data analysis was performed using 95% confidence intervals (ANOVA) and post hoc Tukey's test (p < 0.05). Results: The modified PDTplus showed the highest statistically significant synergistic antimicrobial activity for TBO-based systems evidenced by a CFU reduction of 9 log10 units to 0 for planktonic pathogens and a 4 log10 CFU reduction for biofilm bacteria. The MB-based PDTplus was superior mainly against biofilm pathogens. By comparison, the default TBO-based PDT achieved colony growth reductions of 2 and 1 log10 units concerning planktonic and biofilm cells. Conclusions: Compared to conventional PDT, PDTplus showed superior antibacterial efficacy based on its synergistic effect, promising vast application possibilities.