Abstract
Introduction Antibacterial photodynamic therapy (aPDT) and antibacterial blue light (aBL) are emerging treatment methods auxiliary to mechanical debridement for periodontitis. APDT provided with a near infrared (NIR) light in conjunction with an indocyanine green (ICG) photosensitizer has shown efficacy in several dental in-office-treatment protocols. In this study, we tested Streptococcus mutans biofilm sensitivity to either single-light (aPDT or aBL) or dual-light aPDT (simultaneous aPDT and aBL) exposure.
Materials and Methods Biofilm was cultured by pipeting diluted Streptococcus mutans suspension with growth medium on the bottom of well plates. Either a single-light aPDT (810-nm aPDT or 405-nm aBL) or a dual-light aPDT (simultaneous 810-nm aPDT and 405nm aBL) was applied, in both cases together with the ICG photosensitizer, while keeping the total light energy constant at 100J/cm2. Single-dose light exposures were given after one-day, four-day or fourteen-day biofilm incubations. Also, a repeated daily dose of the same light energy was applied during biofilm incubations on four-day and fourteen-day biofilms. Finally, antibacterial action of the dual-light aPDT with different relative ratios of 810 nm and 405 nm of light energy was examined on the single-day and four-day protocols. Biofilms were scraped, diluted into ratios between 1:1 to 1:100 000, and plated. After re-incubation, colony-forming units (CFUs) were counted, and confocal 3D biofilm imaging was performed.
Results On a one-day biofilm, dual-light aPDT was significantly more efficient than single-light aBL or aPDT, although all modalities were bactericidal. On a four-day maturated biofilm, a single exposure of aPDT or dual-light aPDT was more efficient than aBL, resulting in a four logarithmic scale reduction in bacterial counts. Surprisingly, when the same amount of aPDT was repeated with a daily dosing on a four-day or a fourteen-day biofilm, bacterial viability improved significantly. A similar but milder response of improved bacterial viability was seen after repetitive aBL application. The viability improvement was eliminated when dual-light aPDT was applied. By changing the relative light energy ratios in dual-light aPDT, a relative increase in aBL improved the antibacterial action of dual-light aPDT when the biofilm was older.
Conclusion When aPDT is administered repeatedly to S. mutans biofilm, a single wavelength-based aBL or aPDT leads to a significant biofilm adaptation and increased S. mutans viability. The combined use of aBL light in synchrony with aPDT arrests the adaptation and provides significantly improved and sustained antibacterial efficacy.
