The impact of antimicrobial photodynamic therapy in an artificial biofilm model

The susceptibility of bacterial cultures in biofilm formations is important for a variety of clinical treatment procedures. Therefore, the aim of the study was to assess the impact of laser-induced antimicrobial photodynamic therapy on the viability of Streptococcus mutans cells employing an artificial biofilm model. Using sterile chambered coverglasses, a salivary pellicle layer was formed in 40 chambers. Streptococcus mutans cells were inoculated in a sterile culture medium. Employing a live/dead bacterial viability kit, bacteria with intact cell membranes stained fluorescent green. Each pellicle-coated test chamber was filled with 0.7 ml of the bacterial suspension and analysed using a confocal laser scanning microscope within a layer of 10 μm at intervals of 1 μm from the pellicle layer. Phenothiazine chloride was used as a photosensitizer in all 40 test chambers. A diode laser (wavelength 660 nm, output power 100 mW) was used to irradiated 20 chambers for 2 min. Fluorescence values in the test chambers after laser irradiation (median 2.1 U, range 0.4-3.4 U) were significantly lower than baseline values after adding the photosensitizer (median 3.6 U, range 1.1-9.0; p < 0.05). The non-irradiated control chambers showed no change in fluorescence at the end of an additional photosensitizer residence time of 2 min without laser irradiation (median 1.9 U, range 0.7-3.6; median 1.9 U, range 0.8-6.0, respectively; p > 0.05). The present study indicated that laser irradiation is an essential part of antimicrobial photodynamic therapy to reduce bacteria within a layer of 10 μm. Further studies are needed to evaluate the maximum biofilm thickness that still allows a toxic effect on microorganisms.