Objectives: To explore the potential synergistic effects between blue light (wavelength, 400-500 nm) and antibacterial agents commonly used in dentistry. Methods: Streptococcus mutans where grown in BHI media and incubated over night. The bacteria were transferred to PBS. The bacteria (in PBS) were transferred to a microplate with different concentrations of hydrogen peroxide(0.1%, 0.01%, 0.001%). Each sample contained 50 µl bacteria and 50 µl hydrogen peroxide. Each sample was irradiated by a plasma arc curing (PAC) light source during different time exposures (1, 2, 3 and 10 minutes). After irradiation 100 µl BHI, at double the normal concentration, was added to each sample. The samples where incubated for 24 hours at 37C and bacterial growth was determined using a microplate reader. The effectiveness of the procedure was assessed by the light absorbance of the samples (wavelength of 650 nm) in comparison with the control samples. The same procedure was repeated with cetylpyridinium chloride (CPC) (0.05, 0.5, 2.5 and 5 µM). Results: In all tested time exposures we found that using hydrogen peroxidein concentrations lower than the MIC, immediately followed by irradiation with blue light, had a bactericidal affect; whereas the control samples, containing the same hydrogen peroxide concentrations, showed normal bacterial growth. The experiments using CPC showed bacterial killing only with 10 minutes of irradiation as did the control samples. Conclusion: These results show a synergistic effect between the blue light and hydrogen peroxide. On the other hand, CPC demonstrated no synergistic effect with blue light.

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