Antimicrobial Effect of Potassium Iodide Combined with Methylene Blue-Mediated Photodynamic Therapy on Cultured Subgingival Plaque-Derived Polymicrobial Suspensions

This study evaluated the antimicrobial efficacy of potassium iodide (KI) combined with methylene blue-mediated photodynamic therapy (MB-PDT) against cultured planktonic polymicrobial suspensions derived from clinical subgingival plaque samples. Bacterial suspensions were assigned to untreated control, light-only, KI-only, MB-PDT, and MB + KI-PDT groups. Neither light irradiation alone nor KI alone significantly affected viable bacterial counts compared with the untreated control. Antibacterial activity was assessed using colony-forming unit (CFU) assays and LIVE/DEAD fluorescence staining, and the virulence-related genes kgp, cdtB, and ltxA were evaluated by RT-qPCR. MB-PDT significantly reduced viable bacterial counts, and the addition of 80 mM KI further enhanced the antibacterial effect. The MB + KI-PDT group showed reductions of approximately 1.89 Log10 CFU/mL compared with the untreated control and 0.45 Log10 CFU/mL compared with MB-PDT alone. LIVE/DEAD staining supported the enhanced antibacterial activity of the combined treatment. The relative transcript levels of kgp, cdtB, and ltxA were also lower after MB + KI-PDT. These findings demonstrate that KI enhanced the antimicrobial activity of MB-PDT in a cultured subgingival plaque-derived planktonic polymicrobial suspension model. Further validation in structured multispecies biofilm models is required.