Acid-Tolerant Photosensitizer: Photodynamic Inactivation of Porphyromonas gingivalis by 2′,4′,5′,7′-Tetraiodofluorescein

Porphyromonas gingivalis(P. gingivalis) is a primary pathogen in periodontitis, yet its elimination is limited by complex anatomical structures. Photodynamic therapy (PDT) is a promising adjunct, but its antimicrobial efficacy is compromised in the acidic microenvironment induced byP. gingivalis. Given that 2′,4′,5′,7′-tetraiodofluorescein (TIF) exhibits robust and stable photodynamic activity under acidic conditions, this study investigated the antibacterial effect of TIF-mediated PDT (TIF-PDT) againstP. gingivalis.P. gingivalisATCC 33277 was treated with TIF (5, 10, 20, and 40 μM), light (525 nm), or both. Reactive oxygen species (ROS) generation was assessed at pH 4.5 or 7.4. Bacterial viability and membrane integrity were evaluated by colony-forming unit (CFU) assay and LIVE/DEAD staining. CFU assays demonstrated that TIF-PDT groups achieved an approximately 4-log reduction in bacterial viability compared to the DEMI, Light, and TIF groups, with no dark cytotoxicity and light-alone effects. LIVE/DEAD staining revealed bright yellow fluorescence in the TIF-PDT (40 μM), indicating membrane damage and significantly lower survival rates than controls. TIF-PDT at 10, 20, and 40 μM produced ROS under both neutral and acidic conditions, exhibited low dark cytotoxicity, and demonstrated potent antibacterial activity againstP. gingivalisin vitro, suggesting its potential as an acid-tolerant photosensitizer for periodontitis adjunctive therapy.