Abstract: Objective:To explore the effect of potassium iodide (KI) on antimicrobial photodynamic therapy (aP-DT) mediated by Photofrin (PF) against Gram-negative bacteria.Methods:Pseudomonas aeruginosa (P.a), Klebsiella pneumoniae (K.p), Proteus mirabilis (P.m) and Acinetobacter baumannii (A.b) were used as experimenta subjects.After photodynamic treatment with varying concentrations of PF and KI, the CFU and survival rate of bacteria in each group were observed and calculated.In order to investigate the mechanism by which KI increased the effect of PF-mediated aPDT, Amplex Red was utilized to detect H₂O₂.Results:For these four Gramnegative bacteria with varying PF concentrations, there was no bactericidal impact when the light energy density was 10 J/cm².When the light energy density was 10 J/cm², 50 mmol/L KI combined with 10μmol/L PF could effectively kill A.b bacteria (P<0.001) and the combination of 100 mmol/L KI and 10μmol/L PF could achieve high bactericidal effect against P.a, K.p and P.m (P<0.001).When the light energy density was the same, the fluorescence amount of the 10μmol/L PF combined with 100 mmol/L KI group was significantly higher than that of the control group (P<0.05).Conclusion:The antibacterial effect of PF combined with KI against Gram-negative bacteria is significantly enhanced, and the enhanced effect may be due to the reaction of¹O₂with KI to generate H₂O₂.