Photodynamic antimicrobial chemotherapy with cationic phthalocyanines against Escherichia coli planktonic and biofilm cultures

Background and Objectives: Photodynamic antimicrobial chemotherapy (PACT) has emerged as a hopeful method for treating many bacteria-related infections. Phthalocyanines (Pcs) are promising photosensitizers with high photosensitivity. The aim of this study was to investigate the antimicrobial activity of new octacationic zinc phthalocyanines bearing 1,2-ethanediamine groups and the quaternized derivatives with different positive charges (ZnPcn+, n = 4 or 8) against Escherichia coli (E. coli) in both planktonic and biofilm states. Methods: The uptake of Pcs in E. coli was evaluated according to photometry after alkaline lysis. The dark-toxicity, light-toxicity and light-mediated antimicrobial effect of the drug were determined by plate count method. Intracellular reactive oxygen species (ROS) production was detected by flow cytometry. Scanning electron microscopy (SEM) and propidium iodide (PI) staining were performed to assess the disruption of the biofilm and membrane integrity, respectively. Results: With the incubation time prolonged, the relative fluorescence intensity of the two Pcs increased and peaked at 40 minutes. Pcs alone and irradiation itself had no evident toxicity to the bacteria while a remarkable survival decrease was observed in the PACT groups in a light dose-dependent manner. ZnPc1 showed a more than 3-log reduction while ZnPc2 caused a nearly 5-log reduction of bacterial counts. Intracellular ROS levels were significantly enhanced by PACT treatment. The disruption of the biofilm and membrane integrity detected by SEM and PI staining suggested that Pcs-PACT can effectively damage the biofilm and cell membrane. Conclusion: All the results indicate that Pcs-PACT presents excellent bactericidal activity.