Development of a phthalocyanine-based photosensitizer for enhanced photodynamic inactivation of Gram-negative and Gram-positive bacteria

Light-triggered antibacterial activity can be achieved with appropriate photosensitizers (PS). Quaternized PS are attractive molecules for antimicrobial photodynamic therapy (aPDT) applications. In this study, we synthesized a water-soluble Zn(II) phthalocyanine derivative (Full-COOH-ZnPc) that contains four quaternized groups on the periphery of the macrocycle as well as carboxylic groups. The photochemical and photophysical properties of the compound were examined to determine its potential as a PS in aPDT applications. The antibacterial activity of Full-COOH-ZnPc was tested against Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Staphylococcus aureus, Enterococcus faecalis, and Bacillus cereus. Analyses showed species- and dose-dependent responses to Full-COOH-ZnPc treatment following photoactivation, likely due to differences in cell membrane structure and protective mechanisms such as capsules. The results also highlighted the strong antibacterial effects of ZnPc, especially against S. aureus and E. faecalis, with no detectable dark toxicity, emphasizing the aPDI potential of the Pc compound against Gram-positive bacteria. Further optimization of the compound’s parameters and structure could improve its antimicrobial effectiveness against other bacterial species.