Photoinactivation of Planktonic and Biofilm Forms of Escherichia coli through the Action of Cationic Zinc(II) Phthalocyanines

Abstract

AbstractNovel water‐soluble zinc(II) phthalocyanines (ZnPc) peripherally substituted with 4‐dimethylaminopyridine (DMAP) units (ZnPc 1–2) and the corresponding quaternized derivatives (ZnPc 1 a–2 a) were synthesized and their photodynamic inactivation (PDI) efficiency against a Gram‐negative bacterium, a recombinant bioluminescent Escherichia coli strain either in planktonic or biofilm form, was investigated. The analysis of the photophysical properties revealed that the increase in the number of DMAP units on ZnPc shifted the absorption and emission band in aqueous media to the red region. The presence of multi‐positive charges on ZnPc derivatives prevented aggregation and enhances the solubility in aqueous media. The quaternized derivatives ZnPc 1 a and 2 a displayed good stability and promising efficacy to generate singlet oxygen (1O2). The affinity of the amphiphilic ZnPc 1–2 and quaternized ZnPc 1 a–2 a to planktonic bacterial cells corresponded to an average uptake of ∼106 PS molecules.CFU−1. The PDI assays conducted with planktonic cells and biofilms of E. coli show that irradiation with red or white light (150 mW.cm−2) in the presence of 20 μM of ZnPc derivatives caused an effective inactivation. ZnPc 1 a and ZnPc 2 a exhibited the highest inactivation efficiency, particularly of the planktonic form, causing a 5 log‐scale (99.99 %) reduction in bioluminescence. The inactivation factor for biofilms was 99 % (2 log). ZnPc 1 a and ZnPc 2 a can be regarded as promising photosensitizers for the photodynamic inactivation of Gram‐negative bacteria.