Bacterial photodynamic inactivation (PDI) is based in the administration of a photosensitizer, which is preferentially accumulated in the microbial cells. The subsequent irradiation with visible light, in the presence of oxygen, specifically produces cell damages that inactivate the microorganisms. Gram-negative bacteria show a remarkable resistance to negatively charged or neutral agents. This resistance has been ascribed to the presence of highly organized outer membrane, which hinders the interaction of the photosensitizer with the cytoplasmic membrane and intercepts the photogenerated reactive species.
In this work, the photodynamic activity of three tetracationic phthalocyanines derivatives were compared in homogeneous medium bearing a photooxidizable substrate and in vitro using a typical Gram-negative bacterium, Escherichia coli.
Phthalocyanines derivatives exhibit a high absorption coefficient in the visible region of the spectrum, mainly in the phototherapeutic window (600-800 nm) and fluorescence quantum yields (fF) of ~0.2-0.3. Production of O2(1Dg) was evaluated using 9,10-dimethylanthracene (DMA) yielding values of FD=0.4-0.6. In vitro studies show that these photosensitizers (2 mM) are rapidly bound to E. coli cells in 5 min, reaching a value of ~0.8 nmol/106 cells. After irradiation with visible light, a higher photoinactivation of cells was found for ZnPPc4+, which produces a ~3.5 log (99.97%) decrease of cell survival. This treatment also produces damages in E. coli plasmids and genomic DNA. Moreover, this photosensitizer was evaluated for sterilization of blood contaminated with microorganisms. The studies show that cationic ZnPPc4+ is an efficient phototherapeutic agent with potential applications in photodynamic inactivation of bacteria.
