Inhibitory effects of calcium or magnesium ions on PDI

Photodynamic inactivation of microorganisms (PDI) finds use in a variety of applications. Several studies report on substances enhancing or inhibiting PDI. In this study, we analyzed the inhibitory potential of ubiquitous salts like CaCl2 and MgCl2 on PDI against Staphylococcus aureus and Pseudomonas aeruginosa cells using five cationic photosensitizers methylene blue, TMPyP, SAPYR, FLASH-02a and FLASH-06a. TMPyP changed its molecular structure when exposed to MgCl2, most likely due to complexation. CaCl2 substantially affected singlet oxygen generation by MB at small concentrations. Elevated concentrations of CaCl2 and MgCl2 impaired PDI up to a total loss of bacterial reduction, whereas CaCl2 is more detrimental for PDI than MgCl2. Binding assays cannot not explain the differences of PDI efficacy. It is assumed that divalent ions tightly bind to bacterial cells hindering close binding of the photosensitizers to the membranes. Consequently, photosensitizer binding might be shifted to outer compartments like teichoic acids in Gram-positives or outer sugar moieties of the LPS in Gram-negatives, attenuating the oxidative damage of susceptible cellular structures. In conclusion, CaCl2 and MgCl2 have an inhibitory potential at different phases in PDI. These effects should be considered when using PDI in an environment that contains such salts like in tap water or different fields of food industry

Interplay of phosphate and carbonate ions with flavin photosensitizers in photodynamic inactivation of bacteria

Photodynamic inactivation (PDI) of pathogenic bacteria is a promising technology in different applications. Thereby, a photosensitizer (PS) absorbs visible light and transfers the energy to oxygen yielding reactive oxygen species (ROS). The produced ROS are then capable of killing microorganisms via oxidative damage of cellular constituents. Among other PS, some flavins are capable of producing ROS and cationic flavins are already successfully applied in PDI. When PDI is used for example on tap water, PS like flavins will encounter various ions and other small organic molecules which might hamper the efficacy of PDI. Thus, the impact of carbonate and phosphate ions on PDI using two different cationic flavins (FLASH-02a, FLASH-06a) was investigated using Staphylococcus aureus and Pseudomonas aeruginosa as model organisms. Both were inactivated in vitro at a low light exposure of 0.72 J cm-2. Upon irradiation, FLASH-02a reacts to single substances in the presence of carbonate or phosphate, whereas the photochemical reaction for FLASH-06a was more unspecific. DPBF-assays indicated that carbonate and phosphate ions decreased the generation of singlet oxygen of both flavins. Both microorganisms could be easily inactivated by at least one PS with up to 6 log10 steps of cell counts in low ion concentrations. Using the constant radiation exposure of 0.72 J cm-2, the inactivation efficacy decreased somewhat at medium ion concentrations but reached almost zero for high ion concentrations. Depending on the application of PDI, the presence of carbonate and phosphate ions is unavoidable. Only upon light irradiation such ions may attack the PS molecule and reduce the efficacy of PDI. Our results indicate concentrations for carbonate and phosphate, in which PDI can still lead to efficient reduction of bacterial cells when using flavin based PS