Effectiveness of photodynamic therapy against Escherichia coli: Influence of light dose fractionation in the presence of porphyrins, methylene blue and zinc phthalocyanine disulfonate

Excessive use of antibiotics contributes to the development of multidrug-resistant microorganisms, making bacterial infections more difficult to treat. As an alternative method, photodynamic therapy is being explored. This therapy relies on generating cytotoxic concentrations of reactive oxygen species (ROS) during the interaction of a photosensitizer with light and molecular oxygen. In the presented work, we investigated the antibacterial efficacy of four different photosensitizers: 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin (TmPyP), 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin zinc (ZnTPPS), zinc phthalocyanine disulfonate (ZnPcS), and tetramethylthionine chloride (methylene blue, MB) on Escherichia coli. The results showed that this type of bacteria is susceptible to all of the studied photosensitizers. Our measurements revealed that a bacteriostatic or bactericidal effect can be achieved either by exposure to high-dose (50 J/cm²) violet (414 nm) light or by combining high-dose violet light with TmPyP or ZnTPPS. However, when using red-light-sensitive ZnPcS or MB, significant inhibition of bacterial growth occurred after three cycles of light exposure at a relatively low individual light dose (10 J/cm²), rather than a single high dose (50 J/cm²). Additionally, the fluorescent probe CellROX Red, which monitors ROS production, showed a significant increase in fluorescence in the presence of porphyrin photosensitizers as early as after the first irradiation.