Growing concern of antibiotic resistance has increased research efforts to find nonspecific treatments to inhibit pathogenic microorganisms. In this regard, photodynamic inactivation is a promising method. It is based on the excitation of a photosensitizer molecule (PS) with UV-Vis radiation to produce reactive oxygen species. The high reactivity of such species nearby the PS leads to oxidation of bacterial cell walls, lipid membranes (lipid peroxidation), enzymes, and nucleic acids, eventually producing cell death. In the last decade, many studies have been carried out with different photosensitizers to suppress the growth of bacteria, fungi, viruses, and malignant tumors. Here, our main motivation is to employ pheomelanin nanoparticles as sensitizers for inhibiting the growth of the Gram-negative bacteria E. coli, exposed to blue and UVA radiation. In order to perform our experiments, we synthesized pheomelanin nanoparticles from L-DOPA and L-cysteine through an oxidation process. We carried out experiments at different particle concentrations and different energy fluences. We found that cultures exposed to UVA at 166 μg/mL and 270 J/cm2, in conjunction with ethylenediaminetetraacetic acid (EDTA) as an enhancer, decreased in the viable count 5 log10. Different reactive oxygen species (singlet oxygen, hydroxyl radicals, and peroxynitrates) were detected using different procedures. Our results suggest that the method reported here is effective against E. coli, which could encourage further investigations in other type of bacteria.

Bacterial resistance is today a matter of great medical concern, so it is urgent to investigate alternatives to alleviate it. Photodynamic inactivation (PDI) is a method that has been endorsed to inactivate different pathogens, including bacteria, fungi and viruses. PDI is achieved by using a photosensitizer (PS) molecule which generates reactive oxygen species under visible or UV radiation. We use visible light and UV-A radiation to excite four commercial PSs (methylene blue, rose bengal, riboflavin and curcumin), and nanoparticles synthesized in our laboratory. Despite these PSs having been thoroughly studied in the past by other research groups, in order to compare their effects in an appropriate way, we matched the number of photons they absorb. We found that methylene blue leads to the major inactivation of Escherichia coli. Furthermore, we evaluated the production of singlet oxygen and hydroxyl radicals in the photoinactivation process.