The role of reactive oxygen species in UVA-mediated killing of Escherichia coli

Solar Water Disinfection (SODIS) is a simple method used for the microbial decontamination of water using ultraviolet-A radiation. Microbial killing in this method is proposed to occur by the light-dependent increase in reactive oxygen species (ROS) production by intracellular photo-sensitizing enzymes. ROS can irreversibly damage DNA leading to cell death. To test the potential role of ROS, strains with different catalase phenotypes were subjected to SODIS. Escherichia coli with wild type and mutated catalase genes were exposed to UV-A radiation for 90 minutes, with vigorous bottle agitation every 15 minutes. Increased killing was observed in kat mutants in absence of catalase. It was demonstrated that the absence of the katE gene (upregulated in aerobic conditions) causes a large increase in UV-A mediated cell killing. Catalase helps prevent bacterial cell death, likely by destroying the oxygen radicals created by UV-A exposure. Adding exogenous catalase to the environment generally caused a dosage-dependent increase in survival rates. However, these results were not reproducible and variation was observed. Varying oxygen concentration did not appear to effect survival rates uniformly; though this might have arisen because the bottles were not agitated to distribute oxygen and bacteria in this part of the experiment.