Effect of Ultraviolet-A Radiation on Water Disinfection and the Activity of Catalase and Superoxide Dismutase in Escherichia coli B23

The lack of safe potable water in many developing countries puts their citizens at high risk of contracting water-borne diseases. Solar water disinfection (SODIS) offers a simple, inexpensive method of sanitizing water by utilizing the ultraviolet-A radiation (UV-A) from sunlight to generate highly reactive, bacteriocidal forms of oxygen known as reactive oxygen species (ROS). However, previous studies have shown that some Escherichia coli can survive extended UV-A exposure. It is known that E. coli possess the enzymes catalase and superoxide dismutase (SOD), which convert specific ROS into less lethal forms for the bacteria. As such, we hypothesized that some E. coli cells can survive prolonged UV-A exposure by up-regulating catalase and/or SOD activity. To test this hypothesis, E. coliinoculated water samples were exposed to UV-A for 1.5 hours. The catalase activity in the exposed cells was assessed by a potassium dichromate colorimetric assay performed on their lysates, while SOD activity was assessed by a nitro blue tetrazolium colorimetric assay. The assays for catalase, SOD and protein yields between similarly-treated samples showed high variability. A two-tailed t-test at 95% confidence revealed no significant difference in catalase activity between control and treatment groups; however, the low number of replicates limits the significance of this finding. Accumulated oxidative stress may explain the lack of up-regulation in treated cells, as excessive accumulated oxidative stress can lead to the repression/inactivation of regulons responsible for catalase expression. The activity of SOD was also assayed, but due to the lack of an optimized protocol and a suitable positive control, the results were inconclusive.