Solar disinfection of drinking water (SODIS)
SODIS is a promising and simple method to improve the microbiological quality of the drinking water in developing countries. Drinking water containing waterborne pathogenic germs is exposed to the sun for one day in PET-bottles. The sunlight (primarily the ultraviolet light in the range of 300-400 nm, i.e., UVA and UVB) inactivates a large part of the germs. However, the mechanism that causes the germs to die is not understood yet.
The inactivation process taking place during exposure of microbial cells to sun and artificial light is now being investigated in our research group. First experiments with Escherichia coli showed that the primary damage of the cells affects transport processes on the cell’s cytoplasmic membrane. Hence, the inactivation mechanism seems to be distinctly different from that caused by UVC light where primarily DNA is damaged. Since a single method cannot give adequate information on the way a cell is injured and finally dying, we are employing a range of different methods to study cell inactivation by sunlight; this includes traditional plating, ATP-content of cells, and several flow cytometry-based methods that can give information at the single cell level.
Irradiation of different waterborne pathogenic bacteria (Salmonella typhimurium, Shigella flexneri) has shown that the cell functions were lost always in the same sequence as in E. coli, which indicates that these enteric pathogens are inactivated by same mechanism. S. typhimurium is by far the most resistant organism and was able to recover again from minor damages.
We are now searching direct evidence for the molecular mechanisms leading to cell inactivation using methods that detect damage at the level of protein, DNA and/or lipids. Indirectly, evidence on what cellular components are affected first and how cells in the end die from sunlight may also be obtained from sunlight-sensitive or resistant mutants.