Systematic study of parameters influencing the action of Rose Bengal with visible light on bacterial cells: comparison between the biological effect and singlet-oxygen production.

As part of a project to study different methods for the disinfection of effluent water, the inactivation of different microorganisms (Escherichia coli, Deinococcus radiodurans and spores of Bacillus subtilis) using a combination of a photosensitizer (Rose Bengal) with simulated sunlight and oxygen was determined under various environmental conditions (temperature, pH index). In parallel, the singlet-oxygen (1O2) production was also measured under the same conditions. Whereas the vegetative cells could be inactivated much more efficiently at increased temperature and altered index of pH, the production of 1O2 remained essentially the same under these alterations. Additionally, the relations among the sensitivities of different cell types to be killed by our photodynamic treatments (PDT) were opposite to those found after exposure to ionizing radiation. The results of photodynamic experiments do not reflect the cells' capacity to repair DNA strand breaks. Spores of B. subtilis, as a nonvegetative system, could not be inactivated by illuminations up to 100 J cm-2. Together, these findings indicate that DNA is not the primary target, the inactivation of which leads to the killing of our test organisms. Instead, the cellular envelope appears to be the component being assaulted by our PDT.