Publication
The increase of bacterial resistance against various disinfection processes is a worrying phenomenon. As a consequence, the search for alternative techniques for fighting micro-organisms has become one of the major issues of these last few years. In this domain, water treatment is an important preoccupation for many states, either to guarantee the quality of drinking water supply or to detoxify industrial effluents, charged with chemically or biologically pollutants. The financing of this thesis project was provided by the European project AQUACAT. This project aims at evaluating an emerging technology, the titanium dioxide (TiO2) photocatalysis, as an alternative to the traditional water treatments. This technique is based on the illumination of the catalyst, which thus generates reactive oxygen species (ROS), particularly hydroxyls radicals. TiO2 photocatalysis can be used as a water treatment by detoxifying organic compounds and inactivate micro-organisms. Moreover, its bactericidal activity have a promising biomedical future, by killing cancerous cells for example. Although the bactericidal action of the TiO2 photocatalysis is well documented, its mode of action on bacteria was not yet well defined. The objective of this thesis is to contribute to a better comprehension of the mode of action of the TiO2 photocatalysis on the bacterium Escherichia coli. Few microbiologists ever worked on this subject. A review of published articles on the mode of action of TiO2 photocatalysis showed some experimental failures. Moreover, dogmas related to TiO2 photocatalysis showed contradictions, which will be discussed in this thesis. This thesis is divided in four chapters, including two major sections: i) interaction between TiO2 particles, bacteria, biomolecules and salts and ii) the defence mechanisms of the bacterial cells against TiO2 photocatalysis. The first chapter shows that bacterial cells are adsorbed onto TiO2 particles. So as to demonstrate this feature, bacterial cells and TiO2 particles were mix in the presence of two salts affecting at different degrees the effectiveness of TiO2 photocatalysis rate. In the presence of NaCl-KCl, photocatalysis was very effective and the cultivability of the cells decreased at the beginning of the illumination phase. On the contrary, in a phosphate solution, a latency phase of 20 minutes was observed. Using flow cytometry and microscopic observations, we observed very distinctly that the bactericidal effect begun when the cells started to be adsorbed on TiO2 particles. Bacterial adsorption on TiO2 particles could be correlated to the loss of cell cultivability and also to the loss of membrane integrity, measured by flow cytometry. We then studied the interaction between biomolecules (proteins and DNA), bacterial cells and TiO2 particles. Biomolecules strongly adsorbed onto TiO2 particles in a NaCl-KCl solution (up to 60 µg protein per ml of catalyst). A desorption buffer composed of phosphate (50mM, pH 7.0) and