Studies on photokilling of bacteria on TiO2 thin film

In order to elucidate the mechanism for photokilling of Escherichia coli (E. coli) cells on titanium dioxide (TiO2) thin film, the survival of intact cells and the spheroplasts was investigated as a function of photo-illumination time. The photokilling reaction for intact cells was observed to involve two steps, an initial lower rate photokilling step followed by a higher rate one. In contrast, the reaction for spheroplasts, which do not have cell wall, exhibited only a single step kinetics with a higher rate, suggesting that the cell wall of E. coli cell acts as a barrier to the photokilling process. Changes in concentration of the cell wall components during illumination further showed that the outer membrane serves as a barrier, while the peptidoglycan layer does not have a barrier function. Moreover, atomic force microscopy measurements of cells on illuminated TiO2 film showed that the outer membrane decomposed first, and with further illumination, the cells completely decomposed. These results suggest that the photokilling reaction is initiated by a partial decomposition of the outer membrane, followed by disordering of the cytoplasmic membrane, resulting in cell death.