Photocatalysis and disinfection of water: Identification of potential bacterial targets

In order to identify some of the potential bacterial targets, the effects of TiO2 nanoparticles on bacteria in aqueous suspension were assessed in the dark and under UV-A (λ > 340 nm) radiation exposure, using the microorganism model Escherichia coli K-12. Illumination was produced with a HPK 125 W lamp and suspended TiO2 Degussa P-25 was used as the photocatalyst, absorbing all the incident UV-A radiations.The impact of the photocatalyst on the bacteria was investigated by monitoring cell cultivability, cell wall integrity and nucleic acid stability. The contact of TiO2 particles with bacteria in the dark increased the bacterial sensitivity to membrane-perturbing agents, suggesting an increase in outer membrane permeability. In contrast, the contact between SiO2 particles, with an average particle size similar to that of TiO2 P-25, and bacteria did not induce any alteration of the cell permeability. The TiO2 deleterious action on the envelope integrity continued during the UV-A radiations exposure. Impacts on bacterial permeability precede the total loss of cultivability. After 2.5 h of photocatalytic treatment at 3.45 mW/cm2, bacteria were no longer cultivable on their standard growth medium. However, some of them could become cultivable again under specific environmental conditions appropriate to their survival. These resilient bacteria exposed again to UV-A photocatalysis were more resistant to the treatment.Some chemical by-products released during photocatalytic inactivation of the bacteria were also monitored. The appearance of oxamic and oxalic acids as well as ammonium cations, sulfate and nitrate anions were observed. Transmission electron microscopy (TEM) was used to study the morphological damages to E. coli structure during the photocatalytic inactivation of the microorganisms. After 1.5 h of treatment, bacteria showed disorganized membrane structures, while bacteria were still visible although they were no longer cultivable after a longer exposure time. These results were correlated with damages of nucleic acids at in vivo level. An analysis by electrophoresis revealed that bacterial DNA and RNA molecules completely disappeared after 7 h of photocatalytic treatment.

TEM pictures of E. coli cells in suspension in water exposed to 180 min of UV-A in presence of TiO2 P-25 (0.25 g/l) and IUV-A = 3.45 mW/cm2 illustrating the disruption of the membrane.Figure optionsDownload as PowerPoint slideHighlights
► TiO2 particles have deleterious effects on outer membrane permeability in the dark.
► Damages to the bacterial cell envelope precede the total loss of cultivability.
► Some ions and carboxylic acids are released during the photocatalytic treatment.
► TEM illustrates different steps of the bactericidal mechanism of photocatalysis.
► Nucleic acids are potential targets of ROS at in vivo level.