*2.4. Stirring Experiment*

For the stirring experiment; TiO2 nanoparticles (KronoClean7050) were suspended in a sterile beaker in 27 mL of 1/500 NB or sterile distilled water; depending on the test; and 3 mL of the cell suspension were added to make the final test suspension. Final concentrations of the suspension were 1 g/L for TiO2 and 7 × 104 to 1 × 105 CFU/mL for bacteria. A suspension of bacterial cells without TiO2 was prepared as a control. The beakers (test sample and control) were placed in a sterile flow hood; covered with a Pyrex lid and illuminated with an 8-W black-light bulb at a light intensity of 5 W/m2 .

An aliquot of 1 mL was taken from each beaker every 30 min during 4 h and, when necessary, diluted in phosphate buffer before inclusion in trypticase soy agar as in the deposited-drop experiment. Controls were also carried out without TiO2 and in the dark, with/without TiO2. The data presented are the average of three experiments with the corresponding standard errors.

To assess the influence of the nature of the water during the experiment, two solutions were used: 1/500 nutrient broth and sterile distilled water. The two conductivities of the solutions were compared using a conductivity meter before the test. At room temperature (~21 °C), the conductivities were 4.437 mS/m for 1/500 NB and 1.1 mS/m for distilled water.
