Toxicity of TiO2 nanoparticles to Escherichia coli: effects of particle size, crystal phase and water chemistry

Abstract

Controversial and inconsistent results on the eco-toxicity of TiO2 nanoparticles (NPs) are commonly found in recorded studies and more experimental works are therefore warranted to elucidate the nanotoxicity and its underlying precise mechanisms. Toxicities of five types of TiO2 NPs with different particle sizes (10∼50 nm) and crystal phases were investigated using Escherichia coli as a test organism. The effect of water chemistry on the nanotoxicity was also examined. The antibacterial effects of TiO2 NPs as revealed by dose-effect experiments decreased with increasing particle size and rutile content of the TiO2 NPs. More bacteria could survive at higher solution pH (5.0-10.0) and ionic strength (50-200 mg L(-1) NaCl) as affected by the anatase TiO2 NPs. The TiO2 NPs with anatase crystal structure and smaller particle size produced higher content of intracellular reactive oxygen species and malondialdehyde, in line with their greater antibacterial effect. Transmission electron microscopic observations showed the concentration buildup of the anatase TiO2 NPs especially those with smaller particle sizes on the cell surfaces, leading to membrane damage and internalization. These research results will shed new light on the understanding of ecological effects of TiO2 NPs.

Figure 1. TEM images of the as-received TiO₂ NPs.

Figure 2. Changes of zeta potentials of the TiO₂ NPs against a solution pH.

Figure 3. EDS (left column) and XRD (right column) figures of the as-received TiO₂ NPs.

Figure 4. Variations of the bacteria viability with concentrations of the TiO₂ NPs.

The viability was the ratio of bacterial cell number under the NP treatment to the blank control.

Figure 5. Selected TEM images of the unsliced (A to F) and sliced (G to L) E.coli cells without (A and G) and with the treatments of TiO₂-NP 10A (B and H), TiO₂-NP 25A (C and I), TiO₂-NP 25AR (D and J), TO₂-NP 50A (E and K) and TiO₂-NP 50R (F and L).

The blue arrows point to the cells and the red arrows direct to the NP aggregates.

Figure 6. Relative contents of intracellular ROS (A) and MDA (B) in the bacterial cells after 3 h exposure to the TiO₂ NPs (50 mg L⁻¹).

a–e stand for TiO₂-NP 10A, TiO₂-NP 25A, TiO₂-NP 25AR, TiO₂-NP 50A, and TiO₂-NP 50R, respectively. Asterisk indicates a significant difference relative to the control (*, p<0.05; **, p<0.01) based on the Student’s t test. Error bars represent standard deviation (n = 3).

Figure 7. The effects of pH (A) and NaCl (B) on the relative viability of E.coli exposed to 10 mg L⁻¹ TiO₂-NP 10A for 3 h.

Significant difference (p<0.05) between two treatments is presented by different lowercase letters a, b and c. Error bars represent standard deviation (n = 3).