A Comparative Toxicity Study of TiO2 Nanoparticles in Suspension and Adherent Culture Under the Dark Condition

The present study focused on the different acute toxicity of TiO₂ nanoparticles(TiO₂ NPs) towards the bacteria in suspension culture and adherent culture under the dark conditions. The study investigated the bacteria toxicity with TiO₂ NPs at different concentrations(1—2000 mg/L), sizes(10 nm, 35 nm) and specific surface areas in unit volume solution(0—224 m²/L) characterized by the cell viability, extracellular polymeric substances(EPS) release and biofilm formation. The bacteria in adherent culture was found to be more resistant against the toxicity of TiO₂ NPs compared to that in suspension culture. An NP dose and surface area dependent(rather than the size) bacterial viability was observed in suspension culture, specifically the surface area positively correlated with the toxicity of TiO₂ NPs. The size of TiO₂ NPs, however, played a more critical role in toxicity of TiO₂ NPs in adherent culture. Therefore, the surface area dependent toxicity of TiO₂ NPs is a comprehensive parameter describing the dose and size dependent toxicity of TiO₂ NPs. The electron microscopic(SEM, TEM, EDX) observations suggested the EPS release and biofilm formation, during aggregation of TiO₂ NPs on the bacteria after 12 h cultivation in adherent culture under the dark condition. A possible toxic mechanism could be that “effective surface areas” that directly contact with the bacterial membrane greatly contributed to the toxicity of TiO₂ NPs in both suspension culture and adherent culture. Therefore, as for the possible resistance mechanism, EPS secretion and subsequent biofilm formation may protect the bacteria against the toxicity of TiO₂ NPs.