| The increasing use of engineered nanoparticles (NPs) in industrial and household applications will very likely lead to the release of such materials into the environment. Assessing the risks of these NPs in the environment requires an understanding of their mobility, reactivity, ecotoxicity and persistency. This study examined and compared the ecotoxicity of nano-scaled aluminum, silicon, titanium and zinc oxides to Escherichia coli during their aggregation and deposition processes.A favorable linearity (r≥0.995) was found between NP concentration and optical density (OD) value, suggesting spectrophotometry is an appropriate method assessing the concentrations of NPs. Results from the spectrophotometric analysis revealed that these four oxide NPs (TiC>2, ZnO, Al2O3 and SiO2) deposited completely in≤10 h. TEM images of nanoscale Al2O3, SiO>2, TiO2, and ZnO showed that they could aggregate to large particles. And the large aggregates of NPs would deposit quickly in the suspension.The antibacterial activity of NPs to E. coli was assessed by counting the number of colony forming units (CFU). Al2O3 NPs showed significant (p< 0.05) toxicity to E. coli at 40 mg/L, SiO2 showing significant (p< 0.05) toxicity at 10 mg/L and highly significant (p < 0.01) toxicity when above 20 mg/L, and ZnO NPs showing highly significant (p< 0.01) toxicity at all concentrations. ZnO was the most toxic among the three NPs, and all E. coli died at 40 mg/L. SiO2 appeared to kill more E. coli (53% death rate) than Al2O3 (46.5%) at 40 mg/L, while their mortality did not show a significant difference at the 95% confidence level. TiO2 NPs did not affect E. coli populations.The toxic effect of NPs on the growth of E. coli was studied by spectrophotometry and microcalorimetry, respectively. Results from the spectrophotometric analysis indicated that the growth of E. coli was significantly inhibited, and the bacteria could not reach the stationary growth stage when the initial concentration of nano-ZnO was higher than 20 mg/L. When the concentration of nano-ZnO in Luria-Bertani (LB) media increased to 40 mg/L, the number of E. coli hardly increased within 10 h. Compared with nano-ZnO, the other three oxide NPs showed no significant inhibition on the growth of E. coli. The thermokinetic parameters of Ppeak and k from the microcalorimetric analysis showed that nano-ZnO had a significant (p< 0.05) effect at 10 mg/L, and a highly significant (p< 0.01) effect at 5,20, and 40 mg/L. However, the other three oxide NPs showed no significant (p > 0.05) effect on the growth of E. coli.TEM images of NP-bacteria complexes in 1% NaCl displayed that individual and small aggregate NPs were present on the surface of E. coli, except TiO2 NPs. TEM images of normal E. coli cells and those treated with nano-ZnO and Zn2+ ions in ultrapure water showed that the morphologies of E. coli cells treated with nano-ZnO or Zn2+ ions were significantly different from those of normal E. coli cells, and the cytoplasmic membranes deformed, wherein some cells swelled and the intracellular substances leaked out under both Zn stress and osmotic stress. And SEM analysis revealed that the treatment with ZnO nanoparticles has led to considerable damage to some E. coli and the damage has caused the breakdown of the membrane of E. coli.Considering the published studies have not been designed to mimic natural systems and therefore provide poor understanding of the likely consequences of intentional or unintentional environmental release. Our study presented the understanding of the likely antibacterial activity of some nanomaterials under natural conditions. |
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