(612b) Assessment of Toxicity of Metal Oxide Nanoparticles to Microbial Species
Nanomaterials have unique chemical and physical properties compared to the larger counterparts because of their high specific surface area and quantum size effects. To safely apply nanotechnology in biology, medicine, food and cosmetic industries, it is necessary to understand nanoparticles' toxicity and develop remediation methods to safely remove nanoparticles. This study focuses on the toxicity of metallic nanoparticles. Specifically, aerosol reactors were used to carefully synthesize nanomaterials with different metal compositions: TiO2 nanoparticles (NPs), Cu-doped TiO2 NPs, and ZnO NPs (all in 20-40nm size). The well characterized nanomaterials were then used to evaluate the toxicity by studying their effects on several microbial species: E.coli, a member of Enterobacteria that is commonly found in the lower intestine; Mycobacterium smegmatis, a model pathogenic strain used for the study of Tuberculosis; Shewanella oneidensis MR-1, a gram-negative environmental bacterium found in sediments; Cyanothece sp. ATCC 51142, a photosynthetic bacterium capable of hydrogen production; Saccharomyces Cerevisiae, a type of yeast often used for ethanol fermentation.
We have systematically examined microbial responses to nanomaterials by monitoring cell growth rates, morphological changes, key enzyme functions, gene expression, and photosynthesis activities (for Cyanothece sp.). Some of the results are:
1. TiO2 NPs did not affect the growth of any tested microbial species, while Cu-doped TiO2 NPs and ZnO NPs significantly reduced M. smegmatis growth (particles concentration = 0.02 g/L). Such inhibition was mainly mediated by released Cu2+ or Zn2+ from the NPs and addition of EDTA alleviated the NPs inhibitory effect on cellular growth.
2. TEM images indicated that nano-sized Cu-doped TiO2 and ZnO particles could not penetrate inside the cell and physically damage the cellular structure since they tended to aggregate in the medium solution, and thus the ?nano? associated toxicity was reduced.
3. The tested microbial species responded to toxic NPs differently. M. smegmatis could not produce significant amount of extracellular polymeric substances (EPS, i.e. extracellular proteins and polysaccharides), so M. smegmatis was very sensitive to toxic NPs and may be used as a model strain for toxicity testing. On the other hand, S. oneidensis MR-1 and E.coli produced more EPS under the NP stress. The extracellular protein production improved the resistance of both bacteria to the toxic NPs.
4. S. oneidensis MR-1 reduced ionic metals released from NPs, thus it can be considered for metal oxide NP bioremediation.
5. Although metal oxide NPs did not inhibit cellular growth or protein functions due to particle aggregation in aqueous culture, NPs' toxicity can be clearly revealed if NPs were electrosprayed onto a bacterial biofilm so that nano-sized NPs could be uniformly deposited on the cell surface. For example, electrospraying 20nm ZnO NPs dramatically reduced cellular viability compared with the control cases of electrospraying ZnCl2 or buffer solutions. On the other hand, electrospraying large-size ZnO particles (480 nm) caused much less toxicity to cells. This observation implies that the exposure schemes of NPs to biological system are also important considerations for NPs toxicity.