Bioavailability Of Silver Nanoparticles Based On Food Chain From Esherichia Coli To Caenorhabditis Elegans

Silver nanoparticles(AgNPs)and its related commercial products unique in physical and chemical properties,have been widely used.However it should be noted that the accumulation of AgNP-related products would eventually cause the environmental contamination.Growing evidences showed that AgNPs become harmful to both environment and organisms such as mouse,earthworm,C.elegans,and zebrafish,but the bioavailability of AgNPs remains largely unexplored.The food chain transport should be taken into account when ecological toxicity of engineered nanoparticles(ENMs)is evaluated.In this thesis,the physiological behavior and toxicity of commercially manufactured polyvinyl pyrrolidone(PVP)-coated AgNPs were carefully examined using a food chain model from Escherichia coli(E.coli)to Caenorhabditis elegans(C.elegans).The ecological toxicity of nanometer particles with different size was compared upon the food chain.The transmission and toxicity were compared on the silver nanoparticles through food chain in the presence and absence of natural organic matter(Fulvic acid,FA).It should be mentioned that the semifluid cultivating as a new method was adopted to directly expose worms to the silver nanoparticles.The bio-accessibility and toxicity of polyvinyl pyrrolidone(PVP)-coated AgNPs(25 nm,75 nm)were evaluated using the food chain assay.Transmission electron microscopy(TEM)imaging and inductively coupled plasma mass spectrometry(ICP-MS)analysis showed that AgNPs were transmitted to organisms of higher trophic level through food chain.After being transferred to C.elegans,the accumulated AgNPs caused severe toxicity to higher trophic level(C.elegans),and the germ cell death,reproductive integrity and lifespan were adversely affected.Small AgNPs(25 nm)were more easily accumulated in the food chain and exhibited a more obvious toxicity to the higher trophic level than larger AgNPs(75 nm).Moreover.AgNPs were accumulated in C.elegans through the food chain and the consequent impairment of' germ cells could be transferred to the next generation,indicating that AgNPs could cause genetic damage inter-generations.Ubiquitous FA(Fulvic acid,FA)in the environment affected transmission of AgNPs from E.coli to C.elegans through food chain and its ecological toxicity.The influence of environment conditions to transmission and toxicity on the AgNPs in the food chain was also examined in this thesis.Results showed that natural organic matter(FA)in the environmental ecological system promoted the accumulation of AgNPs in E.coli and C.elegans,and led an increase ecological toxicity of AgNPs.These findings revealed that the environment-related factors might have a significant effect on the bioavailability of nanomaterials in the ecological system.To further investigate the mechanism of AgNPs' food chain transfer,the role of insulin signaling pathway in the toxic effects of AgNPs in food chain transmission was analyzed.Results showed that the daf-16/FOXO contributes to the induction of apoptosis after being exposed to AgNPs through the food chain.Inhibition of Daf-2,AKT-1,and AGE-1 by AgNPs exposure would make nematodes more susceptible to cell apoptosis.With the inactivation of AKT-2 or the successive activation of PDK-1,SGK-1 and AKT-1,daf-16 mutation would be increased,thereby effectively reduced cell apoptosis.Additionally,daf-2/IGF-1 R/PI3K.PDK-1,AGE-1/PKB/AKT and PDK1-1 also negatively regulated the apoptosis induced by silver nanoparticles.Among the insulin signaling pathways involving in the apoptosis induced by AgNPs,daf-16/FOXO exerts an obvious negative effect.Thus it was concluded that the daf-16 is highly susceptible to AgNPs induced stress effect.The most difficult problem in the evaluation of ecotoxicity of nano-materials by C.elegans is to determine the dispersion of nanomaterials in the culture media.AgNPs-exposed in the nematode growth medium(NGM)and conventional experimental exposure medium K-medium were used as the control.Results obtained with ICP-MS.TEM and DLS techniques shows that AgNPs could disperse evenly in NGG.Conventional toxicity end points were used to evaluate the effect of AgNPs on C.elegans.A comparison on the toxicity effects of AgNPs among the NGG,standard NGM and KM were performed,and AgNPs in NGG exhibited a concentration dependent-change of toxic sensitivity.The discoveries of this thesis provided the evidence that the NGG will distribute AgNPs more uniformly,which make it an ideal system for evaluating the bioavailability of nanomaterials.