Nano-silver induces dose-response effects on the nematode Caenorhabditis elegans

Toxicity of nano-formulated silver to eukaryotes was assessed by exposing nematodes (Caenorhabditis elegans) to two types of silver nanoparticles (AgNPs): with average primary particle diameters of 1 nm (AgNP1) and 28 nm (AgNP28, PVP coated), respectively. Tests were performed with and without presence of Escherichia coli to evaluate how the presence of a food bacterium affects the AgNP toxicity. A pre-exposure experiment was also conducted with nematodes pre-exposed to 0 and 1 mg AgNP L−1, respectively, for 20 h prior to exposure at higher concentrations of AgNP. Both AgNP1 and AgNP28 showed adverse dose-response effects and mortality on C. elegans. LC50 for AgNP28 was lower than for AgNP1 and, hence, at the present test conditions the PVP-coated AgNP28 was more toxic than AgNP1. Including E. coli in the test medium as a food source increased AgNPs toxicity towards nematodes compared to when bacteria were not present. Pre-exposure to a low-level AgNP1 concentration made the nematodes slightly more sensitive to further exposure at higher concentrations compared to no pre-exposure, indicating that nematodes have no efficient physiological ability to counteract nano-silver toxicity by acclimation. The amount of dissolved Ag+ was 0.18 to 0.21 mg L−1 after 20 h at the highest AgNP1 (10 mg L−1) and AgNP28 (3 mg L−1) doses in the exposure medium, respectively. The upper limit of Ag+ solubility cannot immediately explain the dose-response-related toxic effects of the AgNP nor the difference between AgNP1 and AgNP28. Higher toxicity of AgNP28 than AgNP1 may be explained by a combination of effects of coating, Ag-solubility and higher uptake rates due to agglomeration into μm-size agglomerates in the exposure medium.

► Silver nanoparticles (AgNPs) have lethal effects on the nematode Caenorhabditis elegans.
► LC50 or EC50 for the nematodes is affected by way of exposure to AgNPs (presence of food bacteria).
► Pre-exposure to low-level AgNP concentration makes nematodes more sensitive to further AgNP exposure.
► The concentration of Ag ions in the exposure medium was similar with two types of nanosilver added.