Pollution- induced community tolerance framework - disc diffusion method to assess the impact of silver nanoparticles in soils: Potential relevance for risk assessment

Abstract The toxicity of silver nanoparticles (AgNPs) in soil bacterial communities (SBC) has been widely reported. However, the bacterial related endpoints have not been effectively considered in the environmental risk assessment (ERA) for nanomaterials. Thus, we aimed to study the long-term effects of AgNPs, or AgNO3 (counterpart), [5 μg (Ag) kg−1] on SBC: (1) at structural level (using denaturing gradient gel electrophoresis-DGGE) and, (2) in terms of bacterial induced tolerance (which is the foundation of the pollution-induced community tolerance - PICT) using a disc diffusion assay (0.1 μg, 1 μg or 10 μg of each silver form). After day-56 of exposure, the structure of SBC was affected by AgNPs and AgNO3, though still sharing 71.9% similarity with the non-exposed SBC. Also, the SBC homogeneity significantly decreased after exposure to silver, suggesting a possible tolerance effect. Regarding tolerance assays, AgNO3 was more effective in inhibiting the SBC growth than AgNPs, and both in a dose-dependent manner (10 μg > 1 μg > 0.1 μg). A second exposure revealed a decrease in tolerance of SBC to AgNO3. Also, a lower amount of AgNO3 is needed to change SBC tolerance. PICT analysis revealed that previous exposure to AgNPs did not increase tolerance-effect in SBC. Overall, AgNPs changes the SBC structure and tolerance, but does not increase the tolerance on a second exposure. Thus, this study highlights the usefulness of combined bacterial endpoints (DGGE, and PICT-disc diffusion tests) for potential inclusion in the ERA of AgNPs in soil ecosystems.