Is sulfidation a true detoxification process for silver nanoparticles?: from the perspective of chronic exposure

Sulfidation has been regarded as a primary transformation and detoxification process for silver nanoparticles (AgNPs). However, considering the high stability of silver sulfide (Ag2S) and its potential persistence in the environment, it is imperative to evaluate how sulfidation impacts the long-term effect of AgNPs. The model bacterium Escherichia coli was exposed to 0.02 mg L-1 Ag2S-coated AgNPs (s-AgNPs) for more than 100 subcultures (>200 days), and both phenotypic and genotypic changes were monitored. The transcriptomic results showed that even though no discernable effect could be detected at this concentration in 48 h, chronic exposure induced a significant stress response due to respiration disturbance. The intracellular stress induced notable upregulation of multidrug-resistant genes; thus, the bacteria showed decreased susceptibility to various antibiotics. A variety of mutations in genes related to SOS response, DNA repair and oxidative stress indicated genetic instability driven by oxidative stress. Interestingly, although sulfidation considerably hindered Ag+ dissolution, after chronic exposure, the cells still developed observable tolerance to both Ag+ and s-AgNPs, as indicated by a change of approximately 2-fold in the minimal inhibitory concentration, which was ascribed to the overexpression of copper efflux transporters. Collectively, sulfidation may effectively decrease acute toxicity; however, the chronic effects of sulfided AgNPs, such as accumulative changes in physiological characteristics, the development of silver tolerance and the triggering of pathogenic evolution, may cast a shadow on the role of sulfidation as an antidote.