Although the toxicity of silver nanoparticles (Ag NPs or nanosilver) to model bacteria has been reported, the effects of Ag NPs on microbial denitrification under anoxic conditions and the mechanism of Ag NPs induced-toxicity to denitrification remain unclear. In this study, the effects of Ag NPs on Paracoccus denitrificans under anoxic conditions were investigated, and the mechanism was explored by analyzing the transcriptional and proteomic responses of bacteria to Ag NPs. The presence of 5 mg/L Ag NPs led to excessive nitrate accumulation (232.5 versus 5.3 mg/L) and increased nitrous oxide emission. Transcriptional analysis indicated that Ag NPs restrained the expression of key genes related to denitrification. Specifically, the genes involved in denitrifying catalytic reduction and electron transfer were significantly down-regulated. Moreover, the expression of the genes responsible for polyhydroxybutyrate synthesis was enhanced, which was adverse to denitrification. Proteomic profiling revealed that the syntheses of the proteins involved in catalytic process, electron transfer, and metabolic process were inhibited by Ag NPs. The activities of nitrate reductase and nitrite reductase in the presence of 5 mg/L Ag NPs were only 42% and 61% of those in the control, respectively, indicating the inhibition of denitrifying enzymes. These results improve understanding of the inhibitory mechanism of Ag NPs toward bacterial denitrification. (C) 2017 Elsevier B.V. All rights reserved.