Integrated induction of silver resistance determinants and production of extracellular polymeric substances in Cupriavidus metallidurans BS1 in response to silver ions and silver nanoparticles

Although the antimicrobial mechanisms of nanomaterials have been extensively investigated, bacterial defense mechanisms associated with AgNPs have not been fully elucidated. We here report that dissolved Ag⁺ (>0.05 μg mL⁻¹) displayed higher toxicity on cell growth of strain Cupriavidus metallidurans BS1 (GCA_003260185.2) in comparison to 2 and 20 nm AgNPs. The genes necessary for synthesis of distinct abundance and composition of extracellular polymeric substances (EPS) were induced in strain BS1 exposed to Ag stress. This resulted in 20.1% (Ag(I)-EPS) and 24.2% (2 nm AgNPs-EPS) of the C[dbnd]O band integrated intensities being converted into C-OH/C-O-C group vibrations and the Ag-O bond was formed between EPS and 20 nm AgNPs. Meanwhile, the expression of primary resistance genes of the cus, sil and cup operon encoding HME-RND-driven efflux systems as well as a P_IB1-type ATPase (CupA) were significantly induced after exposure to Ag(I), 2 and 20 nm AgNPs, respectively. Furthermore, distinct genes involved in biosynthesis pathways responsible for production of EPS were induced to relieve the toxicity of Ag(I), 2 nm and 20 nm AgNPs. This combined action is one potential reason why strain BS1 displayed distinct resistances in response to Ag(I) compared to 2 and 20 nm AgNPs. This work will help in understanding processes important in bacterial defensive mechanisms to AgNPs.