The cytotoxicity of endogenous CdS and Cd²⁺ ions during CdS NPs biosynthesis

In the present study, cadmium-based nanoparticles (NPs) were biosynthesized by incubating their precursor salts with E. coli CD-2. Transmission electron microscopy (TEM) revealed the morphology of the NPs and confirmed that the NPs were formed via an intracellular growth. Energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) determined the elemental composition of the NPs and identified the NPs as CdS. The contents of extracellular Cd²⁺, intracellular Cd²⁺ and intracellular CdS NPs were determined during the whole CdS biosynthetic process. The results demonstrated that the contents of Cd²⁺ and CdS NPs changed during the biosynthetic process. The colony-forming capability test showed that strain CD-2 could maintain its growth during CdS biosynthesis. Protein oxidation levels confirmed that the E. coli cells faced oxidative stress induced both by Cd²⁺ and CdS. Both Cd²⁺ and CdS NPs affected the cellular antioxidative system by upregulating related gene expression. However, different pathways might be involved to eliminate ROS induced by Cd²⁺ ions or CdS NPs, respectively. The expression levels of ef-tu, ftsZ, mutS and dnaK were enhanced together with CdS accumulation, indicating that the cells had to overexpress certain related genes to respond to the NPs-induced stress.