The cytotoxicity of endogenous CdS and Cd 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 Cd, intracellular Cd and intracellular CdS NPs were determined during the whole CdS biosynthetic process. The results demonstrated that the contents of Cd 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 Cd and CdS. Both Cd and CdS NPs affected the cellular antioxidative system by upregulating related gene expression. However, different pathways might be involved to eliminate ROS induced by Cd 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.