Protective role of selenium against mercury in bacterial populations: Characterization of biogenic Se and Hg (nano)particles by spICP-MS, TEM and XRD

Abstract

This study evaluates the protective role of Se against Hg-induced toxicity in Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) by analyzing bacterial viability, bioaccumulation and biogenic formation of nano/microparticles. Bacteria were exposed to Hg(II) and Se(IV), alone or combined, under co-treatment and Se pre-treatment conditions. Bacterial viability was assessed through turbidity measurements, while bioaccumulation was quantified using ICP-MS. Nano/microparticles formation was evaluated by TEM, XRD, and spICP-MS. Results indicate that the protective effect of Se on Hg-induced toxicity was highly dependent on the exposure conditions. The supplementation of Hg at the beginning of the experiment significantly reduced bacteria cell viability, and this effect could not be reversed by co-exposure to Se. However, the addition of Hg after 8 h of bacteria growth did not affect E. coli availability, while it decreased S. aureus by up to 60 %. The presence of Se was able to reduce the Hg-induced toxicity observed in S. aureus. TEM and spICP-MS measurements detected the formation of biogenic micro/nanoparticles under all treatment conditions, regardless of bacterial metabolic activity. Particles were composed of Se and Hg, agglomerated or monodispersed and with different sizes. The spICP-MS results further confirmed the presence of both Se- and Hg-containing nano/microparticles and contributed to elucidating the interaction between Hg and Se within bacteria population. XRD analysis revealed the formation of HgSe micro/nanoparticles only when Hg and Se were added simultaneously to the culture medium at the beginning of the study. In contrast, when bacteria were pretreated with Se followed by the addition of Hg only Se-containing particles were detected.