Abstract
In the present study, we examined the ability of Enterobacter cloacae Z0206 to reduce toxic sodium selenite and mechanism of this process. E. cloacae Z0206 was found to completely reduce up to 10 mM selenite to elemental selenium (Se°) and form selenium nanoparticles (SeNPs) under aerobic conditions. The selenite reducing effector of E. cloacae Z0206 cell was to be a membrane-localized enzyme. iTRAQ proteomic analysis revealed that selenite induced a significant increase in the expression of fumarate reductase. Furthermore, the addition of fumarate to the broth and knockout of fumarate reductase (frd) both significantly decreased the selenite reduction rate, which revealed a previously unrecognized role of E. cloacae Z0206 fumarate reductase in selenite reduction. In contrast, glutathione-mediated Painter-type reactions were not the main pathway of selenite reducing. In conclusion, E. cloacae Z0206 effectively reduced selenite to Se° using fumarate reductase and formed SeNPs; this capability may be employed to develop a bioreactor for treating Se pollution and for the biosynthesis of SeNPs in the future.
Highlights
Selenium (Se) is an important element for life and exhibits redox activity in the environment[1]
To determine the toxicity of selenite to the microorganism, the growth profile of E. cloacae Z0206 was studied under various concentrations of selenite (0, 0.5, 1, 5, 10, 15 mM)
According to the apparent changes in the spent broth shown in Fig. 1A, we found that the strain formed a reddish cell suspension, which indicated its ability to reduce the toxic, colorless, soluble selenite ions to the non-toxic, red, insoluble elemental form of Se (Se°)
Summary
Growth profile and selenite-reducing ability of E. cloacae Z0206 under different selenite concentrations. SeNPs ranging from 100–300 nm were observed scattered around the cells and occurred as aggregates attached to the bacterial biomass in the presence of selenite, and the particle density grew with the increase of the selenite concentration These results indicated that selenite merely reduced the growth rate of E. cloacae Z0206 rather than decreasing the final amount of bacteria. The lack of peaks corresponding to other metals indicated that Se occurred in its elemental state (Se°) rather than as a metal selenide This was confirmed by X-ray photoelectron spectroscopy (XPS) analysis, which shows clearly the 3D spectral peak of Se° (Fig. 2F). These results suggested that E. cloacae Z0206 reduces Se(IV) to Se(0) and assembles it into nanoparticles. The reaction mixture with the total cell fraction and without selenite served as positive and negative controls, respectively
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