In contrast, elements carbon

(C) (Figure 4B) and copper (Cu) (Figure 4E) were distributed both inside and outside of cells because cells were embedded by carbon-contained plastic Epon before section in order to maintain the cell shape, as well as sectional samples were coated by copper grids to support thin slicing of bio-samples. However, strong signals of selenium as shown by orange color were only observed outside of cells whereas the color in cells was black background even the white dots in cells signaling pathway suspected to be SeNPs were not similar to SeNPs outside of cells (Figure 4D), indicating that SeNPs were only formed outside of cells rather than inside of cells. The EDS map of elemental selenium was consistent with TEM-EDX result focusing on high density particles, i.e., SeNPs did not occur in the interior of C. testosteroni S44 cells. In addition, it was clear that small SeNPs aggregated into bigger particles outside of cells (Additional file 1: Figure S1). Figure 3 EDX analysis of electron dense particles formed by cultures of C. testosteroni S44 amended with 1.0 mM sodium selenite. (A) Extracellular particles pointed out by arrows. The emission lines for selenium are shown at 1.37 keV (peak

SeLα), 11.22 keV (peak SeKα) and 12.49 keV (peak SeKβ). (B) Intracellular particles pointed out by arrows. No emission peaks of Se. Figure 4 Localization of selenium particles using EDS Elemental Mapping. (A) The box showed the check details mapping area of B-E, where the K series peaks of the elements was used for mapping. The arrow points to an extracellular selenium particle. B, C, D and E show the distribution of different elements of C (from cell and Epon), Cl, Se and Cu (from Cu grids), respectively. Tungstate inhibited Se(VI) but not Se(IV) reduction Tungsten has been used as

an inhibitor of the molybdoenzymes, since it replaces molybdenum (Mo) in the Mo-cofactor (MoCo) of these enzymes. Tungstate did not affect Astemizole reduction of Se(IV) (Figure 5A) since the same red color of the SeNPs could be observed whether tungstate was added to cells of C. testosteroni S44 or not. In contrast, addition of tungstate and Se(VI) resulted in no development of red colored nanoparticles as in the negative control with no added Se(VI) and tungstate. In contrast, addition of Se(VI) without tungstate resulted in red-colored colonies on LB agar plates (Figure 5B). Therefore, tungstate only inhibited molybdenum-dependent Se(VI) reduction and subsequent reduction to elemental selenium and formation of nanoparticles. Similar results were obtained in different media such as LB, TSB and CDM. Figure 5 Comparison of Se(IV) and Se(VI) reduction and tungstate inhibition in C. testosteroni S44. Cultures were amended with 0.2 mM Se(IV) (A), 5.0 mM Se(VI) (B), respectively, and with or without 10 mM tungstate.