Fig. 2

EM micrographs  of R. capsulatus in the presence (A) or absence (B) of selenite, are shown. EM micrographs presented in A and B were obtained using scanning-electron microscopy (scanning-EM). In A, (cell grown in the presence of 0.5 mM selenite), an elemental selenium particle (approx 55 nm size) is still slightly embedded into the OM layer (black arrow) while the cell membrane seems to be slightly modified at the place where the selenium particle is located (white arrow). In B, (control cell) extracellular membrane vesicles (EMV) are seen attached to the OM layer of the cells (see black arrows) with diameters varying between 30 and 36 nm . Technical details as in Materials and Methods of Wild et al. [167]. EM micrographs presented in C and D were obtained using cryo-electron microscopy (cryo-EM). In C (cell grown in the presence of 0.5 mM selenite) a particle of about 50 nm, likely containing elemental selenium, is protruding from one of the poles of the cell (white arrow), while on the control cell (D), significantly smaller structures are seen on the cell surface. Comparison of this image with that presented in B suggests that the small structures present on the cell surface of D, likely represent EMV. The small cavities, or membrane alterations seen on the cell surface of C, may represent membrane areas damaged by the passage of membrane coated SeNPs (see Additional files 5, 6 and 7 in [120]), and those present on the cell surface of D, are proposed to represent membrane areas damaged by the excretion of EMV. (These EM micrographs were obtained in the Center for Microscopy and Image analysis of the University of Zurich, with bacterial cells produced in J Kessi’s laboratory; unpublished material) [56, 158]