<p>Transformed MC4100 <i>hfq<sup>−</sup> E. coli</i> cells were induced with different concentrations of arabinose (ARA) and grown in the presence of gold salts. Ultrathin-sections of vitrified and freeze-substituted cells are shown in all panels. (A) A bacterium induced with 0.001% ARA. This concentration reproduces the expression levels of Hfq during the stationary growth phase. Electron-dense particles are abundant both in the interior (asterisk) and the periphery (arrow) of the cell. The inset is an enlargement of the peripheral region marked with the dashed rectangle. (B) A transformed bacterium induced with 0.01% ARA showing a heavy accumulation of particles both in the interior (asterisk) and periphery (arrows) of the cell. The inset is a higher magnification view of the area marked with the dashed rectangle where it can be appreciated that some particles are slightly bigger than in bacteria induced with lower ARA concentrations. (C) Detail of the wall of a bacterium induced with 0.01% ARA and stained for 30 s with uranyl acetate (UA). Staining shows the real location of the outer membrane (OM) and confirms that the peripheral electron-dense particles delineate the inner membrane (IM). Extracellular granules are due to deposition of uranium on resin protrusions. (D) and (E) A transformed bacterium induced with 0.001% ARA showing intense signal in intracellular regions compatible with being the nucleoid (asterisk). (E) A high magnification view of the central area of the cell in (D) shows abundant small electron-dense particles, apparently arranged with a filamentous-like pattern (the arrows point to some of several such fibers). Bars: 100 nm (A, B, C and D), 50 nm (E and insets in A and B).</p
