Gram-negative bacteria such as Shewanella and Geobacter are capable of reducing U(VI) at the cell surface as well as in inner compartments (Richter et al., 2011). Similarly to chromium, uranium changes its solubility upon reduction. This extracellular reduction allows the cell to localize precipitation partially to the cell surface, effectively reducing the accumulation of toxins from entering the cell. Findings have demonstrated that the conductive pili of Geobacter are essential for extracellular reduction, and the reduced uranium preferentially precipitates at the cell surface and in the periplasm (Cologgi et al., 2011).