Uranium-contaminated groundwater is a concern in regions with naturally high levels of uranium-rich sediments, and also in areas that have a legacy of nuclear weapons use or production. One possible, relatively noninvasive strategy for cleaning up these former sites of mobile U(VI) ions is the use of certain types of bacteria to promote the reductive precipitation of tiny U(IV) mineral grains that are insoluble and presumably immobile. However, much remains unknown about how these meta l reducing bacteria reduce U(VI) and what U(IV) phases ultimately form, especially in light of the recent discovery that some bacteria can pass electrons across long distances away from the cell through extended hairlike pili structures. Cologgi et al. discovered that for one particular species, Geobacter sulfurreducens, the pili, not redox-active cytochrome proteins, do most of the work reducing U(VI) to U(IV). When pili growth was encouraged, solid-phase U(IV) aggregates tended to form extensive networks away from the cell and along the pili. Because this reduction strategy prevents U(IV) encrustations from enveloping the cell or forming in the periplasm, this species may use its pili to reduce U(VI) as a cellular protection mechanism.