One prospect for remediation of groundwater contaminated with heavy metals is to use microbes to convert soluble metal ions into insoluble minerals that are immobilized in soil. A research team led by James K. Fredrickson of Pacific Northwest National Laboratory has added to understanding of this process by partially unraveling the mechanism by which Shewanella oneidensis bacteria convert U6+ ions into 5-nm UO2 nanoparticles (PLoS Biol. 2006, 4, e268). The researchers developed mutant strains of the bacterium that enabled them to observe the importance of cytochrome enzymes localized on the cells' exterior membranes and pinpoint a specific cytochrome that they believe is responsible for U6+ reduction and UO2 nanoparticle formation. The cells also produce an extracellular polymeric substance, a sticky slime that may serve as a medium for the reduction and further act as a glue to bind the nanoparticles and possibly prevent reoxidization back to U6+. This property could be the basis of a remediation process to halt underground migration of uranium and other heavy metals at contaminated sites.