Microbial dissimilatory reduction of metals is the process by which certain microbes utilize various metallic and metalloid reagents as terminal electron acceptors in cellular respiration. Metal reduction depends on the organism's ability to use a variety of extracellular electron-directing mechanisms potentially including direct electron transfer via outer membrane enzymes, nanowires, or pili, or through the use of soluble electron shuttles. Some bacterial genera, including Geobacter, Aeromonas, Rhodoferax, Desulfobulbus, and Shewanella, each noted for their ability to reduce insoluble extracellular metal ions, have proven useful for electricity production in experimental microbial fuel cells. However, the current range of dissimilatory metal reducing bacteria (DMRB) is unknown. Past studies have demonstrated that under the appropriate conditions, electrically conductive nanowires are not limited to DMRB and can develop in cyanobacteria and thermophilic fermentative bacteria. This finding greatly increases the range of potential metal reducing bacterial species. The objective of the current proposal is to develop a colorimetric plate assay capable of large-scale testing of bacterial cultures for dissimilatory metal-reduction activity. Past studies have also demonstrated that Shewanella oneidensis MR-1, grown anaerobically on lactate with silica ferrihydrite as the sole terminal electron acceptor, yielded a color change of red to black, which was indicative of the reduction of Fe(III) to Fe(II). Other metals, including vanadium, copper, chromium and cobalt exhibit color-changing properties based on oxidation state and may prove useful in testing for DMRB
