Proc. Natl. Acad. Sci. USA 10.1073/pnas.1410551111

Credit: SAHAND PIRBADIAN AND MOH EL-NAGGAR

Metal-reducing bacteria use extracellular nanowires to send electrons to external metal acceptors. However, the mechanisms involved in generating these wires are not clear, in part because characterization of these wires has been conducted under nonphysiological conditions and in part because the molecular composition of the wires remains unknown. Pirbadian et al. now report measurements using live cells that shed light on these intriguing structures. Fluorescence imaging of Shewanella oneidensis MR-1 upon O2 limitation using a protein-dependent dye highlighted filament formation and production of membrane vesicles, in agreement with previous work, whereas labeling with a redox-sensing dye showed that filament formation occurred with an increase in cellular reductase activity. Though previous proposals had suggested that nanowires are constructed using pilus proteins, expression of pilin genes did not increase during filament formation. Instead, the use of a lipid bilayer dye and fluorescent protein fusions demonstrated that the filaments include both membranes and periplasmic components, suggesting that the wires are outer membrane extensions. The authors confirmed that outer membrane cytochromes—previously shown to be required for electron transport by S. oneidensis—were not only expressed in higher amounts upon O2 limitation but also located along the wires. AFM images collected at different time points captured a nanowire maturation process that progressed from vesicle chains to continuous filaments. Given the common structural elements used to form the nanowires, the authors hypothesize that additional extracellular extensions might await discovery.