The ultrastructure of Shewanella oneidensis MR-1 nanowires revealed by electron cryo-tomography
AbstractBacterial nanowires have garnered recent interest as a proposed Extracellular Electron Transfer (EET) pathway that links the bacterial electron transport chain to solid-phase electron acceptors away from the cell. In vivo fluorescence Light Microscopy (fLM) imaging recently showed that Shewanella oneidensis MR-1 nanowires are extensions of the outer membrane that contain EET components. However, their fine structure and distribution of cytochrome electron carriers remained unclear, making it difficult to evaluate the electron transport mechanism along the nanowires. Here, we report high-resolution images of nanowires using Electron Cryo-Tomography (ECT). We developed a robust method for fLM imaging of nanowire growth on electron microscopy grids and used correlative light and electron microscopy to identify and image the same nanowires by ECT. Our results confirm that S. oneidensis nanowires are outer membrane extensions, and further reveal that nanowires are dynamic chains of interconnected Outer Membrane Vesicles (OMVs) with variable dimensions, curvature, and extent of tubulation. Junction densities that potentially stabilize OMV chains are seen between neighboring vesicles in cryotomograms. Our ECT results also provide the first hints of the positions and packing of periplasmic and outer membrane proteins consistent with cytochromes. We observe tight packing of putative cytochromes along lateral patches that extend tens of nanometers, but not across the micrometer scale of whole nanowires. We therefore propose that electron transfer along nanowires involves a combination of direct hopping and diffusive events that link neighboring redox proteins.