Bacterial Nanowires of Shewanella Oneidensis MR-1 are Outer Membrane and Periplasmic Extensions of the Extracellular Electron Transport Components

Bacterial nanowires offer a pathway for extracellular electron transfer (EET) by linking the respiratory chain of bacteria to external surfaces, including oxidized metals in the environment and engineered electrodes in renewable energy devices. Specifically, nanowires of the model metal-reducing bacterium Shewanella oneidensis MR-1 were previously shown to be conductive under non-physiological conditions. Despite the global, environmental, and technological consequences of bacterial nanowire-mediated EET, the composition, electron transport mechanism, and physiological relevance of these appendages remain unclear. The nanowires of S. oneidensis MR-1 were previously thought, but never shown, to be bacterial pili. In addition, the transport mechanism through bacterial nanowires has been the subject of intense debate, with “metallic-like” band transport and multistep redox hopping between multiheme cytochromes as the two proposed mechanisms. Here we report the first in vivo observations of the formation and respiratory impact of nanowires in S. oneidensis MR-1. Using live fluorescence measurements and quantitative gene expression analysis, we demonstrate that S. oneidensis MR-1 nanowires are extensions of the outer membrane and periplasm, rather than pilin-based structures. We show, through immunolabeling, that multiheme cytochromes localize to nanowires, in turn supporting the multistep redox hopping model as the transport mechanism. Furthermore, these bacterial nanowires are associated with outer membrane vesicles, structures ubiquitous in Gram-negative bacteria, and occasionally appear as membrane vesicle chains that transition to smoother filaments. Redox-functionalized membrane and vesicular extensions may represent a general microbial strategy for electron transport and energy distribution.