Resolving Shewanella vesicular nanowire structure during microbial extracellular electron transfer to a poised electrode

Abstract

The nanowires of the model electroactive bacterium Shewanella oneidensis have been the subject of numerous studies to elucidate their structure and function. These previous reports have elegantly utilised advanced microscopic techniques to investigate nanowires formed in response to oxygen limitation. However, the detailed structure of nanowires formed on electrodes during extracellular electron transfer has not been reported and it is imperative to determine whether they possess the same vesicular structure that has been reported in the absence of extracellular electron transfer. Using an acetone hexamethyldisilazane dehydration method to preserve soft biological materials, we employed the relatively uncomplicated technique of secondary electron field emission-scanning electron microscopy to visualise the vesicular nanowire structure while attached to an electrode from an operating bioelectrochemical system. Early-stage nanowires appear to consist of intact chains of outer-membrane vesicles forming connections with the electrode surface and with neighbouring cells. Relying on secondary electrons from the inherently conductive carbon felt electrode, sputter coating could be avoided and the delicate structure of the vesicles was preserved with increased detail. The findings inform the fundamental understanding of nanowires during electron transfer and the simple protocol will allow their examination on a variety of existing and emerging electrode materials.