The blind men and the filament: Understanding structures and functions of microbial nanowires

Abstract

Extracellular electron transfer via filamentous protein appendages called 'microbial nanowires' has long been studied in Geobacter and other bacteria because of their crucial role in globally-important environmental processes and their applications for bioenergy, biofuels, and bioelectronics. Thousands of papers thought these nanowires as pili without direct evidence. Here, we summarize recent discoveries that could help resolve two decades of confounding observations. Using cryo-electron microscopy with multimodal functional imaging and a suite of electrical, biochemical, and physiological studies, we find that rather than pili, nanowires are composed of cytochromes OmcS and OmcZ that transport electrons via seamless stacking of hemes over micrometers. We discuss the physiological need for two different nanowires and their potential applications for sensing, synthesis, and energy production.

Keywords: Atomic force microscopy; Biofilms; Conformational change; Cryo-electron microscopy; Cytochromes; Electrochemically-active bacteria; Electron transport; Geobacter; Interspecies electron transfer; Microbial fuel cells; Microbial nanowires; Multimodal imaging; Pili; Protein structure.