On the electrical conductivity of microbial nanowires and biofilms

Dissimilatory metal-reducing bacteria (DMRB), such as Geobacter and Shewanella spp., occupy a distinct metabolic niche in which they acquire energy by coupling oxidation of organic fuels with reduction of insoluble extracellular electron acceptors (i.e., minerals). Their unique extracellular electron transfer (EET) capabilities extend to reduction of anodes (electrodes maintained at sufficiently positive potentials) on which they form persistent, electric current generating biofilms. One hypothesis describing the mechanism of EET by Geobacter and Shewanella spp. involves superexchange in which electrons are conducted by a succession of electron transfer reactions among redox proteins associated with the outer cell membranes, aligned along pilus-like filaments (e.g. pili), and/or throughout the extracellular matrix. Here we present theory, previously developed to describe superexchange within abiotic redox polymers, to describe superexchange within DMRB biofilms grown on anodes. We show that this theory appears to apply to recent ex situ measurements of electrical conductivity by individual pilus-like filaments of S. oneidensis MR-1 and G. sulfurreducens DL1, referred to as microbial nanowires. Microbial nanowires have received much recent attention because they are thought by some to impart electrical conductivity to DMRB biofilms and because of the prospect of microbe-produced conductive nanomaterials. We also show that this theory appears to apply to preliminary in situ demonstration of electrical conductivity of an anode-grown G. sulfurreducens DL1 biofilm. Based on these results we suggest a role for nanowires of S. oneidensis and G. sulfurreducens in biofilm conductivity.