Hydrated cable bacteria exhibit protonic conductivity over long distances

This study presents the direct measurement of proton transport along filamentous Desulfobulbaceae, or cable bacteria. Cable bacteria are filamentous multicellular microorganisms that have garnered much interest due to their ability to serve as electrical conduits, transferring electrons over several millimeters. Our results indicate that cable bacteria can also function as protonic conduits because they contain proton wires that transport protons at distances >100 µm. We find that protonic conductivity (σ_P) along cable bacteria varies between samples and is measured as high as 114 ± 28 µS cm^-1 at 25 °C and 70% relative humidity (RH). For cable bacteria, the protonic conductance (G_P) and σ_P are dependent upon the RH, increasing by as much as 26-fold between 60% and 80% RH. This observation implies that proton transport occurs via the Grotthuss mechanism along water associated with cable bacteria, forming proton wires. In order to determine σ_P and G_P along cable bacteria, we implemented a protocol using a modified transfer-printing technique to deposit either palladium interdigitated protodes (IDP), palladium transfer length method (TLM) protodes, or gold interdigitated electrodes (IDE) on top of cable bacteria. Due to the relatively mild nature of the transfer-printing technique, this method should be applicable to a broad array of biological samples and curved materials. The observation of protonic conductivity in cable bacteria presents possibilities for investigating the importance of long-distance proton transport in microbial ecosystems and to potentially build biotic or biomimetic scaffolds to interface with materials via proton-mediated gateways or channels.