Charge Injection and Inter-Fiber Electrical Conduction in Cable Bacteria

Cable bacteria are multicellular microorganisms capable of charge transport over centimeterscale distances through a network of conductive fibers embedded in the periplasm. Understanding the charge injection mechanism in cable bacteria is essential to form a complete picture of their long-distance charge transport, and a crucial step for their application in bio-based electronics. To this aim, we fabricated ”crosses” of two filaments, either native bacteria or extracted fiber skeletons, placed one on top of each other. By probing charge transport both through single cables and in cross-cable configurations, i.e., with current flowing from one filament to the other, it is possible to isolate the charge injection contribution. The results indicate that charge transfer between two contacting fibers is possible, albeit with increased resistance. We characterized the crosses at different temperatures, from 300 down to 50 K, observing thermally activated Arrhenius behavior both for single cables and crossconduction.The corresponding activation energy for cable-to-cable transport ranged from 15 to 40 meV, slightly smaller than that of individual cable bacteria filaments. We conclude that charge injection into the fibers must rely on the same mechanism of charge transport along the fibers. A structural model of the fibers is proposed in which the conductive channels are embedded in a protein matrix but can locally reach the surface of the fibers, where they establish electrical contact with the external environment.