Nanoscale electrical characterization reveals an anisotropic resistor network inside the conductive fibers of cable bacteria

Cable bacteria are multicellular microorganisms that stand out in the biological world for their ability to transport electrons over centimeter-scale lengths. These long-distance currents are internally conveyed through an array of fibers embedded in the cell envelope, but the detailed internal architecture of this transport network is presently unknown. Here, we charted the current pathways within these highly conductive fibers via detailed two-probe and four-probe electrical characterization of short (50 nm) segments of cable bacteria fiber skeletons. Four-probe measurements reveal that segments can exhibit a negative current-voltage relationship, i.e., a negative voltage drop is measured for a positive bias current. Moreover, the four-probe segment resistance is shown to be strongly dependent on the current injection position. This unconventional behavior suggests the presence of a complex current path in the conductive fibers, which is modeled as an anisotropic resistor network consisting of high-Ohmic interconnections between parallel low-Ohmic conduits. Analog circuit simulations demonstrate that such network can reproduce the experimental results. The electrical model concurs with recent microscopy observations, which indicate that current in the fibers is conveyed through a bundle of parallel, interconnected metal-organic nanoribbons. The experimentally observed electrical behavior of a fiber is hence determined by the topology of the nanoribbon network and the scale at which it is probed.