  Multicellular cable bacteria display an exceptional form of biological
conduction, channeling electrical currents across centimeter distances through
a regular network of protein fibers embedded in the cell envelope. The fiber
conductivity is among the highest recorded for biomaterials, providing a
promising outlook for new bio-electronic technologies, but the underlying
mechanism of electron transport remains elusive. Here, we use detailed
electrical characterization down to cryogenic temperatures, which reveals that
long-range conduction in these bacterial protein wires is based on a unique
type of quantum-assisted multistep hopping. The conductance near room
temperature reveals thermally activated behavior, yet with a low activation
energy, suggesting that substantial delocalization across charge carrier sites
contributes to high conductivity. At cryogenic temperatures, the conductance
becomes virtually independent of temperature, thus indicating that quantum
vibrations couple to the charge transport. Our results demonstrate that quantum
effects can manifest themselves in biological systems over macroscopic length
scales.
