Hopping, Tunneling, or Wire? How Bacteria Move Electrons Across Micrometers — and What Would Prove It

Some bacteria take electrons from their food and must transfer them onto something outside the cell — a rust mineral, or the electrode of a battery — along protein filaments micrometers long. A single tunneling jump dies beyond about 2 nanometers, so the electron either crosses as a long chain of short hops between heme groups (Marcus-theory hopping) or flows through the filament as through a metal. This paper reviews the evidence fairly: the filament long assumed to carry the current (the pilus) was shown in 2019 to be scaffolding, while the real conductors are cytochrome polymers — OmcS (2019) and OmcZ (2022) — whose hemes sit 3.5–6 ångströms apart, inside easy hopping range. The metallic-like conductivity reported in 2011–2012 was measured on mixed filament populations whose identity was assumed, not proven, and is re-read here as measurements of crowds, not single wires. The paper states a position — Marcus hopping with short-range coherence where hemes stack almost touching — together with an explicit defeat condition, and proposes a decisive test: temperature-resolved conductivity of single, genetically identity-certain filaments (OmcS-only, OmcZ-only, PilA-only), where hopping and metallic conduction make opposite-signed predictions. A companion mud-battery measurement is described together with a plain statement of its limits. Theory preprint; not peer-reviewed.