Proof positive: Protons move along filamentous bacteria

The bacteria of the family Desulfobulbaceae are like living electric cables—they can conduct electrons over centimeter-scale distances along their filamentous structures. These electric currents can make geochemical changes in the sediment. Now, for the first time, scientists have measured these microorganisms’ ability to conduct protons as well. (Proc. Natl. Acad. Sci. U.S.A. 2025, DOI: 10.1073/pnas.2416008122).

Research scientist Bradley Lusk, first author of the study, was curious about what happens to protons in the microenvironments of these bacteria during biodegradation: “If you produce too many protons, the pH gets low and you pickle yourself. . . . So my thought was, Is there any way these protons are being transported outside of these microenvironments?”

First, Lusk and colleagues had to come up with a way to measure protonic conductivity along the surface of Desulfobulbaceae (also known as cable bacteria). This has so far been difficult because protodes (the protonic counterpart of electrodes) show poor contact with bacteria, and the processes involved are unsuitable for biological material.

To overcome this, the researchers modified a technique developed for an earlier study on proton transfer between the biopolymer chitosan and palladium metal contacts. (Palladium absorbs hydrogen and breaks it down into protons.)

“We had to get a palladium protode with 10 µm gaps, align it with a cable bacterial cell, stamp it on top, and then find a way to pull off the stamp while leaving the metal behind,” Lusk says. For this, they used a sticky material called polydimethylsiloxane on which the palladium was printed. A custom-designed 3D-printed arm transferred the palladium to the bacterial filament under an optical microscope. To travel the entire length of the protrode, the organism itself would need to conduct protons across the protrode’s gaps.

The protode-stamped bacteria were then transferred to a sealed chamber, to which hydrogen was added. Electrical probes at two ends of the protode measured conductance. “The only way the protons could get from one side to the other was through the cable bacteria,” Lusk says.

Protonic conductivity varied among the 12 types of cable bacteria analyzed at 21 different conditions, with the highest being 114 ± 28 µS/cm at 25 °C and 70% relative humidity. Conductance increased by up to 26-fold, as relative humidity increased from 60% and 80%.

Curiously, the evolutionary benefit of protonic conductance in cable bacteria is unknown, the researchers point out. “But it’s really fascinating in terms of potential,” Lusk says, such as developing bioprotonic devices to act as interfaces between biotic and abiotic materials. These applications are a decade or two away, though, he adds.

Clara Santato, a researcher at Montreal Polytechnic who wasn’t involved in the study, says that this seems a very rigorous work. “It gives us new information on the functional properties of bacteria,” she says, including insights into developing sustainable organic electronics.