Cable bacteria: Living electrical wires with record conductivity

Bacteria that power themselves using electricity and are able to send electrical currents over long distances through highly conductive power lines. It sounds like the way we charge our TVs and refrigerators, but it effectively occurs in centimeter-long cable bacteria .

Go to the profile of Filip Meysman
Sep 12, 2019
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The discovery of electricity is considered one of the greatest scientific achievements of humankind, and was pioneered by creative minds like Benjamin Franklin and Allesandro Volta. Electricity experienced a real breakthrough in 1879, when Thomas Edison produced the first durable electric light bulb. Nowadays, modern society thrives on electricity, and an extensive network of power lines and copper wiring ensures that our offices, kitchens and bedrooms can be brightly illuminated at night.

However, bacteria have discovered and exploited the advantages of such electrical networks way before we humans did. This is demonstrated by cable bacteria, which are centimeter-long multicellular microbial filaments that live in the surface layer of marine and freshwater sediments. From their very first discovery (Pfeffer et al, 2012), it was clear these cable bacteria were doing something exceptional. Detailed investigations of the sediment chemistry showed that electrical currents must be running through the seafloor, and all data suggested that the cable bacteria were generating and conducting these currents. Such an electricity-based metabolism gives the cable bacteria a considerable advantage, being able to harvest energy from sulfide in deeper layers of the seafloor (see review in Meysman, 2018).

Microscopy image of a bundle of cable bacterium filaments (Silvia Hidalgo Martinez, University of Antwerp)

Although the indirect evidence was convincing (Bjerg et al., 2018), there was yet no direct proof that cable bacteria were indeed conductive. Attempts to connect the cable bacteria directly to electrodes remained unsuccesful. Nor did we know the conductive structure that enabled the centimetre-long currents in cable bacteria. These two questions are now resolved in our new study (Meysman et al., Nat. Comm, 2019). We found out that the cell envelope of cable bacteria contains a parallel network of conductive fibers that connects all cells across the filament. Further investigation revealed that these fibers have an extraordinary high conductivity. Hence, this conductive network in cable bacteria truly operates in comparable way as the copper wiring that we use at home to efficiently supply electricity to our fridge and washing machine .

On the way to these results, we encountered two important hurdles. The first challenge was to demonstrate that individual cable bacterium filaments were indeed conductive. To this end, we invented a procedure to carefully extract a single bacterial filament out of a sediment enrichment and transfer this thin filament to a custom-made setup with two electrodes connected to a source meter (like physicists would test the conductivity of a wire – see accompanying video). It took us considerable effort to optimize and fine-tune the handling procedure of the cable bacteria. Yet, after many trial-and-failure moments, we eventually mastered the trick, and the Eureka moment suddenly happened: a nice current (I) versus voltage (V) was evolving on the screen. The proof was given that single filaments were conductive. Moreover, when we gradually increased the non-conductive spacing between the electrodes, we were able to show currents in a filament that was more than 1 cm long. This extends the known length scale of biological electron transport by orders of magnitude.

This observation of conduction in an intact cable bacterium filament, however, immediately brought on the next question: what are the conductive structures inside the bacteria that can sustain such high electrical currents? Electron microscopy had already previously revealed that the cell wall of the cable bacteria contains a parallel network of fibers which run along the whole length of the bacteria (Cornelissen et al., 2018). So it was time to take the second hurdle: investigating whether the fibers were indeed the conductive structures. To this end, we invented a kind of chemical car-wash procedure that removes cell material in a sequential fashion, and eventually only leaves the fiber structure behind. When we put this so-called “fiber sheath” onto our electrode set-up, we again saw high currents, demonstrating that the fiber network in the cell envelope is actually the conductive structure.

However, the surprise was not over yet. Our electrical measurements revealed that the fibres sustained an extremely high electric current per unit of cross-sectional area, which readily compares to the current density that passes through the copper wiring in household appliances. Even more exciting, the conductivity of the fibres was unusually high, with values exceeding 20 S cm-1. For reference, this conductivity rivals that of the state-of-the-art conductive polymer materials used today in organic electronics (think of flexible solar panels or foldable phones). We were totally surprised when we first obtained these high numbers. Somehow, biological evolution has invented a structure with extraordinary electrical properties. 

The discovery of the highly conductive fibers in cable bacteria is remarkable, as all known biological materials (like proteins, carbohydrates, lipids, nucleic acids) are typically very poor in terms of electrical conduction. It could open a window on new technology. The prospect of a bio-based material with exceptional electrical properties could perhaps push material science and electronics far beyond its current limits.

More information on cable bacteria can be found at: www.microbial-electricity.eu

References

 

Go to the profile of Filip Meysman

Filip Meysman

Professor, University of Antwerp

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