What Are Bacterial Nanowires and How Do They Work?

Certain bacteria construct their own electrical wires, not from metal, but from biological filaments assembled from proteins. Known as bacterial nanowires, these structures are produced by specific microbes, enabling them to perform feats of survival and interaction. They represent an intersection of biology and electricity, where living cells have developed a method to conduct electrical charges.

These nanowires are one of many strategies bacteria have evolved to produce energy. Their existence reveals a microscopic world with electrical activity, challenging previous understandings of biological processes. The study of these living cables offers insight into complex microbial behaviors and novel technological possibilities.

The Physical Nature of Bacterial Nanowires

Bacterial nanowires are protein-based appendages that extend from the cell’s surface. Many are a specialized form of pili, which are hair-like structures bacteria use for movement and attachment. Unlike ordinary pili, however, these nanowires have the unique ability to conduct electricity. Their structure is fine, with diameters measured in nanometers, and they can be exceptionally long relative to the bacterial cell.

Two of the most studied bacteria that produce these conductive filaments are Geobacter sulfurreducens and Shewanella oneidensis. In Geobacter, the nanowires are assembled from a protein called PilA. The structure of Shewanella nanowires is different, consisting of outer membrane extensions containing electron-transporting proteins called cytochromes. The specific arrangement of proteins in these filaments allows them to act as biological wires.

One hypothesis suggests that aromatic amino acids within the protein structure of Geobacter pili overlap, creating a continuous pathway for electrons to move. This mechanism is similar to synthetic organic metals. For Shewanella, the model involves electrons “hopping” between the metal-containing cytochrome proteins embedded along the filament. These discoveries show that bacteria have evolved distinct molecular strategies to build conductive appendages.

Extracellular Respiration

The primary function of bacterial nanowires is to facilitate extracellular respiration. This is a form of anaerobic respiration, which occurs in environments lacking oxygen. Bacteria in oxygen-deprived settings, such as deep soil or sediment, must find other “electron acceptors” to complete their respiratory cycle and generate energy.

Nanowires allow a bacterium to transfer electrons from its internal metabolic processes to an external acceptor located some distance away. After a microbe like Geobacter metabolizes its food source, it generates electrons. These electrons then travel along the nanowire to reach a suitable acceptor in the environment, often a solid mineral like iron or manganese oxide.

This process of extracellular electron transfer (EET) is for the survival of these bacteria. By “breathing” minerals, the bacteria can thrive in environments that would otherwise be uninhabitable. The nanowire extends the cell’s metabolic reach, bridging the physical gap between the bacterium and the minerals it needs to respire.

Biofilm Conductivity and Communication

Beyond individual cell survival, nanowires play a role in microbial communities known as biofilms. A biofilm is a collection of microorganisms that attach to a surface and each other, encased in a self-produced slimy matrix. Within these communities, nanowires connect different bacterial cells, creating a living electrical grid.

This network allows for the efficient transfer of electrons between bacteria. A cell deep within a biofilm can pass its electrons to a neighboring cell. This process can continue, with electrons shuttled from cell to cell until they reach a bacterium at the edge of the biofilm that can transfer them to an external acceptor.

This electron-sharing capability supports the growth of thicker and more complex biofilms. It represents a form of metabolic cooperation where the entire community benefits from the electrical connections. The network also aids in respiration and is thought to function as a signaling mechanism, allowing bacteria to coordinate their behavior.

Harnessing Nanowires for Technology

The properties of bacterial nanowires have potential in various technological applications, one of which is microbial fuel cells (MFCs). In an MFC, bacteria with nanowires are used to break down organic waste in wastewater. As the bacteria metabolize the waste, they transfer electrons to an electrode via their nanowires, generating a direct electrical current while cleaning the water.

Another application is in bioremediation, which uses microorganisms to clean up environmental pollutants. Nanowire-producing bacteria like Geobacter can be used to treat sites contaminated with toxic metals. For instance, they can transfer electrons to soluble uranium, reducing it to an insoluble form that is less mobile and less of an environmental hazard. This process effectively immobilizes the contaminant in the soil.

Researchers are also exploring the field of bioelectronics. The idea is to use these naturally conductive and self-replicating wires to create microscopic electronic components. These could one day be integrated into biocompatible sensors or self-healing circuits. By understanding and manipulating the genetic basis of nanowire production, scientists hope to engineer strains of bacteria that produce wires with even higher conductivity, paving the way for a new generation of sustainable and living electronics.