Microorganisms Creating Currents As Their Metabolism

Unlike most bacteria, which thrive on sugars and amino acids, Shewanella oneidensis is able to transfer electrons to metals such as iron, creating energy for its metabolic processes.

How It Works

Shewanella oneidensis makes use of a specialized process called extracellular electron transport (EET).

Instead of relying solely on soluble electron acceptors, like many other bacteria, Shewanella can use solid, insoluble metal oxides as electron acceptors.

The Key Proteins Involved

Cytochromes are the primary components involved in the EET process.

1. When metabolic processes generate electrons, e.g., through the oxidation of organic compounds like lactate and pyruvate, they are initially transferred to inner membrane proteins, creating a pool of reducing equivalents (such as NADH).

2. Electrons are passed from NADH to the inner membrane electron transport chain to get them to the periplasmatic space.

3. In the periplasm, the multi-heme cytochrome MtrA receives electrons and transfers them to the outer membrane.

4. The MtrCAB complex facilitates the movement of electrons across the outer membrane. MtrC and OmcA on the outer surface then directly transfer the electrons to external metal oxides, reducing them in the process – in essence it is a similar underlaying principle we know
from the electron transport chains in our mitochondria.

Why it matters - Environmental Impact

Shewanella oneidensis has a significant role in the natural cycling of metals within various ecosystems. In freshwater and marine environments, as well as deep-sea sediments, this bacterium contributes to the breakdown and transformation of metal compounds.

It might also be used for Bioremediation - its ability to reduce toxic metals such as chromium (Cr(VI)) means it can be used to clean up contaminated sites.

Talking Bioelectricity
One of the most intriguing applications of Shewanella’s unique metabolism is in the field of bioelectricity. Currently, many strategies to use Shewanella to convert chemical energy from organic compounds directly into electrical energy are under development.

To boost Shewanella's natural abilities various approaches are tested: genetic engineering to enhance protein expression for better electron transfer, incorporating biologically generated nanoparticles, and using synthetic compounds to improve transmembrane electron transport.

Other studies have even suggested ‘electro-genetics (EG)’, in which gene expression can be driven by intracellular redox-sensing systems that are controlled by using electrodes!

Isn’t that a fascinating topic