Electron transport across bacterial cell envelopes

Extracellular electron transfer is an ancient and ubiquitous process that is used by a range of microorganisms to exchange electrons between the cell and environment. These electron transfer reactions can impact the solubility and speciation of redox-active molecules in the environment, such as metal oxides, while allowing bacteria to survive in areas of limited nutrient availability. Controlled transfer of electrons across the cell envelope requires assembly of electron transport chains that must pass through the outer membrane of Gram-negative bacteria or the S-layer of Gram-positive bacteria, but the mechanisms used by bacteria are still far from understood. Here, we review the literature surrounding characterized extracellular electron transfer pathways and use protein modeling tools to investigate novel electron transfer proteins and protein complexes. While these protein models are hypothetical, they provide new insight into features that may explain how extracellular electron transfer complexes interact with a range of different environmental substrates.