Shewanella oneidensis and Extracellular Electron Transfer to Metal Oxides

Abstract

Anaerobic metal reduction by bacteria plays an important role in biogeochemical cycles, bioremediation, and in biotechnological applications such as electricity generation. Shewanella oneidensis is one of the best-studied metal reducing bacteria and its analysis led to the identification of the mechanisms this bacterium uses for respiratory metal reduction. The major proteins involved in metal reduction in S. oneidensis consist of an outer membrane decaheme c-type cytochrome (MtrC), an outer membrane porin (MtrB) and a periplasmic decaheme c-type cytochrome (MtrA). These proteins form a complex that is located on the outer cell surface and transfers electrons extracellularly to the metal oxides. Although other proteins, such as the outer membrane decaheme c-type cytochrome OmcA, are thought to be involved in metal reduction, their role in this process appears to be minor. Several mechanisms to explain the extracellular electron transfer to metal oxides have been proposed. These include direct contact of cells with metal oxides, the use of flavins or electron shuttles, and the use of conductive appendages or nanowires. Flavins, which are thought to allow metal reduction at a distance from the cells, have been shown to function as cofactors that bind to the outer membrane cytochromes and mediate electron transfer. Conductive appendages or pili, also known as nanowires, have been implicated in mediating electron transfer at a distance. However, S. oneidensis mutants that lack pili are able to reduce metals similar to the wild type. Recently, these appendages have been shown to consist of membrane extensions and membrane vesicles. Thus, metal reduction by S. oneidensis appears to be mostly the result of direct contact of cell’s outer membrane cytochromes with the insoluble metal oxides.