Dr. Liang Shi from Pacific Northwest National Laboratory, USA paid a visit to Institute of Hydrobiology, Chinese Academy of Sciences (IHB) on July 14th. During his visit, he gave an oral presentation with the title of “Microbial Extracellular Electron Transfer Pathways” for IHB researchers and graduate students. 

In his presentation, Dr. Liang Shi put forward that the Gram-negative bacterium Shewanella oneidensis MR-1 is a very good model for studying the adaptive mechanisms of microorganisms to the absence of O₂ and other electron acceptors, and the mechanisms transferring electrons. In the absence of O₂ and other electron acceptors, S. oneidensis MR-1 can use ferric [Fe(III)] (oxy)(hydr) oxide minerals as the terminal electron acceptors for anaerobic respiration. At circumneutral pH and in the absence of strong complexing ligands, Fe(III) oxides are relatively insoluble and thus are external to the bacterial cells. S. oneidensis MR-1 and related strains of metal-reducing Shewanella have evolved an machinery (i.e., metal-reducing or Mtr pathway) for transferring electrons from the inner-membrane, through the periplasm and across the outer-membrane to the surface of extracellular Fe(III) oxides.  

The protein components identified to date for the Mtr pathway include CymA, MtrA, MtrB, MtrC and OmcA. CymA is an inner-membrane tetraheme c-type cytochrome (c-Cyt) that belongs to the NapC/NrfH family of quinol dehydrogenases. CymA oxidizes the quinol in the inner-membrane and transfers the released electrons to MtrA. A decaheme c-Cyt, MtrA is thought to be embedded in the trans outer-membrane and porin-like protein MtrB. Together, MtrAB deliver the electrons through the outer-membrane to the MtrC and OmcA on the outmost bacterial surface.  

MtrC and OmcA are the outer-membrane decaheme c-Cyts that are translocated across the outer-membrane by the bacterial type II secretion system. Functioning as terminal reductases, MtrC and OmcA can bind the surface of Fe(III) oxides and transfer electrons directly to these minerals via their solvent-exposed hemes. Although the understanding of the Mtr pathway is still far from complete, it is the best characterized microbial pathway used for extracellular electron exchange. Characterizations of the Mtr pathway have made significant contributions to the molecular understanding of microbial extracellular electron transfer reactions. 

Further survey of microbial genomes has identified homologues of the Mtr pathway in other Fe(III)-reducing bacteria as well as in several Fe(II)-oxidizing bacteria. The apparent widespread distribution of Mtr pathways in both Fe(III)-reducing and Fe(II)-oxidizing bacteria suggests a bi-directional electron transfer role, and emphasizes the importance of this type of extracellular electron transfer pathway in microbial iron redox cycle. The key organizational and electron transfer characteristics of the Mtr pathways are also be shared by other pathways used by microorganisms for exchanging electrons with their extracellular environments. The microorganisms with extracellular electron transfer capabilities have great potential in bioremediation of contaminants in subsurface sediments, in bioenergy production and in electrobiosynthesis of valuable chemicals.