Role of outer membrane cytochromes OmcA and MtrC in electron transfer to oxide electrodes by Shewanella oneidensis MR-1

Dissimilatory Metal Reducing Bacteria (DMRB) play a primary role in reduction of Fe(III) and Mn(IV) oxides and the global cycling of Fe(III) and Mn(IV) in nature. DMRB are known to couple oxidation of organic substrates with electron transfer to insoluble electron acceptors. The study of the mechanisms that DMRB employ has applications in bioremediation of organic and toxic metal contaminants and in the development of Microbial Fuel Cells (MFCs) for energy production. One such widely studied metal-reducing bacterium is Shewanella oneidensis MR-1, which is postulated to use redox enzymes called c-type cytochromes to carry out electron transfer to solid oxide surfaces; c-type cytochromes are proteins containing one or more covalently bound hemes in their structure. Two outer membrane cytochromes in S. oneidensis MR-1, OmcA and MtrC, are hypothesized to act as terminal reductases during electron transfer. This hypothesis was tested in this study by comparing the currents generated in chronoamperometric experiments by S. oneidensis MR-1 wild type with currents generated by deletion mutants deficient in OmcA or MtrC or both OmcA and MtrC. These experiments were more like a natural system insofar as Fluorine-doped Tin Oxide (FTO) was used as a conductive anode in an MFC setup. FTO is an n-type extrinsic oxide semiconductor with electrochemical properties distinct from the conductive carbon used in similar studies by other researchers. In our experiments, the wild type S. oneidensis MR-1 (WT) produced higher current densities than any of the mutants. The mutants lacking both OmcA and MtrC exhibited markedly reduced current densities as compared to the WT. The same was also true for the mutants lacking either OmcA or MtrC. In addition, the mutant lacking MtrC seemed to be more severely affected in its electron transfer abilities as compared to the mutant lacking only OmcA, which suggests that MtrC is more essential for electron transfer by S. oneidensis MR-1 than OmcA. The results obtained in this study support the hypothesis that OmcA and MtrC act as terminal reductases in electron transfer by S. oneidensis MR-1 to solid FTO electrodes.