Electrons selective uptake of a metal-reducing bacterium Shewanella oneidensis MR-1 from ferrocyanide

The extracellular electron transfer of Shewanella oneidensis MR-1 (MR-1) has been extensively studied due to the importance of the biosensors and energy applications of bioelectrochemical systems. However, the oxidation of metal compounds by MR-1, which represents the inward extracellular electron transfer from extracellular electron donors into the microbe, is barely understood. In this study, MR-1 immobilized on an electrode electrocatalyzes the oxidation of [Fe(CN)₆]^4- to [Fe(CN)₆]^3- efficiently and selectively. The selectivity depends on midpoint potential and overall charge(s) of redox molecules. Among 12 investigated redox molecules, the negatively charged molecules with high midpoint potentials, i.e., [Ru(CN)₆]^4- and [Fe(CN)₆]^4-, show strong electrocatalysis. Neither reference bacteria (Escherichia coli K-12 nor Streptococcus mutans) electrocatalyze the oxidation of [Fe(CN)₆]^4-. The electrocatalysis decays when MR-1 is covered with palladium nanoparticles presumptively involved with cytochromes c. However, cytochromes c MtrC and OmcA on MR-1 do not play an essential role in this process. The results support a model that [Fe(CN)₆]^4- donor electrons to MR-1 by interacting with undiscovered active sites and the electrons are subsequently transferred to the electrode through the mediating effect of [Fe(CN)₆]^4-/3-. The selective electron uptake by MR-1 provides valuable and fundamental insights of the applications of bioelectrochemical systems and the detection of specific redox molecules.