The Role of Shewanella oneidensis MR‐1 Outer Surface Structures in Extracellular Electron Transfer

Abstract

The ability of the metal reducer Shewanella oneidensis MR-1 to generate electricity in microbial fuel cells (MFCs) depends on the activity of a predicted type IV prepilin peptidase; PilD. Analysis of an S. oneidensis MR-1 pilD mutant indicated that it was deficient in pili production (Msh and type IV) and type II secretion (T2S). The requirement for T2S in metal reduction has been previously identified, but the role of pili remains largely unexplored. To define the role of type IV or Msh pili in electron transfer, mutants that lack one or both pilus biogenesis systems were generated and analyzed; a mutant that lacked flagella was also constructed and tested. All mutants were able to reduce insoluble Fe(III) and to generate current in MFCs, in contrast to the T2S mutant that is deficient in both processes. Our results show that loss of metal reduction in a PilD mutant is due to a T2S deficiency, and therefore the absence of c cytochromes from the outer surface of MR-1 cells, and not the loss of pili or flagella. Furthermore, MR-1 mutants deficient in type IV pili or flagella generated more current than the wild type, even though extracellular riboflavin levels were similar in all strains. This enhanced current generating ability is in contrast to a mutant that lacks the outer membrane c cytochromes, MtrC and OmcA. This mutant generated significantly less current than the wild type in an MFC and was unable to reduce Fe(III). These results indicated that although nanofilaments and soluble mediators may play a role in electron transfer, surface exposure of outer membrane c cytochromes was the determining factor in extracellular electron transfer in S. oneidensis MR-1.

Spectroelectrochemical Investigation on Biological Electron Transfer Associated with Anode Performance in Microbial Fuel Cells

“…We confirmed that deletion of the gene encoding PilD (pilD), a predicted prepilin peptidase, also resulted in a comparable reduction with omcA/mtrC (Fig. 10) analysis of cell extracts of strain pilD has demonstrated that PilD is involved in the processing of type IV, Msh, and T2S prepilin proteins, and this strain is also reported to lack OMCs, including OmcA and MtrC (Bouhenni et al, 2010). Our electrochemical comparisons among WT, omcA/mtrC, and pilD cells clearly indicate that OmcA and MtrC function as the major reductases for EET from attached cells to electrode surfaces (i.e., DET reaction).…”

“…It should be noted that the E m of 50 mV detected for intact MR-1 cells is a markedly more positive potential compared to those reported for purified OM c-Cyts (170 mV for MtrC and 210 mV for OmcA) (Eggleston et al, 2008;Firer-Sherwood et al, 2008;Hartshorne et al, 2009). In addition to OM c-Cyts, MR-1 cells are known to synthesize several flavin and quinone derivatives that function as electron shuttles, a few of which strongly bind to the outer cell surface and provide redox signals characteristic of immobilized electroactive species (Saffarini et al, 2002;Okamoto et al, 2009;Bouhenni et al, 2010). Therefore, the large difference in E m between purified OM c-Cyts and intact cells has led many researchers to speculate that the observed redox wave at 50 mV is attributable to cell-attached menaquinone, whose midpoint potential is approximately 80 mV (Li et al, 2010;Zhao et al, 2010), rather than OM c-Cyts.…”

The direct electrocatalysis of phenazine-1-carboxylic acid excreted by Pseudomonas alcaliphila under alkaline condition in microbial fuel cells

“…To date, many exoelectrogens have been employed in MFCs, including Shewanella putrefaciens (Kim et al, 1999), Geobacter sulfurreducens (G. sulfurreducens) (Bond and Lovley, 2003), Rhodoferax ferrireducens (Chaudhuri and Lovley, 2003), Klebsiella pneumoniae (K. pneumoniae) (Zhang et al, 2008), etc. These exoelectrogens, which are all cultivated in MFCs under neutral pH conditions, primarily depend on four mechanisms of electron transfer from fuels to electrodes, including membrane-bound cytochromes (Bond and Lovley, 2003;Bouhenni et al, 2010;Zhang et al, 2008), conductive bacterial pili (Gorby et al, 2006;Reguera et al, 2006), oxidation of reduced secondary metabolites (Zhuang et al, 2010a) or soluble redox mediators (Bond and Lovley, 2005;Rabaey et al, 2004;Wang et al, 2007). Furthermore, previous literatures have proved that flavin (Marsili et al, 2008), 2-amino-3-carboxy-1,4-naphthoquinon (Freguia et al, 2009) and 2,6-di-tert-butyl-p-benzoquinone (Deng et al, 2010) could be excreted by Pseudomonas aeruginosa (P. aeruginosa), Lactococcus lactis, K. pneumoniae, respectively, facilitating electron transfer in MFCs.…”

Microbial nanowires: an electrifying tale

“…Extended periplasmic and outer membranes S. oneidensis possess three different types of extracellular proteinaceous appendages: (1) Msh pili, (2) TFP and (3) flagella (Bouhenni et al, 2010), but it was not clear which one of these acts as MNWs. Msh pili have been shown to be necessary for extracellular electron transfer (Fitzgerald et al, 2012), while TFP and flagella have been shown to be dispensable (Bouhenni et al, 2010).…”

“…Msh pili have been shown to be necessary for extracellular electron transfer (Fitzgerald et al, 2012), while TFP and flagella have been shown to be dispensable (Bouhenni et al, 2010). However, MNWs in S. oneidensis are made up of outer membrane vesicle chains which subsequently elongate and become MNWs ( Fig.…”