Outer Membrane c -Type Cytochromes OmcA and MtrC Play Distinct Roles in Enhancing the Attachment of Shewanella oneidensis MR-1 Cells to Goethite

Abstract

The outer membrane c-type cytochromes (c-Cyts) OmcA and MtrC in Shewanella are key terminal reductases that bind and transfer electrons directly to iron (hydr)oxides. Although the amount of OmcA and MtrC at the cell surface and their molecular structures are largely comparable, MtrC is known to play a more important role in dissimilatory iron reduction. To explore the roles of these outer membrane c-Cyts in the interaction of S. oneidensis MR-1 with iron oxides, the attachment processes of S. oneidensis MR-1 wild type and c-type cytochrome-deficient mutants (ΔomcA, ΔmtrC and ΔomcA-ΔmtrC) to goethite are compared via quartz crystal microbalance with dissipation monitoring (QCM-D). Strains with OmcA exhibit a rapid initial attachment. The quantitative model for QCM-D responses reveals that MtrC enhances the contact area and contact elasticity of cells with goethite by more than one and two times, respectively. In situ ATR-FTIR 2D-CoS analysis shows that MtrC promotes the initial interfacial reaction via an inner-sphere coordination. AFM analysis demonstrates that OmcA enhances the attractive force between cells and goethite by about 60%. As a result, OmcA contributes to higher attractive force with goethite and induces a rapid short-term attachment, while MtrC is more important in the longer-term interaction, through an enhanced contact area, which promotes interfacial reactions. These results reveal c-Cyts OmcA and MtrC adopt different mechanisms for enhancing the attachment of S. oneidensis MR-1 cells to goethite. It improves our understanding of the function of outer membrane c-Cyts and the influence of cell surface macromolecules in cell-mineral interactions. IMPORTANCE Shewanella species are one group of versatile and widespread dissimilatory iron-reducing bacteria, which are capable of respiring insoluble iron minerals via six multiheme c-type cytochromes. Outer membrane c-type cytochromes (c-Cyts) OmcA and MtrC are the terminal reductases in this pathway which share comparable protein structures. In this study, we elucidate the different roles of OmcA and MtrC in the interaction of S. oneidensis MR-1 with goethite at the whole-cell level. OmcA confers enhanced affinity towards goethite and results a rapid attachment. Meanwhile, MtrC increases the contact area of bacterial cells with goethite significantly and promotes the interfacial reaction which may explain its central role in extracellular electron transfer. This study provides novel insights into the role of bacterial surface macromolecules in the interfacial interaction of bacteria with minerals, which is critical to develop a comprehensive understanding of cell-mineral interactions.

In SituOptical Quantification of Extracellular Electron Transfer using Plasmonic Metal Oxide Nanocrystals

“…It is notable that even the fully induced mtrC complement did not completely recapture the wild-type EET phenotype, exhibiting ~4.5-fold less activity compared to the empty vector control. This may highlight the importance of other missing EET components in this strain, specifically omcA and mtrF , in binding and electron transfer to metal oxide substrates (50, 51). Tiered electron flux based on varying gene expression was also quantified as current output, ranging from 0.12 ± 0.04 (uninduced) to 0.68 ± 0.08 (fully induced) fA • cell −1 at steady-state (Table 2).…”

In Situ Optical Quantification of Extracellular Electron Transfer Using Plasmonic Metal Oxide Nanocrystals**

“…It is notable that even the fully induced mtrC complement did not completely recapture the wild‐type EET phenotype, exhibiting ∼4.5‐fold less activity compared to the empty vector control. This may highlight the importance of other missing EET components in this strain, specifically omcA and mtrF , in binding and electron transfer to metal oxide substrates [56,57] . Tiered electron flux based on varying gene expression was also quantified as current output, ranging from approximately 0.12±0.04 (uninduced) to 0.68±0.08 (fully induced) fA ⋅ cell −1 at steady‐state (Table 2).…”

Evidence for Horizontal and Vertical Transmission of Mtr-Mediated Extracellular Electron Transfer among the Bacteria

“…Genetic and biophysical analysis suggests that substrate specificity could be an accessory function for these proteins ( 31 , 101 ). OmcA, for example, is thought to enhance adherence to solid substrates like electrodes ( 31 , 125 ), hematite ( 105 , 126 ), and goethite ( 127 ), while UndA may specialize in facilitating electron transfer to soluble substrates like ligand-bound Fe 3+ ( 102 , 108 ). MtrCAB and MtrDEF may be adapted to different conditions as well, as MtrCAB is preferentially expressed under iron-limited or O 2 -limited conditions, while MtrDEF prevails under iron-replete conditions or when cells are aggregated ( 128 – 130 ).…”