Spatially Resolved Chemical Analysis of Geobacter sulfurreducens Cell Surface

Abstract

Geobacter sulfurreducens is of interest for the highest efficiency of power generation and extremely long extracellular electron transfer (EET) between the bacterium and electrodes. Despite more than 15 years of intensive molecular biological research, there is still no clear answer which molecules are responsible for these processes. In the present work, we look at the problem from another (atomic) perspective and identify the location and shape of the compounds that are known to be conductive, particularly those containing Fe atoms. By using highly sophisticated energy dispersive X-ray spectroscopy combined with highangle annular dark-field transmission electron microscopy enabling detection, identification, and localization of chemical compounds on the surface at nearly atomic spatial resolution, we analyze Fe spatial distribution within the G. sulfurreducens community. We discover the presence of small Fe-containing particles on the surface of the bacterium cells. The size of the particles (diameter 5.6 nm) is highly reproducible and comparable with the size of a single protein. The particles cover about 2% of the cell surface, which is similar to that expected for molecular conductors responsible for electron transfer through the bacterium cell wall. We find that G. sulfurreducens filaments ("bacterial molecular wires") also contain Fe atoms in their bundles. We observe that the bacterium enable changing the distance between the Fe-containing bundles in the filaments from separated to attached (the latter is needed for the efficient electron transfer between the Fe-containing particles), depending on the bacterium metabolic activity and attachment to extracellular substrates. These results are consistent with the recently published research about the role of Fe atoms in protein molecular conductance (Phys. Chem. Chem. Phys., 2018, 20, 14072−14081) and show what type of Fe-containing particles are involved in the bacterial extracellular communication. They can be used for the design and construction of artificial biomolecular wires and bioinorganic interfaces.

Geobacter protein nanowires

“…The elemental composition 522 of the filaments was consistent with that expected for filaments comprised of PilA pilin 523 monomer (Lebedev et al, 2019). Energy dispersive X-ray spectroscopy (EDXS) detected 524 iron, which appeared to be associated with protein "bundles" randomly distributed along 525 the filaments (Lebedev et al, 2019). This pattern of putative iron-containing proteins 526 along the filaments was similar to the patterns described above for immunogold labeling for OmcS (Leang et al, 2010) and the distribution of globules along apparent e-pili associated with OmcS expression (Figure 8).…”

“…diameter) of 3.5 nm. The elemental composition 522 of the filaments was consistent with that expected for filaments comprised of PilA pilin 523 monomer (Lebedev et al, 2019). Energy dispersive X-ray spectroscopy (EDXS) detected 524 iron, which appeared to be associated with protein "bundles" randomly distributed along 525 the filaments (Lebedev et al, 2019).…”

“…This pattern of putative iron-containing proteins 526 along the filaments was similar to the patterns described above for immunogold labeling for OmcS (Leang et al, 2010) and the distribution of globules along apparent e-pili associated with OmcS expression (Figure 8). Cells grown at 30 °C, a temperature that inhibits e-pili expression (Reguera et al, 2005), expressed filaments in which iron was more closely packed (Lebedev et al, 2019). However, the height of these filaments was 2-3 nm, too thin for OmcS.…”

Geobacter Protein Nanowires

“…Elemental mapping of filaments extending from G. sulfurreducens provided additional insights into the potential interactions of cytochromes and pili (Lebedev et al, 2019). Cells grown at 25°C with fumarate as the electron acceptor, a temperature that induces e-pili expression (Reguera et al, 2005), expressed filaments that atomic force microscopy indicated had a height (i.e.…”

Peak selection matters in principal component analysis: A case study of syntrophic microbes

“…5,6 The presence of small Fe-containing particles on the surface and microbial nanowires of the Geobacter sulfurreducens are considered responsible for extracellular electron transfer. 7 The role of conductive nanowires in DIET of Geobacter species can be promoted by the addition of conductive nanoparticle materials. 8,9 However, the details of chemical composition and structure properties on the syntrophic Geobacter biofilm surface remain unclear.…”