Genomics:GTL

Highlights of Research Progress

Bacteria Use “Nanowires” to Facilitate Extracellular Electron Transfer

GTL science and capabilities are being leveraged to identify and characterize the composition, function, and expression of extracellular appendages grown by some bacteria to facilitate electron transfer in challenging environments important to DOE missions. These appendages are electrically conductive and are hypothesized to function as biological “nanowires.”Below are some highlights of research on nanowires in Shewanella and Geobacter species. In addition to providing insights into microbes with potential uses in bioremediation strategies, these remarkable structures may one day have commercial applicability.

Shewanella

Nanowires were revealed in S. oneidensis MR-1 cells experiencing electron-acceptor limitation (EAL) using scanning tunneling microscopy (STM) and tunneling spectroscopy. Shewanella is a metabolically versatile is a metabolically versatile bacterium that uses a variety of electron acceptors, including nitrate, metals such as solid-phase iron and manganese oxides, and radionuclides such as uranium and technetium. A GTL Shewanella collaborative team uses an integrated approach to study this organism’s electron-transport and energy-transduction systems.

Nanowires Facilitate Extracellular Electron Transfer via c-Type Cytochromes. Shewanella nanowires were observed using scanning electron microscopy (SEM) and STM of MR-1 cells grown in chemostats under EAL. The ability to be imaged by STM indicated that the material is conductive, allowing electrons to tunnel from the probe tip to the underlying graphite surface. Peptide-specific antibodies against outer membrane cytochromes MtrC and OmcA were used in immunoEM experiments to investigate their cellular location. ImmunocytoTEM (transmission electron microscopy) analysis of MR-1 cells grown under EAL revealed that MtrC and OmcA are associated with extracellular structures morphologically identical to the MR-1 nanowires observed by SEM and STM.

Nanocrystalline Magnetite Particles are Associated with Nanowires. Since nanowires can conduct electrons in vitro, investigations have been made of the association between nanowires and the Fe(III) mineral ferrihydrite in vivo. TEM analyses of MR-1 grown anaerobically in the presence of ferrihydrite revealed nanocrystalline magnetite arranged in linear arrays along features consistent with nanowires.

In addition, a mutant deficient in the outer membrane decaheme cytochromes MtrC and OmcA was unable to reduce hydrous ferric oxide or transfer electrons directly to electrodes in a mediator-less fuel cell, directly linking these cytochromes to extracellular electron transfer in MR-1. Also observed was the production of nanowires in several other microbes in direct response to electron-acceptor limitation, including Geobacter sulfurreducens and Desulfovibrio desulfuricans, suggesting that nanowires may be common to other bacteria and microbial consortia dependent on electron transfer. Furthermore, nanowires could be responsible for cell-to-cell electron-transfer processes in biofilms and complex microbial mat communities. [Yuri Gorby and Jim Fredrickson, Pacific Northwest National Laboratory]

Reference

A. S. Beliaev et al., “MtrC, an Outer Membrane Decahaem c Cytochrome Required for Cytochrome Required for Metal Reduction in Shewanella putrefaciens MR-1,”Mol. Microbiol. 39, 722–30 (2001).

Geobacter

Field experiments have demonstrated that stimulating the growth of Geobacter species in uranium-contaminated subsurface environments precipitates the uranium from the groundwater and prevents its spread. To support their growth, Geobacter species require Fe(III) oxide minerals, naturally present in species require Fe(III) oxide minerals, naturally present in the subsurface, as an electron acceptor. Transferring electrons outside the cell onto an insoluble mineral represents a physiological challenge not faced by microorganisms that use such commonly considered, soluble electron acceptors as oxygen, nitrate, and sulfate. Understanding electron transfer to Fe(III) oxide is essential to optimize strategies for the in situ bioremediation of uranium-contaminated groundwater.

Pili Extend as Nanowires to Transfer Electrons. Investigators noted that Geobacter species specifically produced species specifically produced fine, hair-like structures known as pili on one side of the cell during growth on Fe(III) oxide.1 Knocking out a key gene for pili production prevented G. sulfurreducens from growing on from growing on insoluble Fe(III) oxides but had no effect on growth with soluble electron acceptors. Although pili in other organisms often function in attachment to surfaces, the mutant strain could attach to Fe(III) oxides as well as the wild-type strain. Further investigation with an atomic force microscope fitted with a tip capable of conducting electrical current demonstrated that the pili of G. sulfurreducens are highly conductive. These results suggest that Geobacter species are able to transfer electrons onto Fe(III) oxide with conductive pili that extend as nanowires from the cell.² Mechanisms for pili conductivity and electron transfer have yet to be eludicated.

Potential Applications of Nanowires. Conductive pili produced by G. sulfurreducens are only 3 to 5 nm wide. A wire this thin that can be mass-produced biologically may have a variety of nanoelectronic applications. Furthermore, genetically modifying G. sulfurreducens pili structure or composition to generate nanowires with different functionalities may have significant commercial value. [Derek Lovley, University of Massachusetts]

References

1. S. E. Childers, S. Ciufo, and D. R. Lovley, "Geobacter metallireducens Accesses Fe (III) Oxide by Chemotaxis," Nature 416, 767-69 (2002).

2. G. Reguera et al., "Extracellular Electron Transfer Via Microbial Nanowires," Nature 435, 1098-1101 (2005).