Geobacter metallireducens

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Classification

Higher order taxa

Bacteria; Proteobacteria; Delta Proteobacteria; Desulferomonadales; Geobacteraceae

Species

Geobacter metallireducens

Description and significance

Geobacter metallireducens is a rod shaped, Gram-negative, anaerobic bacteria and can be seen to have flagella and pili. The first G. metallireducens (initially known as strain GS-15) was first isolated from freshwater sediment, and was able to gain energy through dissimilatory reduction of iron, manganese, uranium and other metals [1]. This organism was the first organism found to oxidize organic compounds to carbon dioxide with iron oxides as the electron acceptor [1]. G. metallireducens can also oxidize short chain fatty acids, alcohols, and monoaromatic compounds such as toluene and phenol using iron as its electron acceptor [2]. G. metallireducens also plays a role in carbon and nutrient cycling and bioremediation, enabling the metabolism of soluble harmful (sometimes radioactive) contaminants into insoluble harmless forms.

Genome structure

The genome of G. metallireducens is 4.01 Mbp long with a (circular) chromosome length of 3,997,420 bp encoding 3,621 genes with a GC content of 59.51%. It also has a plasmid of 13,762 bp encoding 13 genes with a GC content of 52.48% [3]. The plasmid contains a gene for an addiction module toxin, RelE/StbE, which produces toxin and gives resistance to the bacteria. The plasmid also contains a gene that encodes for a plasmid stabilization system protein, RelE/ParE, that allows the bacteria to adapt to new environmental conditions (ie: a change in nutrients) [4]. The chromosome encodes for various housekeeping pathways including metabolism, organism cell structure, sensor proteins (chemotaxis), as well as genes that encode for flagella and pili synthesis [3].

Cell structure and metabolism

G. metallireducens contains genes for flagella synthesis. G. metallireducens was originally thought to be immotile because they were grown in labs under ideal conditions where the bacteria had plenty of soluble metals. Flagella synthesis does not initiate unless nutrient conditions are poor. Under conditions where soluble metals were replaced with less favorable iron oxide, G. metallireducens was seen to grow flagella and swim [5].

G. metallireducens also contains genes that allow the bacteria the ability of chemotaxis. Chemotaxis allows G. metallireducens to sense compounds, favorable and unfavorable, in its surrounding environment. Together with the ability to perform chemotaxis and produce and use flagella, G. metallireducens has the ability to move towards metallic compounds or favorable environments where nutrient supply is favorable and away from less favorable environments where nutrient supply is poor [5]. G. metallireducens favors or is chemotactic to soluble electron acceptors Fe(II) and Mn(II) and expresses flagella and pili only when grown on insoluble Fe(III) or Mn(IV) oxide [5]. These results suggest that G. metallireducens senses when soluble electron acceptors are depleted and will promote synthesis of flagella and pili allowing it to search for, and establish contact with, insoluble Fe(III) or Mn(IV) oxide [5].

Previous experiments on bacteria that can perform electron transfer to Fe(III) have focused on the role of outer-membrane c-type cytochromes [6]. However, some Fe(III) reducers lack c-cytochromes. Geobacter species are examples of such Fe(III) reducers that lack c-cytochromes and must directly contact Fe(III) oxides to reduce them [6]. They produce pili that were proposed to aid in establishing contact with the Fe(III) oxides [6].

Ecology

G. metallireducens has been known to take part in bioremediation of organic and metal contaminants in groundwater and participates in the carbon and nutrient cycles of aquatic sediments. Aside from using Fe(III) oxides, the G. metallireducens uses metals such as plutonium and uranium to metabolize food [5]. G. metallireducens consumes these radioactive elements and breaks down the contaminants. When G. metallireducens metabolizes uranium, it changes the metal from a soluble to an insoluble form. The insoluble uranium drops out of the groundwater--decontaminating streams and drinking water. The insoluble uranium remains in the soil and could then be extracted [5]. The use of an insoluble electron acceptor may explain why Geobacter species predominate over other dissimilatory iron-reducing bacterial species in a wide variety of sedimentary environments.

Application to Biotechnology

In G. metallireducens' relative species, Geobacter sulfurreducens, conducting-probe atomic force microscopy revealed that the pili were highly conductive. Results from experimentation on pili of G. sulfurreducens might serve as biological nanowires, transferring electrons from the cell surface to the surface of Fe(III) oxides [6]. The ability of electron transfer through pili has implications for other unique cell-surface and cell-cell interactions, and for bioengineering of new conductive materials [6].

Current Research

Wiatrowski (et al) determined that some dissimilatory reducing bacteria, such as Shewanella oneidensis, Geobacter sulfurreducens, and Geobacter metallireducens, can also reduce ionic mercury (Hg[II]) to elemental mercury (Hg[0]) without having to use a mercury reductase [7]. The reduction of mercury was determined to be metabolically similar to how Fe(III) is reduced in the tested dissimilatory reducing bacteria where reduction required the presence of electron donors and acceptors. It was concluded that the discovery of mercury reduction indicated possibilities of mobilizing mercury and producing methylmercury in anoxic environments [7].

Tang (et al) analyzed the central metabolic pathway in G. metallireducens, specifically the carbon fluxes using isotopic carbon (13C). Acetate was used as the primary carbon source and Ferric nitrilotriacetate (Fe-NTA) was the electron acceptor. G. metallireducens was found to have complete biosynthesis pathways for essential metabolism and an additional (and unusual) isoleucine pathway, which used acetyl-CoA and pyruvate as precursors [8]. The isotopomer modeling indicated that acetate was oxidized to carbon dioxide via TCA cycle while also reducing iron. The main biosynthesis pathways employed were the pentose phosphate pathway and gluconeogenesis but only accounted for less than 3% of carbon consumption [8]. The model also indicated high reversibility in the reaction between oxoglutarate and succinate, which was not expected. This metabolic step where oxoglutarate is converted to succinate was found to be the rate limiting reaction/step [8]. All in all, it was concluded that this rate limiting step determines G. metallireducens' carbon metabolism and explains why it is low.

Jahn (et al) discovered that even very stable iron complexes could be reduced by dissimilatory iron-reducing bacteria (G. metallireducens). Cyanide-metal complexes are frequent contaminants found in the soil or aquifers of industrial sites that may be released and spread by outgassing or transport with the groundwater. Cyanide forms very stable complexes with iron called Prussian Blue (Fe(4)[Fe(CN)(6)](3)). Jahn (et al) indicated that Prussian Blue could be used as the electron acceptor in dissimilatory iron-reducing bacteria such as G. metallireducens and Shewanella alga strain BrY [9]. Prussian Blue is reduced to Prussian White (Fe(2)[Fe(CN)(6)]). However, Prussian White could reoxided by exposure to air and become Prussian Blue again [9]. Both G. metallireducens and Shewanella alga strain BrY were shown to be able to grow with Prussian Blue, using it as its primary electron acceptor.