Geobacter metallireducens

Geobacter metallireducens with flagella from the Genome News Network.

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 (Lovely et al 1988). This organism was the first organism found to oxidize organic compounds to carbon dioxide with iron oxides as the electron acceptor (Lovely and Phillips 1988). G. metallireducens can also oxidize short chain fatty acids, alcohols, and monoaromatic compounds such as toluene and phenol using iron as its electron acceptor (Lovely et al 1993). 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. metallireducensis 4.01Mbp long with a chromosome length of 3,997,420bp encoding 3,621 genes and a plasmid of 13,762bp encoding 13 genes (Schneider et al 2006).

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. Synthesis for flagella do 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 (Childers et al 2002).

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 (Childers et al 2002).

Previous experiments on bacteria that can perform electron transfer to Fe(III) have focused on the role of outer-membrane c-type cytochromes (Reguera et al 2005). 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 (Reguera et al 2005). They produce pili that were proposed to aid in establishing contact with the Fe(III) oxides (Reguera et al 2005).

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 (Childers et al 2002). 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 (Childers et al 2002).

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. G. metallireducens consumes these radioactive elements and breaks down the contaminants. In the case of uranium, it changes the metal from a soluble to an insoluble form. The insoluble uranium drops out of the groundwater, thus decontaminating streams and drinking water. It remains in the soil and could then be extracted (Childer 2002). The use of an insoluble electron acceptor may explain why Geobacter species predominate over other Fe(III) oxide-reducing microorganisms in a wide variety of sedimentary environments.

Describe any interactions with other organisms (included eukaryotes), contributions to the environment, effect on environment, etc.

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 (Reguera et al 2005). The ability of electron transfer through pili indicates possibilities for other unique cell-surface and cell-cell interactions, and for bioengineering of new conductive materials (Reguera et al 2005).

Current Research

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References

Lovley DR and Phillips EJ. "Novel Mode of Microbial Energy Metabolism: Organic Carbon Oxidation Coupled to Dissimilatory Reduction of Iron or Manganese". Applied and Environmental Microbiology. 1988. Volume 54, No. 6, p. 1472-1480.

Lovley DR, Giovannoni SJ, White DC, Champine JE, Phillips EJ, Gorby YA,Goodwin S. "Geobacter metallireducens gen. nov. sp. nov., a microorganism capable of coupling the complete oxidation of organic compounds to the reduction of iron and other metals." Arch Microbiol. 1993. Volume 159. No.4. p. 336-344.

Childers SE, Ciufo S, Lovley DR. "Geobacter metallireducens accesses insoluble Fe(III) oxide by chemotaxis". Nature. 2002. Volume 416. p. 767-769.

Reguera G, McCarthy KD, Mehta T, Nicoll JS, Tuominen MT, Lovley DR. "Extracellular electron transfer via microbial nanowires". Nature. 2005. Volume 435. p. 1098-1101.

Schneider KL, Pollard KS, Baertsch R, Pohl A and Lowe TM. "Geobacter metallireducens (Geobacter metallireducens GS-15) Genome Browser Gateway". The UCSC Archaeal Genome Browser. 2006. Volume 43. Database issue D407-D410.

Edited by Christine Tang student of Rachel Larsen and Kit Pogliano