Geobacter metallireducens

Geobacter metallireducens is a gram-negative metal-reducing proteobacterium. It is a strict anaerobe that oxidizes several short-chain fatty acids, alcohols, and monoaromatic compounds with Fe(III) as the sole electron acceptor. It can also use uranium for its growth and convert U(VI) to U(IV).

Geobacter metallireducens was discovered by Derek Lovley at UMass Amherst in 1993. It is an iron-reducing bacteria and it has been thought that the microbe could be used to treat industrial sites where "cyanide-metal complexes" have formed to contaminate the site.

The genome of Geobacter metallireducens has a chromosome length of 3,997,420 bp. It has a circular bacterial chromosome, meaning there are no free ends of DNA. The shape is roughly like that of an egg. Geobacter metallireducens also has a GC content of 59.51%. The plasmid has a lower GC content, of 52.48%, and is 13,762 bp in length. The plasmid encodes a stabilizing protein, RelE/ParE, which allows Geobacter metallireducens to adapt and thrive in different and new environmental conditions.

Geobacter metallireducens becomes motile when necessary, producing a flagellum in order to relocate when environmental conditions become unfavorable. Insoluble Fe(II) and Mn (II) are electron acceptors for many chemolithotrophic microorganisms. Fe (II) is produced through the reduction of Fe(III) and Mn (IV) oxides. It is often difficult for these organisms to attain iron and manganese because Fe(III) and Mn (IV) oxides do not freely diffuse through bacterial membranes. Geobacter metallireducens has evolved a unique way to access iron via insoluble Fe(III) and Mn (IV) oxides; they grow motility appendages to help them find and contact the insoluble oxides. According to a study conducted by Childers et. al., cells of G. metallireducens that grew in an environment with insoluble Fe(III) and Mn (IV) oxides grew flagella and pili. Whereas those grown in environments with soluble Fe(III) and Mn (IV) oxides did not have flagella nor pili. G. metallireducens is only motile when there are no soluble Fe(III) and Mn (IV) oxides in its environment to act as the electron acceptor. It is the first known microorganism to display chemotactic tendencies towards iron and manganese, as well as the first microbe discovered that oxidizes organic compounds with the inorganic elements iron and manganese.

G. metallireducens does not solely reduce Fe(III) and Mn(IV) oxides, it can reduce a variety of compounds including those that are toxic or radioactive such as uranium, plutonium, technetium, and vanadium. Vanadium, specifically, can contaminate groundwater in areas near high mining activity. G. metallireducens can utilize vanadium (V) as an energy source by reducing the metal to vanadium (IV). Therefore the bacteria can be used to aid in decontamination of affected groundwaters. G. metallireducens can use a similar mechanism to reduce uranium (VI) to uranium (V) in contaminated groundwaters. However, there is still research to be done on making this process more effective.

G. metallireducens has been demonstrated to reduce chloramphenicol (CAP) to complete dechlorination products under pure culture conditions. Research utilizing cyclic voltammograms and chronoamperometry revealed that the bacteria exhibited a negative correlation CAP removal efficiency with initial CAP dosages, displaying the organism's potential application of bioremediation in environments polluted by antibiotics.

G. metallireducens can make electrical connections with other microbes. This, in turn, allows other microbes to perform anaerobic syntrophic metabolism of organic substrates. This process of this electrical connection is called direct interspecies electron transfer (DIET). DIET is a metabolism that is defined by the movement of free electrons, rather than organisms only receiving electrons via the reduction of other compounds. The pili of G. metallireducens conduct electrical currents. They can transfer electrons to other Geobacter species as well as archaea, specifically methanogens. The DIET connection to methanogens allows these bacteria to contribute to the methane cycle, and convert organic wastes to methane.