Magnetospirillum magnetotacticum
Andrew Wessel

The species Magnetospirillum magnetotacicum was isolated from microaerobic zones in freshwater sediments in 1979 by Blakemore.  What makes this organism so interesting is the ability to create a specialized vacuole that collects iron and, over time, forms single crystals of ferrimagnetite iron oxides like magnetite.  These specialized vacuoles are called magnetosomes.  Through the use of these magnetosomes, the bacteria can use the magnetic field of the environment (usually this will be the earth) to orient themselves in the proper manner for optimum growth. 
Magnetotactic bacteria represent a diverse group of prokaryotes with respect to morphology and physiology that are capable of magnetic orientation.  All known magnetotaxic bacteria belong to the protobacteria and the Nitrospirae phylum.  Most known species are also within the alphaproteobacteria.  Magnetotaxis is the detection of magnetic stimuli while responding in a desired behavior.

In order to produce magnetosomes, many steps have to occur.  The first step in the process is to increase uptake of iron in the cell.  This occurs after a highly conserved segment of magnetosome genes are expressed.  Since this organism lives in freshwater, the standard conditions will provide little overall dissolved iron.  When free iron is present, it is also occupying the oxidized ferric form.  This provides a transportation problem.  In order to transport the iron into the cells, enzyme mediated active transport channels are used.  Most strains utilize siderophores to import iron; although, there are other mechanisms that can import iron but these also utilize active transportation. 

The second step in the process is to create and designate a vacuole for use as a magnetosome.  Specialized proteins become embedded into the vacuole to aid in the transfer of the iron molecules.  The third step in the process is to transport the iron into the magnetosome vesicle.  The iron from the cytosol is passed into the vesicle for further processing.  The iron ions float freely until assembled in the next step.
The fourth step in the process is to mineralize the iron in the magnetosome in the correct way to produce magnetite.  Magnetite, Fe3O4, is comprised of a mixture of ferrous and ferric ions.  Since a majority of the imported iron is in the ferric form, some of the iron needs to be reduced.  Another enzyme from the magnetosome genes called iron reductase is utilized.  In an experimental assay, NADH, FMN and ferric citrate were utilized to prove the activity of the enzyme and find the cofactors.  After the proper concentrations of ferrous and ferric iron are present in the magnetosome, many proteins including the Mms6 protein aid in the magnetite biomineralization. 

Interestingly enough, the magnetosomes form pure magnetite (with few exceptions) even in the presence of other metal ions in the environment.  However, the size, shape and the orientation of the magnetosomes can be influenced by high levels of zinc and nickel.

References

Kundu, S; et al. “On the change in bacterial size and magentosome features for Magnetospirillum magnetotacticum (MS-1) under high concentrations of zinc and nickel”. Biomaterials 30 (2009) 4211-4218

Nouguchi, Y; et al. “Iron Reductase for Magnetite Synthesis in the Magnetotactic Bacterium Magnetospirillum magnetotacticum”. Journal of Bacteriology: 181:7 (1999) 2142-2147

Richter, M; et al. “Comparative Genome Analysis of Four Magnetoactic Bacteria Reveals a Complex Set of Group-Specific Genes Implicated in Magnetosome Biomineralization and Function”. Journal of Bacteriology 189:13 (2007) 4899-4910

Taoka, A; et al. “Identification of Iron Transporters Expressed in the Magnetotactic Bacterium Magnetospirillum magnetotacticum”. Current Microbiology 58 (2009) 177-181

Xia, Meng; et al. “A Novel Ferric Reductase Purified from Magnetospirillum gryphiswaldense MSR-1” Current Microbiology 55 (2007) 71-75

*Disclaimer - This report was written by a student participaring in a microbiology course at the Missouri University of Science and Technology. The accuracy of the contents of this report is not guaranteed and it is recommended that you seek additional sources of information to verify the contents.

 

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