An international consortium, involving researchers from the CEA [1], CNRS, the Université de la Méditerranée and Université Pierre et Marie Curie, together with researchers from the DOE [2] in Ames (USA), the Universities of Nevada (USA), Rio de Janeiro (Brazil), San Luis Obispo (USA) and Pannonia (Hungary), have just characterized a new group of magnetotactic bacteria (MTB) able to produce magnetite and greigite nanomagnets [3]depending on environmental conditions. The phylogenetic, physiological and genomic characterization of one such bacterium, called Desulfamplus magnetomortis BW-1, was achieved thanks to its isolation under culture conditions and led to the identification of two groups of genes essential for nanomagnet formation. The first group is implicated in the formation of magnetite nanomagnets and the second in the production of greigite nanomagnets. This is the first time a magnetotactic bacterium that produces greigite crystals has been isolated under culture conditions. This is a major step forward that could open up the way to a broad range of biotechnology applications for such nanomagnets. These results were published in Science on 23 December 201l.

Magnetotactic bacteria (MTB) possess a single organelle, called a magnetosome, made up of magnetic nanocrystals of greigite (Fe3S4) or magnetite (Fe3O4). Aligned like the needle of a compass, these nanocrystals enable them to swim along magnetic fields looking for the best place to grow. Although they are widely found in nature, it is still difficult to cultivate MTBs in the laboratory. It has only been possible to cultivate a few strains of these bacteria, which are only capable of producing magnetite nanomagnets. Until now, researchers have never succeeded in isolating magnetotactic bacteria that form greigite nanocrystals.

To rise to this challenge, researchers at the CEA, CNRS, the Université de la Méditerranée and Université Pierre et Marie Curie, in collaboration with their American, Brazilian and Hungarian partners, took samples in Nevada and California from freshwater and brackish water aquatic environments. They have shown the presence of magnetotactic bacteria (MTB) producing both greigite and magnetite in these environments. In addition, phylogenetic analysis [4] of these bacteria allowed them to identify two new, previously unknown species belonging to the Deltaproteobacteria class, one of the most widely-studied bacterial classes. As a result of analyzing samples taken from a brackish water basin in Death Valley, California, the researchers successfully isolated and cultivated a bacterium, called Desulfamplus magnetomortis BW-1, which belongs to one of these new bacterial genera and is both greigite- and magnetite-producing. Generally-speaking, magnetosome formation in MTBs is genetically controlled by a group of genes, the mam genes, which are specific to magnetotactic bacteria (MTB). Gene sequencing the BW-1 bacterium confirmed the existence of these mam genes in this new species. Nonetheless, in BW-1, these genes are organized in a different way and form two distinct groups of genes within the genome. The first group is homologous to the genes that enable magnetite nanomagnet formation in MTB. The second has more in common with the genes involved in greigite nanomagnet formation. This implies that the formation of magnetite and greigite magnetosomes, in BW-1, is governed by these two gene groups, whose expression is regulated according to environmental conditions.

A vast range of biotechnology applications are being explored for the use of magnetite nanocrystals produced by MTB, especially for magnetic resonance imaging, pollution cleanup and the use of modified magnetosomes as catalysts. The first cultured production of this new greigite-producing bacterium is a major step forward that will make it possible to characterize these new nanomagnets and extend the range of potential applications of magnetosomes.

This research work was partly funded by the Fondation pour la Recherche Médicale (FRM: SPF20101220993).

Reference:

A cultured greigite-producing magnetotactic bacterium in a novel group of sulfate-reducing bacteria. Christopher T. Lefèvre, Nicolas Menguy, Fernanda Abreu, Ulysses Lins, Mihály Pósfai, Tanya Prozorov, David Pignol, Richard B. Frankel, Dennis A. Bazylinski, Science, Dec/23/2011.

[1] The Institute of Environmental Biology and Biotechnology (iBEB), Life Sciences Division (DSV), Cadarache.

[2]Department of Energy: US equivalent of the CEA.

[3] Greigite is an iron sulfide mineral that has ferromagnetic properties, the equivalent of magnetite.

[4] Identified using sequencing of the 16S ribosomal RNA of these bacteria.