It is not only migratory birds that orient themselves to the magnetic field of the Earth. Also bacteria -- supposedly "simple" organisms -- have evolved to be able to take advantage of the magnetic field in their search for optimal living conditions. Such "magnetotactic"microorganisms use a miniature, cellular compass made of a chain of single nanomagnets, called magnetosomes. The entire bacterium is oriented like a compass needle inside the magnetic field. Until now, it was not clear how the cells organise magnetosomes into a stable chain, against their physical tendency to collapse by magnetic attraction. But using modern molecular-genetic and imaging processes, researchers from the Max Planck Institue for Marine Microbiology in Bremen and Max Planck Institute of Biochemistry in Martinsried, Germany have identified the protein responsible for creating the magnetosome chain. The scientists showed that this protein aligns the magnetosomes along a cytoskeletal structure which was previously unknown. This points to evidence that genetics regulate the magnetosome chain exactly. The structure is one of the most complex that has ever been found in bacterial cells. It is comparable to organelles that, until now, scientists had only been familiar with in higher organisms. (Nature, Advanced Online Publication, November 20, 2005).

Image 1: Magnetotactic bacteria compared. Left: genetically unaltered cells ("wild type") of Magnetospirillum gryphiswaldense contain as many as one-hundred magnetite crystals. They are nanosized permanent magnets, which are arranged into a nearly perfect chain inside the cell. The magnetosome chain orients the bacteria similarly to the way a compass needle in the Earth's magnetic field works. Right: a particular gene was removed from a mutant, resulting in the loss of the MamJ protein. The magnetosomes group together into irregularly ordered clusters. Here, because the magnetic moments of the individual magnetite crystals partially cancel each other out, the bacteria without MamJ can orient themselves only weakly in the magnetic field.

Image: Max Planck Institute for Marine Microbiology

Cyroelectron tomography makes it possible to analyse structures in detail inside an intact, shock-frozen cell (minus 196 degrees celsius), and to display them in three dimensions, at a resolution of just a few nanometres. In Dr Jürgen Plitzko's research group in Martinsried, Manuela Gruska's doctoral work involved investigating magnetic bacteria cells using this technology, and comparing the wild type with the MamJ mutants. She was able to make visible not only the magnetite crystal, but also the surrounding membrane vesicle, in a resolution never achieved before.

Amazingly, the scientists saw a previously unknown filamentous structure along the magnetosome chain of the wild type cells. It resembled a similar structure which structural biologists from Martinsried had already imaged in three dimensions in other cells. This shed light on the crux of the problem with the magnetosome chain: although in the wild type, the magnetosomes lay like pearls on a string along the filament, in the cells missing MamJ, the empty magnetosome vesicles scattered themselves about. It also explains why, as soon as they grow to a particular size, the magnetic crystals clump together in mutant bacteria.

The scientists speculate that the MamJ protein develops, on one hand, on the surface of the magnetosome, and on the other hand, on the newly-discovered filament. It thus enables a close connection between the magnetosome vesicle and structure. This is further evidence of the diverse functions that cytoskeletal structures in bacteria seem to have, and which, for a long time, have only been known in eukaryotes -- i.e., organisms whose cells have nuclei.

The fact that the chain structure of bacterial nanomagnets is exactly regulated by genetics could also have implications for our understanding of the way higher organisms orient themselves to the magnetic field. We have known for some years that certain animals -- like migratory salmon or homing pigeons -- have magnetite crystal chains in particular tissue. These chains are astoundingly similar to those in bacteria, and possibly develop through a related mechanism

Original work:

André Scheffel, Manuela Gruska, Damien Faivre, Alexandros Linaroudis, Juergen M. Plitzko and Dirk Schueler:
An acidic protein aligns magnetosomes along a filamentous structure in magnetotactic bacteria
Nature, Advance Online Publication (AOP), November 20, 2005 (DOI 10.1038/nature04382)