A magnetosome chain viewed as a bio-elastic magnet.

In light of the coarse-grained Monte Carlo numerical simulation method, the magnetosome chain stability of magnetotactic bacteria is analysed and discussed. This discrete chain of magnetic nanoparticles, encapsulated in a lipid membrane and flanked by filaments, orients bacteria in the geomagnetic field as a compass needle. Each magnetosome is a magnetite or greigite nanocrystal encapsulated in a soft lipid shell. This structure is modelled by a hard core with a magnetic dipole embedded and a cloud of electric dipoles which are able to move and rotate over the magnetic spherical core. In the present paper, some of the many possibilities of the model by varying the control parameters of the system are explored. Magnetic particles arrange in long linear clusters when the coating is removed. However, linear but twisted chains of magnetic particles emerge when there are electric dipoles in the coating shell. A unique linear and straight chain is not observed in any 3D numerical simulation; this result is in agreement with a real living system of bacteria in a geomagnetic field when proteins that form the filament are absent. Finally, the stability and magnetization of a magnetosome chain of 30 beads in one dimension set up are discussed resembling a real chain. The results suggest that a magnetosome chain not only orients bacteria but also should be considered as a potential storage of elastic energy.

Ferromagnetic resonance of intact cells and isolated crystals from cultured and uncultured magnetite-producing magnetotactic bacteria.

Most magnetotactic bacteria (MB) produce stable, single-domain magnetite nanocrystals with species-specific size, shape and chain arrangement. In addition, most crystals are elongated along the [111] direction, which is the easy axis of magnetization in magnetite, chemically pure and structurally perfect. These special characteristics allow magnetite crystal chains from MB to be recognized in environmental samples including old sedimentary rocks. Ferromagnetic resonance (FMR) has been proposed as a powerful and practical tool for screening large numbers of samples possibly containing magnetofossils. Indeed, several studies were recently published on FMR of cultured MB, mainly Magnetospirillum gryphiswaldense. In this work, we examined both uncultured magnetotactic cocci and the cultured MB M. gryphiswaldense using transmission electron microscopy (TEM) and FMR from 10 K to room temperature (RT). The TEM data supported the FMR spectral characteristics of our samples. The FMR spectra of both bacteria showed the intrinsic characteristics of magnetite produced by MB, such as extended absorption at the low field region of the spectra and a Verwey transition around 100 K. As previously observed, the spectra of M. gryphiswaldense isolated crystals were more symmetrical than the spectra obtained from whole cells, reflecting the loss of chain arrangement due to the small size and symmetrical shape of the crystals. However, the FMR spectra of magnetic crystals isolated from magnetotactic cocci were very similar to the FMR spectra of whole cells, because the chain arrangement was maintained due to the large size and prismatic shape of the crystals. Our data support the use of FMR spectra to detect magnetotactic bacteria and magnetofossils in samples of present and past environments. Furthermore, the spectra suggest the use of the temperature transition of spectral peak-to-peak intensity to obtain the Verwey temperature for these systems.

Deep-etching electron microscopy of cells of Magnetospirillum magnetotacticum: evidence for filamentous structures connecting the magnetosome chain to the cell surface.

Magnetospirillum magnetotacticum are magnetotactic bacteria that form a single chain of magnetite magnetosomes within its cytoplasm. Here, we studied the ultrastructure of M. magnetotacticum by freeze-fracture and deep-etching to understand the spatial correlation between the magnetosome chain and the cell envelope and its possible implications for magnetotaxis. Magnetosomes were found mainly near the cell envelope, forming chains that were closely associated with the granular cytoplasmic material. The membrane surrounding the magnetosomes could be visualized in deep-etching preparations. Thin connections between magnetosome chains and the cell envelope were observed in deep-etching images. These results strengthen the hypothesis for the existence of structures that transfer the torque from the magnetosome chains to the whole cell during the orientation of magnetotactic bacteria to a magnetic field lines.

Membrane vesicles in magnetotactic bacteria.

Magnetotactic bacteria are microorganisms that respond to magnetic fields. We have studied the surface ultrastructure of Magnetospirillum magnetotacticum and uncultured magnetotactic bacteria from a marine environment using transmission electron microscopy and freeze-etching. Numerous membrane vesicles were observed on the surface of Magnetospirillum magnetotacticum bacteria. All uncultured magnetotactic bacteria presented membrane vesicles on their surface in addition to an extensive capsular material and an S-layer formed by particles arranged in a hexagonal symmetry. We did not observe any indication of electron-dense precipitation on the surface of these microorganisms. Our results indicate that membrane vesicles are a common characteristic of magnetotactic bacteria in natural sediments.