Growing Magnetotactic Bacteria of the Genus Magnetospirillum: Strains MSR-1, AMB-1 and MS-1

Experimental determination of the propulsion matrix of the body of helical Magnetospirillum magneticum cells

Helical-shaped magnetotactic bacteria provide a rare opportunity to precisely measure both the translational and rotational friction coefficients of micron-sized chiral particles. The possibility to align these cells with a uniform magnetic field allows clearly separating diffusion along and perpendicular to their longitudinal axis. Meanwhile, their corkscrew shape allows detecting rotations around their longitudinal axis, after which orientation correlation analysis can be used to retrieve rotational diffusion coefficients in the two principal directions. Using light microscopy, we measured the four principal friction coefficients of deflagellated Magnetospirillum magneticum cells, and compared our results to that expected for cylinders of comparable size. We show that for rotational motions, the overall dimensions of the cell body are what matters most, while the exact body shape has a larger influence on translational motions. To obtain a full characterization of the friction matrix of these elongated chiral particles, we also quantified the coupling between the rotation around and translation along the longitudinal axis of the cell. Our results suggest that for this bacterial species cell body rotation could significantly contribute to cellular propulsion.

Magnetotactic Bacteria and Magnetosomes: Basic Properties and Applications

Magnetotactic bacteria (MTB) belong to several phyla. This class of microorganisms exhibits the ability of magneto-aerotaxis. MTB synthesize biominerals in organelle-like structures called magnetosomes, which contain single-domain crystals of magnetite (Fe3O4) or greigite (Fe3S4) characterized by a high degree of structural and compositional perfection. Magnetosomes from dead MTB could be preserved in sediments (called fossil magnetosomes or magnetofossils). Under certain conditions, magnetofossils are capable of retaining their remanence for millions of years. This accounts for the growing interest in MTB and magnetofossils in paleo- and rock magnetism and in a wider field of biogeoscience. At the same time, high biocompatibility of magnetosomes makes possible their potential use in biomedical applications, including magnetic resonance imaging, hyperthermia, magnetically guided drug delivery, and immunomagnetic analysis. In this review, we attempt to summarize the current state of the art in the field of MTB research and applications.

Self-organisation and convection of confined magnetotactic bacteria

Collective motion is found at all scales in biological and artificial systems, and extensive research is devoted to describing the interplay between interactions and external cues in collective dynamics. Magnetotactic bacteria constitute a remarkable example of living organisms for which motion can be easily controlled remotely. Here, we report a new type of collective motion where a uniform distribution of magnetotactic bacteria is rendered unstable by a magnetic field. A new state of “bacterial magneto-convection” results, wherein bacterial plumes emerge spontaneously perpendicular to an interface and develop into self-sustained flow convection cells. While there are similarities to gravity driven bioconvection and the Rayleigh–Bénard instability, these rely on a density mismatch between layers of the fluids. Remarkably, here no external forces are applied on the fluid and the magnetic field only exerts an external torque aligning magnetotactic bacteria with the field. Using a theoretical model based on hydrodynamic singularities, we capture quantitatively the instability and the observed long-time growth. Bacterial magneto-convection represents a new class of collective behaviour resulting only from the balance between hydrodynamic interactions and external alignment.

Misalignment between the magnetic dipole moment and the cell axis in the magnetotactic bacterium Magnetospirillum magneticum AMB-1

While most quantitative studies of the motion of magnetotactic bacteria rely on the premise that the cells' magnetic dipole moment is aligned with their direction of motility, this assumption has so far rarely been challenged. Here we use phase contrast microscopy to detect the rotational diffusion of non-motile cells of Magnetospirillum magneticum AMB-1 around their magnetic moment, showing that in this species the magnetic dipole moment is, in fact, not exactly aligned with the cell body axis. From the cell rotational trajectories, we are able to infer the misalignment between cell magnetic moment and body axis with a precision of better than 1°, showing that it is, on average, 6°, and can be as high as 20°. We propose a method to correct for this misalignment, and perform a non-biased measurement of the magnetic moment of single cells based on the analysis of their orientation distribution. Using this correction, we show that magnetic moment strongly correlates with cell length. The existence of a range of misalignments between magnetic moment and cell axis in a population implies that the orientation and trajectories of magnetotactic bacteria placed in external magnetic fields is more complex than generally assumed, and might show some important cell-to-cell differences.