Controlling Magnetotactic Bacteria, Their Topological Properties and Generating Skyrmions in Their Polar and Non-Polar Velocity Vector Fields

Magnetotactic bacteria are microorganisms containing magnetite magnetic nanoparticle (MNP) bacteria organelles called magnetosomes. Magnetosomes are often arrayed in a specific pattern for a given strain of magnetotactic bacteria (i.e. a line along the long axis of the bacterium) and are made via biomineralization of ferromagnetic elements [1]. Previous work on the behavior of magneto-tactic bacteria in magnetic fields has assumed that the magnetic structure is a rigid dipole fixed along the bacteria's long axis – the direction of bacterial propulsion [1,2]. Such models have successfully elucidated the bacteria's behavior in rotating external fields, and magnetic field gradients. However, such models fail to explain the bacteria's behavior in a uniform magnetic field, do not allow for direct directional control over a set of bacteria, and do not address the inherent energetic degeneracy present in uniaxial magnetic systems [1,2].<br/><br/>Additionally, in recent years there has been significant research on topological structures, such as skyrmions and hopfions, in both magnetic [3] and soft matter liquid crystal systems [4] due to their stability and technological potential. Magnetic systems and soft matter systems typically differ in the nature of their order parameter: Magnetic systems have a polar order parameter and soft matter systems have a non-polar (axial) order parameter. No topological structures have been observed in magnetotactic bacteria despite both their magnetic and soft matter properties.<br/><br/>In this work we combine nanomagnetic simulations of magnetosome arrays distributions with transmission electron microscopy to simulate the magnetic behavior of the magnetotactic bacteria. Combining this with experiments using phase contrast optical microscopy and in-situ magnetic fields allowed us to understand the magnetic field response of bacteria based on how their magnetosomes are arranged. This allows us to explain previously anomalous behavior in the bacteria, control large, in phase groups of bacteria directly with external uniform magnetic fields, selectively change the topological parameter space from polar to non-polar, and generate topological structures in both the polar and the non-polar velocity vector fields of the bacteria. Being able to directly control these bacteria with external magnetic fields opens up the possibility of using them as guided microrobots in biomedicine (e.g. magnetic hyperthermia, drug delivery) and bio-spintronics.<br/><br/>References:<br/>[1] Faivre, D. & Schuler, D. Chemical Reviews 108, 4875–4898 (2008).<br/>[2] Steinberger, B.,et al.. Journal of Fluid Mechanics 273, 189–211 (1994).<br/>[3] Kent, N., et al. Nat Commun 12 (2021)<br/>[4] Smalyukh, I 2020 <i>Rep. Prog. Phys.</i> <b>83</b> 106601