Magnetotactic bacteria biosynthesize magnetite nanoparticles of high structural and chemical purity that allow them to orientate in the geomagnetic field. In this work we have followed the process of biomineralization of these magnetite nanoparticles. We have performed a time-resolved study on magnetotactic bacteria Magnetospirillum gryphiswaldense strain MSR-1. From the combination of magnetic and structural studies by means of Fe K-edge X-ray absorption near edge structure (XANES) and high-resolution transmission electron microscopy we have identified and quantified two phases of Fe (ferrihydrite and magnetite) involved in the biomineralization process, confirming the role of ferrihydrite as the source of Fe ions for magnetite biomineralization in M. gryphiswaldense. We have distinguished two steps in the biomineralization process: the first, in which Fe is accumulated in the form of ferrihydrite, and the second, in which the magnetite is rapidly biomineralized from ferrihydrite. Finally, the XANES analysis suggests that the origin of the ferrihydrite could be at bacterial ferritin cores, characterized by a poorly crystalline structure and high phosphorus content.

Magnetospirillum gryphiswaldense is a microorganism with the ability to biomineralize magnetite nanoparticles, called magnetosomes, and arrange them into a chain that behaves like a magnetic compass. Rather than straight lines, magnetosome chains are slightly bent, as evidenced by electron cryotomography. Our experimental and theoretical results suggest that due to the competition between the magnetocrystalline and shape anisotropies, the effective magnetic moment of individual magnetosomes is tilted out of the [111] crystallographic easy axis of magnetite. This tilt does not affect the direction of the chain net magnetic moment, which remains along the [111] axis, but explains the arrangement of magnetosomes in helical-like shaped chains. Indeed, we demonstrate that the chain shape can be reproduced by considering an interplay between the magnetic dipolar interactions between magnetosomes, ruled by the orientation of the magnetosome magnetic moment, and a lipid/protein-based mechanism, modeled as an elastic recovery force exerted on the magnetosomes.

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Magnetotactic bacteria synthesize a chain of magnetic nanoparticles, called magnetosome chain, used to align and swim along the geomagnetic field lines. In particular, Magnetospirillum gryphiswaldense biomineralize magnetite, Fe3O4. Growing this species in a Co-supplemented medium, Co-doped magnetite is obtained, tailoring in this way the magnetic properties of the magnetosome chain. Combining structural and magnetic techniques such as transmission electron microscopy, energy-dispersive x-ray spectroscopy, X-ray absorption near edge structure, and X-ray magnetic circular dichroism, we determine that ∼1% of Co2+ substitutes Fe2+ located in octahedral places in the magnetite, thus increasing the coercive field. In the framework of the Stoner–Wohlfarth model, we have analyzed the evolution of the hysteresis loops as a function of temperature determining the different magnetic anisotropy contributions and their evolution with temperature. In contrast with the control magnetosome chains, whose effective anisotropy is uniaxial in the whole temperature range from 300 to 5 K, the effective anisotropy of Co-doped magnetosome chains changes appreciably with temperature, from uniaxial down to 150 K, through biaxial down to 100 K, to triaxial below 100 K.

Magnetotactic bacteria (MTB) are aquatic microorganisms that are able to biomineralize membrane-enclosed magnetic nanoparticles called magnetosomes. Inside the MTB, magnetosomes are arranged in a chain that allows MTB to align and navigate along the Earth's magnetic field. When isolated from the MTB, magnetosomes display a number of potential applications for targeted cancer therapies, such as magnetic hyperthermia, localized drug delivery, or tumor monitoring. The characteristics and properties of magnetosomes for these applications exceed in several aspects those of synthetic magnetic nanoparticles. Likewise, the whole MTB can also be considered as promising agents for cancer treatment, taking advantage of their self-propulsion capability provided by their flagella and the guidance capabilities ensured by their magnetosome chain. Indeed, MTB are envisaged as nanobiots that can be guided and manipulated by external magnetic fields and are naturally attracted toward hypoxic areas, such as the tumor regions, while retaining the therapeutic and imaging capacities of the isolated magnetosomes. Moreover, unlike most of the bacteria currently tested in clinical trials for cancer therapy, MTB are not pathogenic but could be engineered to deliver and/or express specific cytotoxic molecules. In this article, we will review the progress and perspectives of this emerging research field and will discuss the main challenges to overcome before the use of MTB can be successfully applied in the clinic.