Showing papers on "Magnetotactic bacteria published in 2022"

In Situ Generation of Gold Nanoparticles on Bacteria‐Derived Magnetosomes for Imaging‐Guided Starving/Chemodynamic/Photothermal Synergistic Therapy against Cancer

Abstract: There are several attractive opportunities for using magnetic nanomaterials for anticancer applications. Herein, a magnetic nanomaterial platform is successfully developed based on natural Fe3O4 magnetosomes extracted from the bacterium Magnetospirillum magneticum AMB‐1 for anticancer therapy. The authors initially functionalize the magnetosome membranes in situ with gold nanoparticles to construct an attractive core‐satellite structure. Subsequently, the physical properties and application potentials of these structures are characterized as contrast agents for photoacoustic imaging and magnetic resonance imaging and as therapeutic agents with selective magnetic field guidance for diverse antitumor modalities, including starving, chemodynamic, and photothermal therapies. Owing to the high‐performance imaging‐guided synergistic effect, only a single injection and single laser irradiation result in excellent therapeutic efficacy against tumor growth in multiple cell‐derived xenograft tumor models and, most notably, patient‐derived organoid and patient‐derived xenograft tumor models. The demonstrations of the use of natural magnetic nanomaterials to achieve strong and synergistic antitumor performances highlight the promising application potential of this flexible and easy‐to‐prepare platform for developing innovative treatments for diseases in humans.

Conservation of magnetite biomineralization genes in all domains of life and implications for magnetic sensing

Abstract: Significance We present a model of biogenic magnetite formation in eukaryotes and hypothesize this genetic mechanism is used by broad forms of life for geomagnetic sensory perception. Countering previous assertions that salmon olfactory tissues lack biogenic magnetite, we determine that it is present in the form of compact crystal clusters and show that a subset of genes differentially expressed in candidate magnetoreceptor cells, compared to nonmagnetic olfactory cells, are distant homologs to a core suite of genes utilized by magnetotactic bacteria for magnetite biomineralization. This same core gene suite is common to a broad array of eukaryotes and the Asgard clade archaea Lokiarchaeta. Findings have implications for revising our understanding of eukaryote magnetite biomineralization, sensory perception of magnetic fields, and eukaryogenesis. Animals use geomagnetic fields for navigational cues, yet the sensory mechanism underlying magnetic perception remains poorly understood. One idea is that geomagnetic fields are physically transduced by magnetite crystals contained inside specialized receptor cells, but evidence for intracellular, biogenic magnetite in eukaryotes is scant. Certain bacteria produce magnetite crystals inside intracellular compartments, representing the most ancient form of biomineralization known and having evolved prior to emergence of the crown group of eukaryotes, raising the question of whether magnetite biomineralization in eukaryotes and prokaryotes might share a common evolutionary history. Here, we discover that salmonid olfactory epithelium contains magnetite crystals arranged in compact clusters and determine that genes differentially expressed in magnetic olfactory cells, contrasted to nonmagnetic olfactory cells, share ancestry with an ancient prokaryote magnetite biomineralization system, consistent with exaptation for use in eukaryotic magnetoreception. We also show that 11 prokaryote biomineralization genes are universally present among a diverse set of eukaryote taxa and that nine of those genes are present within the Asgard clade of archaea Lokiarchaeota that affiliates with eukaryotes in phylogenomic analysis. Consistent with deep homology, we present an evolutionary genetics hypothesis for magnetite formation among eukaryotes to motivate convergent approaches for examining magnetite-based magnetoreception, molecular origins of matrix-associated biomineralization processes, and eukaryogenesis.

Key Signatures of Magnetofossils Elucidated by Mutant Magnetotactic Bacteria and Micromagnetic Calculations

Abstract: Magnetotactic bacteria (MTB) produce single-stranded or multi-stranded chains of magnetic nanoparticles that contribute to the magnetization of sediments and rocks. Their magnetic fingerprint can be detected in ancient geological samples and serve as a unique biosignature of microbial life. However, some fossilized assemblages bear contradictory signatures pointing to magnetic components that have distinct origin(s). Here, using micromagnetic simulations and mutant MTB producing looped magnetosome chains, we demonstrate that the observed magnetofossil fingerprints are produced by a mixture of single-stranded and multi-stranded chains, and that diagenetically induced chain collapse, if occurring, must preserve the strong uniaxial anisotropy of native chains. This anisotropy is the key factor for distinguishing magnetofossils from other populations of natural magnetite particles, including those with similar individual crystal characteristics. Furthermore, the detailed properties of magnetofossil signatures depend on the proportion of equant and elongated magnetosomes, as well as on the relative abundances of single-stranded and multi-stranded chains. This work has important paleoclimatic, paleontological, and phylogenetic implications, as it provides reference data to differentiate distinct MTB lineages according to their chain and magnetosome morphologies, which will enable the tracking of the evolution of some of the most ancient biomineralizing organisms in a time-resolved manner. It also enables a more accurate discrimination of different sources of magnetite particles, which is pivotal for gaining better environmental and relative paleointensity reconstructions from sedimentary records.

Magnetotactic bacteria and magnetofossils: ecology, evolution and environmental implications

Abstract: Magnetotactic bacteria (MTB) are a group of phylogenetically diverse and morphologically varied microorganisms with a magnetoresponsive capability called magnetotaxis or microbial magnetoreception. MTB are a distinctive constituent of the microbiome of aquatic ecosystems because they use Earth's magnetic field to align themselves in a north or south facing direction and efficiently navigate to their favored microenvironments. They have been identified worldwide from diverse aquatic and waterlogged microbiomes, including freshwater, saline, brackish and marine ecosystems, and some extreme environments. MTB play important roles in the biogeochemical cycling of iron, sulphur, phosphorus, carbon and nitrogen in nature and have been recognized from in vitro cultures to sequester heavy metals like selenium, cadmium, and tellurium, which makes them prospective candidate organisms for aquatic pollution bioremediation. The role of MTB in environmental systems is not limited to their lifespan; after death, fossil magnetosomal magnetic nanoparticles (known as magnetofossils) are a promising proxy for recording paleoenvironmental change and geomagnetic field history. Here, we summarize the ecology, evolution, and environmental function of MTB and the paleoenvironmental implications of magnetofossils in light of recent discoveries.

Magnetotactic bacteria and magnetofossils: ecology, evolution and environmental implications

Abstract: Magnetotactic bacteria (MTB) are a group of phylogenetically diverse and morphologically varied microorganisms with a magnetoresponsive capability called magnetotaxis or microbial magnetoreception. MTB are a distinctive constituent of the microbiome of aquatic ecosystems because they use Earth's magnetic field to align themselves in a north or south facing direction and efficiently navigate to their favored microenvironments. They have been identified worldwide from diverse aquatic and waterlogged microbiomes, including freshwater, saline, brackish and marine ecosystems, and some extreme environments. MTB play important roles in the biogeochemical cycling of iron, sulphur, phosphorus, carbon and nitrogen in nature and have been recognized from in vitro cultures to sequester heavy metals like selenium, cadmium, and tellurium, which makes them prospective candidate organisms for aquatic pollution bioremediation. The role of MTB in environmental systems is not limited to their lifespan; after death, fossil magnetosomal magnetic nanoparticles (known as magnetofossils) are a promising proxy for recording paleoenvironmental change and geomagnetic field history. Here, we summarize the ecology, evolution, and environmental function of MTB and the paleoenvironmental implications of magnetofossils in light of recent discoveries.

A protease-mediated switch regulates the growth of magnetosome organelles in Magnetospirillum magneticum

Abstract: Significance Biomineralization, the process by which elaborate three-dimensional structures are built out of organic and inorganic molecules, is central to health and survival of many organisms. In some magnetotactic bacteria, the growth of magnetosome membranes is closely correlated to the progression of mineral formation. However, the molecular mechanisms of such regulation are not clear. We show that the serine protease MamE links magnetosome membrane growth to the controlled production of magnetite nanoparticles through the processing of mineral-associated MamD protein. Our results indicate that membrane growth directly controls mineral growth and shed light on how an organelle’s size can determine its physiological output. Manipulation of the MamE pathway may also open the door for control of nanoparticle size in future biotechnological applications. Magnetosomes are lipid-bound organelles that direct the biomineralization of magnetic nanoparticles in magnetotactic bacteria. Magnetosome membranes are not uniform in size and can grow in a biomineralization-dependent manner. However, the underlying mechanisms of magnetosome membrane growth regulation remain unclear. Using cryoelectron tomography, we systematically examined mutants with defects at various stages of magnetosome formation to identify factors involved in controlling membrane growth. We found that a conserved serine protease, MamE, plays a key role in magnetosome membrane growth regulation. When the protease activity of MamE is disrupted, magnetosome membrane growth is restricted, which, in turn, limits the size of the magnetite particles. Consistent with this finding, the upstream regulators of MamE protease activity, MamO and MamM, are also required for magnetosome membrane growth. We then used a combination of candidate and comparative proteomics approaches to identify Mms6 and MamD as two MamE substrates. Mms6 does not appear to participate in magnetosome membrane growth. However, in the absence of MamD, magnetosome membranes grow to a larger size than the wild type. Furthermore, when the cleavage of MamD by MamE protease is blocked, magnetosome membrane growth and biomineralization are severely inhibited, phenocopying the MamE protease-inactive mutant. We therefore propose that the growth of magnetosome membranes is controlled by a protease-mediated switch through processing of MamD. Overall, our work shows that, like many eukaryotic systems, bacteria control the growth and size of biominerals by manipulating the physical properties of intracellular organelles.

Virus‐Like Iron Oxide Minerals Inspired by Magnetotactic Bacteria: Towards an Outstanding Photothermal Superhydrophobic Platform on Universal Substrates

Abstract: Magnetic iron oxides, as the typical photothermal materials, possess the advantages of low cost, easy preparation, and biocompatibility, which impart great expectations in broad application prospects. However, the limited photothermal efficiency of iron oxides restricts their further use. Inspired by magnetotactic bacteria, a liquid film‐confined strategy has been developed assisted by a magnetic field for mineralization and assembly of iron oxides on the surface at room temperature. Virus‐like hierarchically micro/nanostructured iron oxides can be obtained on universal substrates which exhibit excellent photothermal performance, the highest among all iron oxide coatings and even comparable with carbon‐based materials. Theoretical simulation demonstrates the promotion of light capture by these particular structures. Moreover, by virtue of this, the surface is endowed with superhydrophobicity by a simple modification to construct a photothermal superhydrophobic platform, which is demonstrated by two challenging scenarios: high‐efficient antibacterial activity and defrosting/deicing ability controlled remotely. There is no need for harsh experimental conditions and templates, the strategy reported here is mild, environmental‐friendly and adopts trace amount of liquid (55 µL cm−2), which can provide a reference for the fabrication and application of other photothermal materials.

Self‐Confirming Magnetosomes for Tumor‐Targeted T1/T2 Dual‐Mode MRI and MRI‐Guided Photothermal Therapy

Abstract: Nanomaterials as T1/T2 dual‐mode magnetic resonance imaging (MRI) contrast agents have great potential in improving the accuracy of tumor diagnosis. Applications of such materials, however, are limited by the complicated chemical synthesis process and potential biosafety issues. In this study, the biosynthesis of manganese (Mn)‐doped magnetosomes (MagMn) that not only can be used in T1/T2 dual‐mode MR imaging with self‐confirmation for tumor detection, but also improve the photothermal conversion efficiency for MRI‐guided photothermal therapy (PTT) is reported. The MagMn nanoparticles (NPs) are naturally produced through the biomineralization of magnetotactic bacteria by doping Mn into the ferromagnetic iron oxide crystals. In vitro and in vivo studies demonstrated that targeting peptides functionalized MagMn enhanced both T1 and T2 MRI signals in tumor tissue and significantly inhibited tumor growth by the further MRI‐guided PTT. It is envisioned that the biosynthesized multifunctional MagMn nanoplatform may serve as a potential theranostic agent for cancer diagnosis and treatment.

Detection of interphylum transfers of the magnetosome gene cluster in magnetotactic bacteria

Abstract: Magnetosome synthesis in magnetotactic bacteria (MTB) is regarded as a very ancient evolutionary process that dates back to deep-branching phyla. MTB belonging to one of such phyla, Nitrospirota, contain the classical genes for the magnetosome synthesis (e.g., mam, mms) and man genes, which were considered to be specific for this group. However, the recent discovery of man genes in MTB from the Thermodesulfobacteriota phylum has raised several questions about the inheritance of these genes in MTB. In this work, three new man genes containing MTB genomes affiliated with Nitrospirota and Thermodesulfobacteriota, were obtained. By applying reconciliation with these and the previously published MTB genomes, we demonstrate that the last common ancestor of all Nitrospirota was most likely not magnetotactic as assumed previously. Instead, our findings suggest that the genes for magnetosome synthesis were transmitted to the phylum Nitrospirota by horizontal gene transfer (HGT), which is the first case of the interphylum transfer of magnetosome genes detected to date. Furthermore, we provide evidence for the HGT of magnetosome genes from the Magnetobacteriaceae to the Dissulfurispiraceae family within Nitrospirota. Thus, our results imply a more significant role of HGT in the MTB evolution than deemed before and challenge the hypothesis of the ancient origin of magnetosome synthesis.

Intracellular silicification by early-branching magnetotactic bacteria

Abstract: Biosilicification—the formation of biological structures composed of silica—has a wide distribution among eukaryotes; it plays a major role in global biogeochemical cycles, and has driven the decline of dissolved silicon in the oceans through geological time. While it has long been thought that eukaryotes are the only organisms appreciably affecting the biogeochemical cycling of Si, the recent discoveries of silica transporter genes and marked silicon accumulation in bacteria suggest that prokaryotes may play an underappreciated role in the Si cycle, particularly in ancient times. Here, we report a previously unidentified magnetotactic bacterium that forms intracellular, amorphous silica globules. This bacterium, phylogenetically affiliated with the phylum Nitrospirota, belongs to a deep-branching group of magnetotactic bacteria that also forms intracellular magnetite magnetosomes and sulfur inclusions. This contribution reveals intracellularly controlled silicification within prokaryotes and suggests a previously unrecognized influence on the biogeochemical Si cycle that was operational during early Earth history.

Magnetic Particle Imaging of Magnetotactic Bacteria as Living Contrast Agents Is Improved by Altering Magnetosome Arrangement.

Abstract: Superparamagnetic iron oxide nanoparticles (SPIONs) can be used as imaging agents to differentiate between normal and diseased tissue or track cell movement. Magnetic particle imaging (MPI) detects the magnetic properties of SPIONs, providing quantitative and sensitive image data. MPI performance depends on the size, structure, and composition of nanoparticles. Magnetotactic bacteria produce magnetosomes with properties similar to those of synthetic nanoparticles, and these can be modified by mutating biosynthetic genes. The use of Magnetospirillum gryphiswaldense, MSR-1 with a mamJ deletion, containing clustered magnetosomes instead of typical linear chains, resulted in improved MPI signal and resolution. Bioluminescent MSR-1 with the mamJ deletion were administered into tumor-bearing and healthy mice. In vivo bioluminescence imaging revealed the viability of MSR-1, and MPI detected signals in livers and tumors. The development of living contrast agents offers opportunities for imaging and therapy with multimodality imaging guiding development of these agents by tracking the location, viability, and resulting biological effects.

Therapeutic Applications of Magnetotactic Bacteria and Magnetosomes: A Review Emphasizing on the Cancer Treatment

Abstract: Magnetotactic bacteria (MTB) are aquatic microorganisms have the ability to biomineralize magnetosomes, which are membrane-enclosed magnetic nanoparticles. Magnetosomes are organized in a chain inside the MTB, allowing them to align with and traverse along the earth’s magnetic field. Magnetosomes have several potential applications for targeted cancer therapy when isolated from the MTB, including magnetic hyperthermia, localized medication delivery, and tumour monitoring. Magnetosomes features and properties for various applications outperform manufactured magnetic nanoparticles in several ways. Similarly, the entire MTB can be regarded as prospective agents for cancer treatment, thanks to their flagella’s ability to self-propel and the magnetosome chain’s ability to guide them. MTBs are conceptualized as nanobiots that can be guided and manipulated by external magnetic fields and are driven to hypoxic areas, such as tumor sites, while retaining the therapeutic and imaging characteristics of isolated magnetosomes. Furthermore, unlike most bacteria now being studied in clinical trials for cancer treatment, MTB are not pathogenic but might be modified to deliver and express certain cytotoxic chemicals. This review will assess the current and prospects of this burgeoning research field and the major obstacles that must be overcome before MTB can be successfully used in clinical treatments.

Detection of interphylum transfers of the magnetosome gene cluster in magnetotactic bacteria

Abstract: Magnetosome synthesis in magnetotactic bacteria (MTB) is regarded as a very ancient evolutionary process that dates back to deep-branching phyla. Magnetotactic bacteria belonging to one of such phyla, Nitrospirota , contain the classical genes for the magnetosome synthesis (e.g., mam , mms ) and man genes, which were considered to be specific for this group. However, the recent discovery of man genes in MTB from the Thermodesulfobacteriota phylum has raised several questions about the inheritance of these genes in MTB. In this work, three new man genes containing MTB genomes affiliated with Nitrospirota and Thermodesulfobacteriota, were obtained. By applying reconciliation with these and the previously published MTB genomes, we demonstrate that the last common ancestor of all Nitrospirota was most likely not magnetotactic as assumed previously. Instead, our findings suggest that the genes for magnetosome synthesis were transmitted to the phylum Nitrospirota by horizontal gene transfer (HGT), which is the first case of the interphylum transfer of magnetosome genes detected to date. Furthermore, we provide evidence for the HGT of magnetosome genes from the Magnetobacteriaceae to the Dissulfurispiraceae family within Nitrospirota. Thus, our results imply a more significant role of HGT in the MTB evolution than deemed before and challenge the hypothesis of the ancient origin of magnetosome synthesis.

McaA and McaB control the dynamic positioning of a bacterial magnetic organelle

Abstract: Abstract Magnetotactic bacteria are a diverse group of microorganisms that use intracellular chains of ferrimagnetic nanocrystals, produced within magnetosome organelles, to align and navigate along the geomagnetic field. Several conserved genes for magnetosome formation have been described, but the mechanisms leading to distinct species-specific magnetosome chain configurations remain unclear. Here, we show that the fragmented nature of magnetosome chains in Magnetospirillum magneticum AMB-1 is controlled by genes mcaA and mcaB . McaA recognizes the positive curvature of the inner cell membrane, while McaB localizes to magnetosomes. Along with the MamK actin-like cytoskeleton, McaA and McaB create space for addition of new magnetosomes in between pre-existing magnetosomes. Phylogenetic analyses suggest that McaA and McaB homologs are widespread among magnetotactic bacteria and may represent an ancient strategy for magnetosome positioning.

Smart Magnetotactic Bacteria Enable the Inhibition of Neuroblastoma under an Alternating Magnetic Field.

Abstract: Magnetotactic bacteria are ubiquitous microorganisms in nature that synthesize intracellular magnetic nanoparticles called magnetosomes in a gene-controlled way and arrange them in chains. From in vitro to in vivo, we demonstrate that the intact body of Magnetospirillum magneticum AMB-1 has potential as a natural magnetic hyperthermia material for cancer therapy. Compared to chains of magnetosomes and individual magnetosomes, the entire AMB-1 cell exhibits superior heating capability under an alternating magnetic field. When incubating with tumor cells, the intact AMB-1 cells disperse better than the other two types of magnetosomes, decreasing cellular viability under the control of an alternating magnetic field. Furthermore, in vivo experiments in nude mice with neuroblastoma found that intact AMB-1 cells had the best antitumor activity with magnetic hyperthermia therapy compared to other treatment groups. These findings suggest that the intact body of magnetotactic bacteria has enormous promise as a natural material for tumor magnetic hyperthermia. In biomedical applications, intact and living magnetotactic bacteria play an increasingly essential function as a targeting robot due to their magnetotaxis.

Prussian blue technique is prone to yield false negative results in magnetoreception research

Abstract: Abstract Perls’s Prussian blue staining technique has been used in magnetoreception research to screen tissues for iron-rich structures as proxies for putative magnetoreceptor structures based on magnetic particles. However, seemingly promising structural candidates in the upper beak of birds detected with Prussian blue turned out to be either irreproducible or located in non-neuronal cells, which has spurred a controversy that has not been settled yet. Here we identify possible pitfalls in the previous works and apply the Prussian blue technique to tissues implicated in magnetic-particle-based magnetoreception, in an effort to reassess its suitability for staining single-domain magnetite, i.e., the proposed magnetic substrate for the interaction with the external magnetic field. In the upper beak of night-migratory songbirds, we found staining products in great numbers, but not remotely associated with fiber terminals of the traced ophthalmic branch of the trigeminal nerve. Surprisingly, staining products were absent from the lamina propria in the olfactory rosette of rainbow trout where candidate magnetoreceptor structures were identified with different techniques earlier. Critically, magnetosome chains in whole cells of magnetotactic bacteria remained unstained. The failure to label single-domain magnetite in positive control samples is a serious limitation of the technique and suggests that two most influential but antipodal studies conducted previously stood little chances of obtaining correct positive results under the assumption that magnetosome-like particles were present in the tissues. Nonetheless, the staining technique appears suitable to identify tissue contamination with iron-rich fine dust trapped in epithelia already in vivo.

Construction of Recombinant Magnetospirillum Strains for Nitrate Removal from Wastewater Based on Magnetic Adsorption

Abstract: Nitrate ion (NO3−) in wastewater is a major cause of pollution in aquatic environments worldwide. Magnetospirillum gryphiswaldense (MSR-1) has a complete dissimilatory denitrification pathway, converts NO3− in water into nitrogen (N2) and simultaneously removes ammonium ions (NH4+). We investigated and confirmed direct effects of regulatory protein factors Mg2046 and MgFnr on MSR-1 denitrification pathway by EMSAs and ChIP-qPCR assays. Corresponding mutant strains were constructed. Denitrification efficiency in synthetic wastewater medium during a 12-h cell growth period was significantly higher for mutant strain Δmgfnr (0.456 mmol·L−1·h−1) than for wild-type (0.362 mmol·L−1·h−1). Presence of magnetic particles (magnetosomes) in MSR-1 greatly facilitates collection and isolation of bacterial cells (and activated sludge) by addition of a magnetic field. The easy separation of magnetotactic bacteria, such as MSR-1 and Δmgfnr, from wastewater using magnetic fields is a unique feature that makes them promising candidates for practical application in wastewater treatment and sludge pretreatment.

Adsorption of Biomineralization Protein Mms6 on Magnetite (Fe3O4) Nanoparticles

Abstract: Biomineralization is an elaborate process that controls the deposition of inorganic materials in living organisms with the aid of associated proteins. Magnetotactic bacteria mineralize magnetite (Fe3O4) nanoparticles with finely tuned morphologies in their cells. Mms6, a magnetosome membrane specific (Mms) protein isolated from the surfaces of bacterial magnetite nanoparticles, plays an important role in regulating the magnetite crystal morphology. Although the binding ability of Mms6 to magnetite nanoparticles has been speculated, the interactions between Mms6 and magnetite crystals have not been elucidated thus far. Here, we show a direct adsorption ability of Mms6 on magnetite nanoparticles in vitro. An adsorption isotherm indicates that Mms6 has a high adsorption affinity (Kd = 9.52 µM) to magnetite nanoparticles. In addition, Mms6 also demonstrated adsorption on other inorganic nanoparticles such as titanium oxide, zinc oxide, and hydroxyapatite. Therefore, Mms6 can potentially be utilized for the bioconjugation of functional proteins to inorganic material surfaces to modulate inorganic nanoparticles for biomedical and medicinal applications.

Magnetosome-inspired synthesis of soft ferrimagnetic nanoparticles for magnetic tumor targeting

Abstract: Significance Magnetic targeted delivery of nanoparticle drugs has become one of the most promising means of tumor imaging and drug therapy. Inspired by magnetosome biomineralization in magnetotactic bacteria (MTB), in this study, we construct a biomimetic nanoreactor similar to that of the magnetosome by integrating Mms6 protein into a reverse micelle system. The magnetosome-like magnetic nanoparticles (MNPs) with a single domain were synthesized in this magnetosome-inspired nanoscale chamber. Their morphology and magnetic property were subsequently characterized and compared with the natural magnetosomes produced by AMB-1 MTB. The small size of magnetosome-like MNPs and their strong magnetic targeting ability produced by soft ferromagnetism improved the tumor penetration by an order of magnitude, showing a positive contrast in the tumor area.

Interplay of surface interaction and magnetic torque in single-cell motion of magnetotactic bacteria in microfluidic confinement

Abstract: Swimming microorganisms often experience complex environments in their natural habitat. The same is true for microswimmers in envisioned biomedical applications. The simple aqueous conditions typically studied in the lab differ strongly from those found in these environments and often exclude the effects of small volume confinement or the influence that external fields have on their motion. In this work, we investigate magnetically steerable microswimmers, specifically magnetotactic bacteria, in strong spatial confinement and under the influence of an external magnetic field. We trap single cells in micrometer-sized microfluidic chambers and track and analyze their motion, which shows a variety of different trajectories, depending on the chamber size and the strength of the magnetic field. Combining these experimental observations with simulations using a variant of an active Brownian particle model, we explain the variety of trajectories by the interplay between the wall interactions and the magnetic torque. We also analyze the pronounced cell-to-cell heterogeneity, which makes single-cell tracking essential for an understanding of the motility patterns. In this way, our work establishes a basis for the analysis and prediction of microswimmer motility in more complex environments.

Magnetic Biosignatures of Magnetosomal Greigite From Micromagnetic Calculation

Abstract: Greigite magnetosomes produced by magnetotactic bacteria (MTB) are widely distributed in natural environments, but large uncertainties remain regarding their magnetic biosignatures. Here, we have constructed micromagnetic models with realistic biogenic greigite particles to quantify the magnetic properties and magnetotaxis efficiency of greigite-producing MTB cells. Our calculations suggest coercivity (Bc) of ∼15–21 mT for intact greigite-producing rod-shaped MTB and many-celled magnetotactic prokaryotes, with Bc decreasing to ∼11 mT for greigite magnetofossils with clumped particles. These magnetic signatures make biogenic greigite distinguishable from typical biogenic magnetite and inorganic greigite, providing reliable magnetic criteria to detect biogenic greigite in a wide range of environmental and geological settings. Our numerical calculations suggest that rod-shaped greigite-producing MTB have a similar magnetotaxis efficiency to magnetite MTB, likely by biomineralizing more greigite crystals to compensate for the lower saturation magnetization of greigite and less ordered chains in greigite MTB cells, demonstrating biological-controlled optimization of their magnetic nanostructure.

Modifying the magnetic response of magnetotactic bacteria: incorporation of Gd and Tb ions into the magnetosome structure

Abstract: Magnetotactic bacteria Magnetospirillum gryphiswaldense MSR-1 biosynthesise chains of cube–octahedral magnetosomes, which are 40 nm magnetite high quality (Fe3O4) nanoparticles. The magnetic properties of these crystalline magnetite nanoparticles, which can be modified by the addition of other elements into the magnetosome structure (doping), are of prime interest in a plethora of applications, those related to cancer therapy being some of the most promising ones. Although previous studies have focused on transition metal elements, rare earth (RE) elements are very interesting as doping agents, both from a fundamental point of view (e.g. significant differences in ionic sizes) and for the potential applications, especially in biomedicine (e.g. magnetic resonance imaging and luminescence). In this work, we have investigated the impact of Gd and Tb on the magnetic properties of magnetosomes by using different complementary techniques. X-ray diffraction, transmission electron microscopy, and X-ray absorption near edge spectroscopy analyses have revealed that a small amount of RE ions, ∼3–4%, incorporate into the Fe3O4 structure as Gd3+ and Tb3+ ions. The experimental magnetic characterisation has shown a clear Verwey transition for the RE-doped bacteria, located at T ∼ 100 K, which is slightly below the one corresponding to the undoped ones (106 K). However, we report a decrease in the coercivity and remanence of the RE-doped bacteria. Simulations based on the Stoner–Wohlfarth model have allowed us to associate these changes in the magnetic response with a reduction of the magnetocrystalline (KC) and, especially, the uniaxial (Kuni) anisotropies below the Verwey transition. In this way, Kuni reaches a value of 23 and 26 kJ m−3 for the Gd- and Tb-doped bacteria, respectively, whilst a value of 37 kJ m−3 is obtained for the undoped bacteria.

Periplasmic Bacterial Biomineralization of Copper Sulfide Nanoparticles

Abstract: Metal sulfides are a common group of extracellular bacterial biominerals. However, only a few cases of intracellular biomineralization are reported in this group, mostly limited to greigite (Fe3S4) in magnetotactic bacteria. Here, a previously unknown periplasmic biomineralization of copper sulfide produced by the magnetotactic bacterium Desulfamplus magnetovallimortis strain BW‐1, a species known to mineralize greigite (Fe3S4) and magnetite (Fe3O4) in the cytoplasm is reported. BW‐1 produces hundreds of spherical nanoparticles, composed of 1–2 nm substructures of a poorly crystalline hexagonal copper sulfide structure that remains in a thermodynamically unstable state. The particles appear to be surrounded by an organic matrix as found from staining and electron microscopy inspection. Differential proteomics suggests that periplasmic proteins, such as a DegP‐like protein and a heavy metal‐binding protein, could be involved in this biomineralization process. The unexpected periplasmic formation of copper sulfide nanoparticles in BW‐1 reveals previously unknown possibilities for intracellular biomineralization that involves intriguing biological control and holds promise for biological metal recovery in times of copper shortage.

Bending and Collapse: Magnetic Recording Fidelity of Magnetofossils From Micromagnetic Simulation

Abstract: Magnetosome chains produced by magnetotactic bacteria are important paleoenvironmental and paleomagnetic recorders. It has been shown that magnetic properties of magnetosome chains are closely related to their morphology and chain structures; however, the in situ structures of magnetosome chains in sediments (magnetofossils) are not known. Magnetosome chains are subject to various deformations after cell dissolution and are therefore unlikely to be fully intact, obscuring their original magnetic signals. Here, we use finite element micromagnetic simulations to quantify changes in magnetic signals in response to chain deformation, in particular, as a function of variable degrees of bending and collapse. Our results indicate that bending/collapse leads to a significant coercivity reduction and domain state transition of the chain. Therefore, hysteresis parameters can be used to assess the degree of chain bending/collapse in magnetofossil-rich sediments. Calculations of the contributions of chain bending/collapse to the post-depositional remanent magnetization (pDRM) of magnetofossils indicate that pDRM remains both faithful to the pre-bending/collapse natural remanent magnetization, and that the remanence of some structurally deformed magnetofossil assemblages remains thermally stable over billion-year timescales, suggesting that even strongly deformed magnetosome chains in ancient geological materials retain faithful paleomagnetic records and thus have potentials for tracing ancient geomagnetic field variations and microbial activities on early Earth.