A Comparison of Methods to Measure the Magnetic Moment of Magnetotactic Bacteria through Analysis of Their Trajectories in External Magnetic Fields

Magnetotactic bacteria possess organelles called magnetosomes that confer a magnetic moment on the cells, resulting in their partial alignment with external magnetic fields. Here we show that analysis of the trajectories of cells exposed to an external magnetic field can be used to measure the average magnetic dipole moment of a cell population in at least five different ways. We apply this analysis to movies of Magnetospirillum magneticum AMB-1 cells, and compare the values of the magnetic moment obtained in this way to that obtained by direct measurements of magnetosome dimension from electron micrographs. We find that methods relying on the viscous relaxation of the cell orientation give results comparable to that obtained by magnetosome measurements, whereas methods relying on statistical mechanics assumptions give systematically lower values of the magnetic moment. Since the observed distribution of magnetic moments in the population is not sufficient to explain this discrepancy, our results suggest that non-thermal random noise is present in the system, implying that a magnetotactic bacterial population should not be considered as similar to a paramagnetic material.     

Magnetite and magnetotaxis in microorganisms

Magnetotactic bacteria from freshwater and marine sediments orient and navigate along geomagnetic field lines. Their magnetotactic response is based on intracellular, single magnetic domains of ferrimagnetic magnetite, which impart a permanent magnetic dipole moment to the cell.     

Reduced Efficiency of Magnetotaxis in Magnetotactic Coccoid Bacteria in Higher than Geomagnetic Fields

Magnetotactic bacteria are microorganisms that orient and migrate along magnetic field lines. The classical model of polar magnetotaxis predicts that the field-parallel migration velocity of magnetotactic bacteria increases monotonically with the strength of an applied magnetic field. We here test this model experimentally on magnetotactic coccoid bacteria that swim along helical trajectories. It turns out that the contribution of the field-parallel migration velocity decreases with increasing field strength from 0.1 to 1.5 mT. This unexpected observation can be explained and reproduced in a mathematical model under the assumption that the magnetosome chain is inclined with respect to the flagellar propulsion axis. The magnetic disadvantage, however, becomes apparent only in stronger than geomagnetic fields, which suggests that magnetotaxis is optimized under geomagnetic field conditions. It is therefore not beneficial for these bacteria to increase their intracellular magnetic dipole moment beyond the value needed to overcome Brownian motion in geomagnetic field conditions.     

Dynamics of magnetotactic bacteria in a rotating magnetic field

The dynamics of the motile magnetotactic bacterium Magnetospirillum gryphiswaldense in a rotating magnetic field is investigated experimentally and analyzed by a theoretical model. These elongated bacteria are propelled by single flagella at each bacterial end and contain a magnetic filament formed by a linear assembly of approximately 40 ferromagnetic nanoparticles. The movements of the bacteria in suspension are analyzed by consideration of the orientation of their magnetic dipoles in the field, the hydrodynamic resistance of the bacteria, and the propulsive force of the flagella. Several novel features found in experiments include a velocity reversal during motion in the rotating field and an interesting diffusive wandering of the trajectory curvature centers. A new method to measure the magnetic moment of an individual bacterium is proposed based on the theory developed.     

A new study of bacterial motion: superconducting quantum interference device microscopy of magnetotactic bacteria.

The recently developed "microscope" based on a high-Tc dc SQUID (superconducting quantum interference device) is used to detect the magnetic fields produced by the motion of magnetotactic bacteria, which have permanent dipole moments. The bacteria, in growth medium at room temperature, can be brought to within 15 micron of a SQUID at liquid nitrogen temperature. Measurements are performed on both motile and nonmotile bacteria. In the nonmotile case, we obtain the power spectrum of the magnetic field noise produced by the rotational Brownian motion of the ensemble of bacteria. Furthermore, we measure the time-dependent field produced by the ensemble in response to an applied uniform magnetic field. In the motile case, we obtain the magnetic field power spectra produced by the swimming bacteria. Combined, these measurements determine the average rotational drag coefficient, magnetic moment, and the frequency and amplitude of the vibrational and rotational modes of the bacteria in a unified set of measurements. In addition, the microscope can easily resolve the motion of a single bacterium. This technique can be extended to any cell to which a magnetic tag can be attached.     

趋磁螺菌中磁小体链装配相关基因及其功能

趋磁细菌(MTB)依赖于体内磁小体结构在磁场中取向,多个磁小体以一定的组 织形式排列是形成菌体内生物磁罗盘的重要环节．多数趋磁细菌中磁小体成链排列,有效增加了细胞磁偶极矩,从而使菌体表现出在环境磁场中定向的能力．趋磁螺菌M. magneticum AMB－1和M. gryphiswaldense MSR－1中磁小体均沿细胞长轴形成一条磁 小体链．通过对相关基因突变体表型的研究,结合对磁小体链形成过程的实时动态观 察,人们已初步了解MamJ、MamK和MamA等基因在磁小体链装配和维护过程中的功能．本文介绍了近年来趋磁螺菌磁小体链装配过程中重要功能性基因的研究进展,并重点分析了AMB-1和MSR-1中磁小体链装配的差异．     

Magnetic-dependent ATP pool in <Emphasis Type="Italic">Escherichia coli</Emphasis>

The ATP pool in Escherichia coli is a magnetic-dependent characteristic of microorganism vital activity. It depends on the values of the external static magnetic field and the existence of magnetic moment of magnesium isotopes nuclei added to the growth nutrient medium. The combined effects of the magnetic field 70–95 mT and magnesium magnetic isotope ²⁵Mg on E. coli bacteria leads to increase intracellular concentration of ATP. Magnetic-field effects in the range of 0.8–16 mT, registered for all bacteria regardless of the magnesium-isotopic enrichment of nutrient medium, evidence about the sensitivity of intracellular processes to weak magnetic fields.     

Conformation of biological macromolecules. Circular dichroism and magnetic circular dichroism studies of metmyoglobin and its derivatives.

The circular dichroism (CD) and magnetic circular dichroism (MCD) spectra of horse heart metmyoglobin and the following derivatives were measured in the Soret and near ultraviolet regions: metmyoglobin and its peroxide compound, and hydroxide, cyanide, azide, and fluoride derivatives. The heme-related CD bands in the Soret and near ultraviolet wavelength regions were altered by ligand substitution, though their relationships to the magnetic moment were quite different. In the Soret region, the CD peak had no definite relation to the magnetic moment, while in the near ultraviolet region the magnitude of the CD peak decreased with the magnetic moment. The MCD peak in the Soret and near Ultraviolet regions also varied with ligand substitution. The magnetic ellipticity decreased with the magnetic moment in both wavelength regions. There was a more quantitative correlation between the magnetic ellipticity and the magnetic moment in the near ultraviolet region than in the Soret region. Metmyoglobin peroxide compound exhibited slightly different behavior in the MCD spectrum from other derivatives. It is suggested that the heme iron of the metmyoglobin peroxide compound is in an oxidation state other than the ferric state and that the porphyrin structure of metmyoglobin may be modified by the reaction with hydrogen peroxide.     

Effects of Hypomagnetic Field on Magnetosome Formation of Magnetospirillum Magneticum AMB-1

Magnetotactic bacteria synthesize intracellular magnetic particles, magnetosomes, which arrange in chain(s) and confer on cell a magnetic dipolar moment. To explore the function of geomagnetic field to magnetotactic bacteria, the effects of hypomagnetic field on magnetosome formation in Magnetospirillum magneticum AMB-1 were studied. Cells were cultivated in a specially designed device where geomagnetic field was reduced by about 100-fold to less than 500nT. AMB-1 cultures were incubated in hypomagnetic field or geomagnetic field. Results showed that hypomagnetic field had no significant effects on the average number of magnetic particles per bacterium and bacterial iron depletion. However, the growth (OD) of cell at stationary-phase was lower and cellular magnetism (R _mag) at exponential growth phase was higher than that of bacteria cultivated in geomagnetic field. Statistic results on transmission electron microscopy (TEM) micrographs showed that the average size of magnetic particles in AMB-1 cells in hypomagnetic field group was larger than that of in geomagnetic field group and more ratio of larger-size magnetic particles (>50 nm) was observed when cultivated 16 h under hypomagnetic field. Furthermore, the influences of hypomagnetic field on gene expression were studied in AMB-1 cells. Quantitative RT-PCR results showed that hypomagnetic field up-regulated mms13, down-regulated mms6 and had no effect on magA. Together, the results showed that hypomagnetic field could affect the growth of AMB-1 at the stationary-phase, the crystallization process of magnetosomes, and mms13, mms6 expressions. In addition, our results suggested that the geomagnetic field plays an important role in the biomineralization of magnetosomes.     

An MCP-Like Protein Interacts with the MamK Cytoskeleton and Is Involved in Magnetotaxis in Magnetospirillum magneticum AMB-1

Magnetotactic bacteria have the unique capacity of aligning and swimming along geomagnetic field lines, a behavior called magnetotaxis. Although this behavior has been observed for 40 years, little is known about its mechanism. Magnetotactic bacteria synthesize unique organelles, magnetosomes, which are magnetic crystals enveloped by membrane. They form chains with the help of the filamentous cytoskeletal protein MamK and impart a net magnetic-dipole moment to the bacterium. The current model proposes that magnetotaxis comprises passive magnetic orientation and active swimming due to flagellar rotation. We thought that magnetic sensing, via the widely used chemotaxis mechanism, might be actively involved in magnetotaxis. We found that the methyl-accepting chemotaxis protein Amb0994 of Magnetospirillum magneticum AMB-1 was capable of carrying out such a function. Amb0994 is encoded by a gene in the magnetosome island, in which genes essential for magnetosome biosynthesis and magnetotaxis are concentrated. Amb0994 lacks periplasmic sensing domain, which is generally involved in sensing stimuli from outside of cells. By constructing fusions with a derivative of yellow-fluorescent-protein, we showed that Amb0994 localizes to the cell poles, where methyl-accepting chemotaxis proteins are usually clustered. We then showed that Amb0994 specifically interacts, via its C-terminal domain, with MamK, using a bimolecular fluorescence complementation assay. Moreover, overproduction of Amb0994 slowed down the response of the bacterium to changes in the direction of the magnetic field. Most importantly, the C-terminal domain of Amb0994, which interacts with MamK, is responsible for this phenotype, suggesting that the interaction between Amb0994 and MamK plays a key role in magnetotaxis. These results lead to a novel explanation for magnetotaxis at the molecular level.     

磁分离仪分离磁性细菌的新方法

磁性细菌胞内可以产生磁性颗粒,因此具有趋磁性,基于这种特性,利用磁分离的原理,本研究开发了一种磁性细菌分离仪,提供了一种分离磁性细菌的新方法。以氧化亚铁硫杆菌为例,使用磁性细菌分离仪进行分离,可以得到强磁菌和弱磁菌。利用透射电镜观察,强磁菌胞内磁性颗粒明显多于弱磁菌;半固体平板磁泳实验也表明强磁菌趋磁性明显强于弱磁菌。各项实验结果表明磁性细菌分离仪可以有效地分离磁性细菌,这是一种分离磁性细菌的新方法,将促进磁性细菌分离培养的研究。     

Magnetic properties of tunicate blood cells. I. Ascidia nigra

The magnetic properties of intact and freeze-dried blood cells of the tunicate Ascidia nigra and of model vanadium(III) and (IV) compounds as polycrystalline solids and in aqueous solution have been measured up to 50 kOe with a SQUID susceptometer. Corrections for the samples' diamagnetism were extracted from the temperature dependence of the data without any further assumptions. For vanadium(IV), measured values of the magnetic moment at different values of the applied magnetic field over the temperature range 2-100 K obey a Brillouin function with spin 1/2. For vanadium(III), the magnetic moment data did not obey a Brillouin function and were analyzed in terms of a spin Hamiltonian with S = 1. Measurements on both whole and freeze-dried blood samples give consistent results with vanadium(III) the predominant species. These results are discussed in terms of the mechanisms of vanadium accumulation and the use of vanadium oxidation states as criteria of ascidian taxonomy.     

Orientational dynamics of magnetotactic bacteria in Earth’s magnetic field—a simulation study

We investigate through simulations the phenomena of magnetoreception to enable an understanding of the minimum requirements of a fail-safe mechanism, operational at the cellular level, to sense a weak magnetic field at ambient temperature in a biologically active environment. To do this, we use magnetotactic bacteria (MTB) as our model system. The magnetic field sensing ability of these bacteria is due to the presence of magnetosomes, which are internal membrane-bound organelles that contain an iron-based magnetic mineral crystal. These magnetosomes are usually found arranged in a chain aligned with the long axis of the bacterial body. This arrangement yields an overall magnetic dipole moment to the bacterial cell. To simulate this orientation process, we set up a rotational Langevin stochastic differential equation and solve it repeatedly over appropriate time steps for isolated spherical shaped MTB as well as for a more realistic model of spheroidal MTB with flagella. The orientation process appears to depend on shape parameters with spheroidal MTB showing a slower response time compared to spherical MTB. Further, our simulation also reveals that the alignment to the external magnetic field is more robust for an MTB when compared to single magnetosome. For the simulation involving magnetosomes, we include an extra torque that arises from the twisting of an attachment tether and enhance the viscosity of the surrounding medium to mimic intracellular conditions in the governing Langevin equation. The response time of alignment is found to be substantially reduced when one includes a dipole interaction term with a neighboring magnetosome and the alignment becomes less robust with increase in inter dipole distance. The alignment process can thereby be said to be very sensitively dependent on the distance between magnetosomes. Simulating the process of alignment between two neighboring magnetosomes, both in the absence and presence of an ambient magnetic field, we conclude that alignment between these dipoles at the distances typical in an MTB is highly probable and it would be the locked unit that responds to changes in the external magnetic field.     

磁泳分离细菌新方法的研究

从酸性矿坑水中富集培养分离到的嗜酸氧化亚铁硫杆菌（Acidithiobacillus ferrooxidans,A.ferrooxidans）[1-2] 菌同趋磁细菌具有一定的相似性。通过显微镜观察发现,部分浸矿细菌在外加磁场的作用下具有微弱的趋磁性,基于菌种的这种特性,设计了磁泳分离仪,对其在磁场作用下泳动（磁泳）进行分析,经磁泳后的近磁、远磁菌的生理特性有较大的差异。从用涂布平板法获得的近磁菌纯培养A. ferrooxidans菌体中,分离得到纳米磁性颗粒,能谱分析表明,其主要成分为Fe和O元素。实验结果证明,A. ferrooxidans具有微弱趋磁性,采用磁泳分离该类菌体内含有磁性颗粒的细菌是可行的,这一分离技术的进一步完善和改进将为传统的微生物菌种分离提供一种新型分离技术,也将大大促进趋磁细菌的研究,而且它与浸矿工艺的结合将大大促进我国生物冶金的研究步伐。     

Elastic stability of chains of magnetosomes in magnetotactic bacteria

Electron micrographs of magnetotactic bacteria reveal that chains of magnetosomes are often bent. This is surprising inasmuch as straight chains are actually the most favourable arrangement for magnetonavigation achieving the maximum value of the bacterial net magnetic moment. In order to answer the question of what causes the chains to bend, we calculated the stability limit of straight magnetosome chains by taking into account elastic and magnetic forces. For several scenarios, the threshold values of external forces leading to elastic instability were computed. From our calculations and observations on freeze-dried cells, we conclude that, under normal conditions, magnetosome chains are straight or only slightly bent, whereas shrinkage during preparation may cause severe artifacts such as kinks or zig-zag structures in the chains. Received: 10 February 1997 / Accepted: 9 April 1997     

Particle confinement in axisymmetric poloidal magnetic field configurations with zeros of B: Methodological note

Collisionless particle confinement in axisymmetric configurations with magnetic field nulls is analyzed. The existence of an invariant of motion—the generalized azimuthal momentum—makes it possible to determine in which of the spatial regions separated by magnetic separatrices passing through the magnetic null lines the particle occurs after it leaves the vicinity of a magnetic null line. In particular, it is possible to formulate a sufficient condition for the particle not to escape through the separatrix from the confinement region to the external region. In the configuration under analysis, the particles can be lost from a separatrix layer with a thickness on the order of the Larmor radius because of the nonconservation of the magnetic moment μ. In this case, the variations in μ are easier to describe in a coordinate system associated with the magnetic surfaces. An analysis is made of the applicability of expressions for the single-pass change Δμ in the magnetic moment that were obtained in different magnetic field models for a confinement system with a divertor (such that there is a circular null line).     

Magnetic configuration model for the multicellular magnetotactic prokaryote Candidatus Magnetoglobus multicellularis

Candidatus Magnetoglobus multicellularis (CMm) is a multicellular organism in which each constituent cell is a magnetotactic bacterium. It has been observed that disaggregation of this organism provokes the death of the individual cells. The observed flagellar movement of the CMm indicates that the constituent cells move in a coordinated way, indicating a strong correlation between them and showing that this aggregate could be considered as an individual. As every constituent cell is a magnetotactic bacterium, every cell contributes with a magnetic moment vector to the resultant magnetic moment of the CMm organism that can be calculated through the vectorial sum of all the constituent magnetic moments. Scanning electron microscopy images of CMm organisms have shown that the constituent cells are distributed on a helix convoluted on a spherical surface. To analyze the magnetic properties of the distribution of magnetic moments on this curve, we calculated the magnetic energy numerically as well as the vectorial sum of the magnetic moment distribution as a function of the number of cells, the sphere radius and the number of spiral loops. This distribution proposes a magnetic organization not seen in any other living organism and shows that minimum energy configurations of magnetic moments are in spherical meridian chains, perpendicular to the helix turns. We observed that CMm has a high theoretical degree of magnetic optimization, showing that its geometrical structure is important to the magnetic response. Our results indicate that the helical structure must have magnetic significance.     

An instrument for sorting of magnetic microparticles in a magnetic field gradient.

BACKGROUND: The goal of our bioassay technique is to demonstrate high throughput, highly parallel, and high sensitivity quantitative molecular analysis that will expand current biomedical research capabilities. To this end, we have built and characterized a magnetophoresis instrument using a flow chamber in a magnetic field gradient to sort magnetic microparticles by their magnetic moment for eventual use as biological labels. METHODS: The flow chamber consists of a sample inlet, differential sheath streams, and eight outlets for collecting the microparticles after they have traversed the chamber. Magnetic microparticles are injected into the flow chamber that is positioned in a linear magnetic field gradient. The trajectory for each microparticle is determined by its total magnetic moment and size. The resulting populations of monodispersed magnetic microparticles in the different outlet bins are sorted by their magnetic moment; with the highest magnetic moments being deflected the furthest. RESULTS: We have characterized the system for sorting both superparamagnetic and ferromagnetic microparticles with approximate diameters of 8 microm and 4.0-4.9 microm, respectively. To characterize the instrument, we used microparticles with a known size distribution and varied the transit time through the chamber. This is equivalent to varying the magnetic moment, while allowing us to hold the particle properties constant from run-to-run. We demonstrated the ability to reproducibly change the distribution of the particles in the collection bins by varying transit time in good agreement with theory. We identified hydrodynamic instabilities responsible for causing dispersion in the flow. Improvements to the flow chamber hydrodynamics such as reducing the aspect ratio between the sample inlet and the chamber depth and stabilizing the sheath flow resulted in narrow sorting distributions. We measured a sorting reproducibility (percentage of particles returning to their original bin upon resorting individual populations) of 84-89%. CONCLUSIONS: We have developed a simple magnetophoresis system for reproducibly sorting magnetic microparticles. This technique will permit the use of microparticles with a wide range of magnetic moments to create a wide range of magnetic labels. Careful consideration of system design and operational parameters enables reliable and reproducible sorting of microparticles with varying size and magnetic content.     

Study of the motion of magnetotactic bacteria

Motion of flagellate bacteria is considered from the point of view of rigid body mechanics. As a general case we consider a flagellate coccus magnetotactic bacterium swimming in a fluid in the presence of an external magnetic field. The proposed model generalizes previous approaches to the problem and allows one to access parameters of the motion that can be measured experimentally. The results suggest that the strong helical pattern observed in typical trajectories of magnetotactic bacteria can be a biological advantage complementary to magnetic orientation. In the particular case of zero magnetic interaction the model describes the motion of a non-magnetotactic coccus bacterium swimming in a fluid. Theoretical calculations based on experimental results are compared with the experimental track obtained by dark field optical microscopy. Correspondence to: H. G. P. Lins de Barros     

Magnetic optimization in a multicellular magnetotactic organism

Unicellular magnetotactic prokaryotes, which typically carry a natural remanent magnetic moment equal to the saturation magnetic moment, are the prime example of magnetically optimized organisms. We here report magnetic measurements on a multicellular magnetotactic prokaryote (MMP) consisting of 17 undifferentiated cells (mean from 148 MMPs) with chains of ferrimagnetic particles in each cell. To test if the chain polarities of each cell contribute coherently to the total magnetic moment of the MMP, we used a highly sensitive magnetization measurement technique (1 fAm(2)) that enabled us to determine the degree of magnetic optimization (DMO) of individual MMPs in vivo. We obtained DMO values consistently above 80%. Numerical modeling shows that the probability of reaching a DMO > 80% would be as low as 0.017 for 17 randomly oriented magnetic dipoles. We simulated different scenarios to test whether high DMOs are attainable by aggregation or self-organization of individual magnetic cells. None of the scenarios investigated is likely to yield consistently high DMOs in each generation of MMPs. The observed high DMO values require strong Darwinian selection and a sophisticated reproduction mechanism. We suggest a multicellular life cycle as the most plausible scenario for transmitting the high DMO from one generation to the next.