Phototaxis in the magnetotactic bacterium Magnetospirillum magneticum strain AMB-1 is independent of magnetic fields

Magnetotactic bacteria (MTB) can rapidly relocate to optimal habitats by magneto-aerotaxis. Little is known about MTB phototaxis, a response that might also aid navigation. In this study, we analyzed the relationship between phototaxis and magnetotaxis in Magnetospirillum magneticum strain AMB-1. Magnotactic AMB-1 cells migrated toward light, and migration increased with higher light intensity. This response was independent of wavelength, as AMB-1 cells migrated equally toward light from 400 to 750 nm. When AMB-1 cells were exposed to zero magnetic fields or to 0.2 mT magnetic fields that were opposite or orthogonal to the light beam, cells still migrated toward the light, indicating that phototaxis was independent of magnetotaxis. The R _mag value and coercive force (H _c) of AMB-1 increased when the bacteria were illuminated for 20 h, consistent with an increase in magnetosome synthesis or in magnetosome-containing cells. These results demonstrated that the M. magneticum AMB-1 responded to light as well as other environmental factors. To our knowledge, this is the first report of phototactic behavior in the bacteria of Magnetospirillum.     

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.     

Quantifying the magnetic advantage in magnetotaxis

Magnetotactic bacteria are characterized by the production of magnetosomes, nanoscale particles of lipid bilayer encapsulated magnetite, that act to orient the bacteria in magnetic fields. These magnetosomes allow magneto-aerotaxis, which is the motion of the bacteria along a magnetic field and toward preferred concentrations of oxygen. Magneto-aerotaxis has been shown to direct the motion of these bacteria downward toward sediments and microaerobic environments favorable for growth. Herein, we compare the magneto-aerotaxis of wild-type, magnetic Magnetospirillum magneticum AMB-1 with a nonmagnetic mutant we have engineered. Using an applied magnetic field and an advancing oxygen gradient, we have quantified the magnetic advantage in magneto-aerotaxis as a more rapid migration to preferred oxygen levels. Magnetic, wild-type cells swimming in an applied magnetic field more quickly migrate away from the advancing oxygen than either wild-type cells in a zero field or the nonmagnetic cells in any field. We find that the responses of the magnetic and mutant strains are well described by a relatively simple analytical model, an analysis of which indicates that the key benefit of magnetotaxis is an enhancement of a bacterium's ability to detect oxygen, not an increase in its average speed moving away from high oxygen concentrations.     

Swimming characteristics of magnetic bacterium, Magnetospirillum sp. AMB-1, and implications as toxicity measurement

To develop a novel toxicity measurement system using the persistent swimming property of magnetic bacteria along an externally applied magnetic field, certain characteristics of Magnetospirillum sp. AMB-1 cells were examined, including their growth pattern, motility, magnetosensitivity, swimming speed, and cell length distribution. In addition, the effect of toxic compounds on the swimming speed was assessed relative to application as a toxicity sensor. With an inoculum of 1.0 x 10(8) cells/mL, the cells reached the stationary phase with a concentration of about 5 x 10(8) cells/mL after 20 h, under both aerobic and anaerobic conditions. The distribution of the cell length did not vary significantly during the growth period, and both aerobically and anaerobically growing cells showed a similar cell length distribution. Although the cells showed similar growth patterns under both conditions, the anaerobically grown cells exhibited higher motility and magnetosensitivity. Actively growing cells under anaerobic conditions had an average swimming speed of 49 microm/s with a standard deviation of 20 microm/s. When the anaerobically growing cells were exposed to various concentrations of toxic compounds, such as 1-propanol and acetone, the swimming speed decreased with an increased concentration of the toxic compound. Accordingly, the relationship between swimming speed and toxicity can be used as an effective quantitative toxicity measurement; furthermore, the relative sensitivity of the proposed system was comparable to Microtox, which is commercially available.     

Magnetosome Formation and Expression of mamA, mms13, mms6 and magA in Magnetospirillum magneticum AMB-1 Exposed to Pulsed Magnetic Field

To investigate the effects of pulsed magnetic field on magnetosome formation in Magnetospirillum magneticum AMB-1, cultures inoculated with either mangetic or non-magnetic pre-cultures were incubated under 1 mT pulsed magnetic field. Magnetism of cells was measured by using spectrophotometer coupled with applied magnetic fields and the values were described as C _mag. Magnetosome in cells was counted by transmission electron microscopy observation. The results showed that pulsed magnetic field did not affect cellular growth, but enhanced magnetosome formation. The applied pulsed magnetic field might exceed the chain of magnetosomes and change the homogeneity of the magnetosome particles. The results implied that magnetite precipitation induced by the adjacent magnetosome was affected by pulsed magnetic field. Moreover, the applied pulsed magnetic field up-regulated the magA and mamA expression in cells, which might account for the increasing number and the exceeding chain of magnetosomes in cells.     

Enhancement of magnetic particle production by nitrate and succinate fed-batch culture of Magnetospirillum sp. AMB-1

Summary Magnetospirillum sp. AMB-1 is a magnetic bacterium, which is capable of growing under air atmosphere. This bacterium was employed to make bacterial magnetic particles (BMPs). AMB-1 only makes BMPs during logarithmic growth phase under anaerobic conditions. Since it requires nitrate as a nitrogen source, control of nitrate concentration in the medium was necessary. The fed-batch culture of AMB-1 was carried out by adding nitric acid and succinate as nitrogen and carbon source respectively. One liter of AMB-1 culture produced 0.34 g of dry cells and 4.5 mg of BMPs. BMP production by AMB-1 cultivated in the fed-batch culture was found to be seven times higher than that cultivated in the batch culture.     

Respiratory inhibitors of a magnetic bacterium Magnetospirillum sp. AMB-1 capable of growing aerobically

Respiratory inhibitors of a magnetic bacterium Magnetospirillum sp. AMB-1, which is able to grow aerobically, were investigated using a microbial electrode system. The respiration of strain AMB-1 was inhibited by 2-heptyl-4-hydroxyquinoline N-oxide (HQNO), KCN and dicumarol. Strain AMB-1 cannot grow in the presence of these inhibitors under aerobic conditions. On the other hand, strain AMB-1 can grow and form magnetite (Fe₃O₄) particles with HQNO and KCN under anaerobic conditions. Growth and magnetite formation of strain AMB-1 were reduced by dicumarol, which also inhibited iron reduction under anaerobic conditions, whereas iron reduction was not inhibited by HQNO and KCN.     

Enhancement of the efficiency of non-viral gene delivery by application of pulsed magnetic field

New approaches to increase the efficiency of non-viral gene delivery are still required. Here we report a simple approach that enhances gene delivery using permanent and pulsating magnetic fields. DNA plasmids and novel DNA fragments (PCR products) containing sequence encoding for green fluorescent protein were coupled to polyethylenimine coated superparamagnetic nanoparticles (SPIONs). The complexes were added to cells that were subsequently exposed to permanent and pulsating magnetic fields. Presence of these magnetic fields significantly increased the transfection efficiency 40 times more than in cells not exposed to the magnetic field. The transfection efficiency was highest when the nanoparticles were sedimented on the permanent magnet before the application of the pulsating field, both for small (50 nm) and large (200–250 nm) nanoparticles. The highly efficient gene transfer already within 5 min shows that this technique is a powerful tool for future in vivo studies, where rapid gene delivery is required before systemic clearance or filtration of the gene vectors occurs.     

Production of luciferase-magnetic particle complex by recombinant Magnetospirillum sp. AMB-1

Luciferase-bacterial magnetic particle (BMP) complexes were produced by recombinant Magnetospirillum sp. AMB-1. We constructed plasmids pKML and pNELM, respectively, by fusing luc to the 5' and 3' terminal of magA, encoding an integral iron translocating protein situated in the BMP membrane, of AMB-1. In addition, we produced bifunctional active-fusion proteins on BMPs by using a plasmid pAcML. In this plasmid, acetate kinase and luciferase genes were fused to the N-terminus and the C-terminus of MagA, respectively. Bacterial magnetic particles isolated from transconjugants for pKML, pNELM and pAcML exhibited luciferase activity. Bacterial magnetic particles isolated from transconjugants for pAcML also exhibited acetate kinase activity. Fed-batch culture of pKML transconjugant yielded 2.6 mg BMPs per liter of culture, and 95% conversion of iron into magnetite was obtained, at a nitrate concentration of 1.4 mM. Continuous feeding of iron as ferric quinate significantly enhanced growth and total magnetic production. Final cell concentration of 1.8 x 10(9) cells/mL and 6 mg per liter of culture was obtained. Magnetite production by fed-batch culture of AMB-1 was about 3 times that obtained by batch culture. There were no significant differences in BMPs yield between recombinant AMB-1 cultivated by fed-batch culture and wild type of AMB-1.     

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.     

The effect of iron-chelating agents on Magnetospirillum magneticum strain AMB-1: stimulated growth and magnetosome production and improved magnetosome heating properties

The introduction of various iron-chelating agents to the Magnetospirillum magneticum strain AMB-1 bacterial growth medium stimulated the growth of M. magneticum strain AMB-1 magnetotactic bacteria and enhanced the production of magnetosomes. After 7?days of growth, the number of bacteria and the production of magnetosomes were increased in the presence of iron-chelating agents by factors of up to ??2 and ??6, respectively. The presence of iron-chelating agents also produced an increase in magnetosome size and chain length and yielded improved magnetosome heating properties. The specific absorption rate of suspensions of magnetosome chains isolated from M. magneticum strain AMB-1 magnetotactic bacteria, measured under the application of an alternating magnetic field of average field strength ??20?mT and frequency 198?kHz, increased from ??222?W/g_Fe in the absence of iron-chelating agent up to ??444?W/g_Fe in the presence of 4???M rhodamine B and to ??723?W/g_Fe in the presence of 4???M EDTA. These observations were made at an iron concentration of 20???M and iron-chelating agent concentrations below 40???M.     

Magnetosomes extracted from Magnetospirillum magneticum strain AMB-1 showed enhanced peroxidase-like activity under visible-light irradiation

Magnetosomes are intracellular structures produced by magnetotactic bacteria and are magnetic nanoparticles surrounded by a lipid bilayer membrane. Magnetosomes reportedly possess intrinsic enzyme mimetic activity similar to that found in horseradish peroxidase (HRP) and can scavenge reactive oxygen species depending on peroxidase activity. Our previous study has demonstrated the phototaxis characteristics of Magnetospirillum magneticum strain AMB-1 cells, but the mechanism is not well understood. Therefore, we studied the relationship between visible-light irradiation and peroxidase-like activity of magnetosomes extracted from M. magneticum strain AMB-1. We then compared this characteristic with that of HRP, iron ions, and naked magnetosomes using 3,3′,5,5′-tetramethylbenzidine as a peroxidase substrate in the presence of H₂O₂. Results showed that HRP and iron ions had different activities from those of magnetosomes and naked magnetosomes when exposed to visible-light irradiation. Magnetosomes and naked magnetosomes had enhanced peroxidase-like activities under visible-light irradiation, but magnetosomes showed less affinity toward substrates than naked magnetosomes under visible-light irradiation. These results suggested that the peroxidase-like activity of magnetosomes may follow an ordered ternary mechanism rather than a ping–pong mechanism. This finding may provide new insight into the function of magnetosomes in the phototaxis in magnetotactic bacteria.     

Noncovalent Immobilization of Streptavidin on In Vitro- and In Vivo-Biotinylated Bacterial Magnetic Particles

Biotinylated magnetic nanoparticles were constructed by displaying biotin acceptor peptide (BAP) or biotin carboxyl carrier protein (BCCP) on the surface of bacterial magnetic particles (BacMPs) synthesized by Magnetospirillum magneticum AMB-1. BAP-displaying BacMPs (BAP-BacMPs) were extracted from bacterial cells and incubated with biotin and Escherichia coli biotin ligase. Then the in vitro biotinylation of BAP-BacMPs was confirmed using alkaline phosphatase-labeled antibiotin antibody. In contrast, BacMPs displaying the intact 149 residues of AMB-1 BCCP (BCCP-BacMPs) and displaying the COOH-terminal 78 residues of BCCP (BCCP78-BacMPs) were biotinylated in AMB-1 cells. The in vivo biotinylation of BCCP-BacMPs and BCCP78-BacMPs was thought to be performed by endogenous AMB-1 biotin ligase. Streptavidin was introduced onto biotinylated BacMPs by simple mixing. In an analysis using tetramethyl rhodamine isocyanate-labeled streptavidin, approximately 15 streptavidin molecules were shown to be immobilized on a single BCCP-BacMP. Furthermore, gold nanoparticle-BacMP composites were constructed via the biotin-streptavidin interaction. The conjugation system developed in this work provides a simple, low-cost method for producing biotin- or streptavidin-labeled magnetic nanoparticles. Various functional materials can be site selectively immobilized on these specially designed BacMPs. By combining the site-selective biotinylation technology and the protein display technology, more innovative and attractive magnetic nanomaterials can be constructed.     

Effect of prolonged exposure to a magnetic field on the haematopoietic stem cell.

The authors studied the effect of prolonged exposure (3, 4 and 5 months) to the action of a magnetic field of 180-200 gauss formed by the poles of a rotating permanent magnet on the haematopoietic stem cells of mouse bone marrow donors. The effect of the field was evaluated from the ability of the donors' bone marrow cells to form haematopoietic colonies in the spleen of lethally irradiated mice. It was found that the number of stem cells was not reduced by the action of the above magnetic field and that proliferative capacity was likewide unimpaired.     

Potentials of radio-frequency field gradient NMR microscopy in environmental science

An understanding of transport, flow, diffusivity and mass transfer processes is of central importance in many fields of environmental biotechnology such as biofilm, bioreactor and membrane engineering, soil and groundwater bioremediation, and wastewater treatment. Owing to its remarkable sensitivity to molecular displacements and to its noninvasive and nondestructive character, pulsed field gradient (PFG) nuclear magnetic resonance (NMR) can be a valuable tool for investigating such processes. In conventional NMR microscopy, spatial encoding is achieved by using static magnetic field gradients (B ₀ gradients). However, an interesting alternative is to use radio-frequency magnetic field gradients (RF or B ₁ gradients). Although the latter are less versatile than the former, RF field gradient microscopy is particularly suitable for dealing with heterogeneous systems such as porous media because of its quasi-immunity to background static magnetic field gradients arising from magnetic susceptibility inhomogeneities, unlike the B ₀ gradients microscopy. Here, we present an overview of basic principles and the main features of this technique, which is still relatively unused. Different examples of diffusion imaging illustrate the potentialities of the method in both micro-imaging and the measurement of global or local diffusion coefficients within membranes and at liquid–solid interfaces. These examples suggest that a number of environmental problems could benefit from this technique. Different future prospects of application of B ₁ gradient NMR microscopy in environmental biotechnology are considered. Journal of Industrial Microbiology & Biotechnology (2001) 26, 53–61. Received 09 February 2000/ Accepted in revised form 07 August 2000     

Characterization of Bacterial Magnetotactic Behaviors by Using a Magnetospectrophotometry Assay

Magnetotactic bacteria have the unique capacity of synthesizing intracellular single-domain magnetic particles called magnetosomes. The magnetosomes are usually organized in a chain that allows the bacteria to align and swim along geomagnetic field lines, a behavior called magnetotaxis. Two mechanisms of magnetotaxis have been described. Axial magnetotactic cells swim in both directions along magnetic field lines. In contrast, polar magnetotactic cells swim either parallel to the geomagnetic field lines toward the North Pole (north seeking) or antiparallel toward the South Pole (south seeking). In this study, we used a magnetospectrophotometry (MSP) assay to characterize both the axial magnetotaxis of “Magnetospirillum magneticum” strain AMB-1 and the polar magnetotaxis of magneto-ovoid strain MO-1. Two pairs of Helmholtz coils were mounted onto the cuvette holder of a common laboratory spectrophotometer to generate two mutually perpendicular homogeneous magnetic fields parallel or perpendicular to the light beam. The application of magnetic fields allowed measurements of the change in light scattering resulting from cell alignment in a magnetic field or in absorbance due to bacteria swimming across the light beam. Our results showed that MSP is a powerful tool for the determination of bacterial magnetism and the analysis of alignment and swimming of magnetotactic bacteria in magnetic fields. Moreover, this assay allowed us to characterize south-seeking derivatives and non-magnetosome-bearing strains obtained from north-seeking MO-1 cultures. Our results suggest that oxygen is a determinant factor that controls magnetotactic behavior.Magnetotactic bacteria are morphologically, metabolically, and phylogenetically diverse prokaryotes (1, 11). They synthesize unique intracellular organelles, the magnetosomes, which are single-domain magnetic crystals of the mineral magnetite or greigite enveloped by membranes. Magnetosomes are usually organized in a chain(s) within the cell and cause the cell to align along geomagnetic field lines while it swims. The highest numbers of magnetotactic bacteria are generally found at, or just below, the oxic-anoxic transition zone (OATZ) or redoxocline in aquatic habitats (1). Early studies showed that Northern Hemisphere magnetotactic bacteria swim preferentially northward in parallel with the geomagnetic field lines (north seeking [NS]) (2) and that those from the Southern Hemisphere swim preferentially antiparallel to the geomagnetic field lines to the magnetic South Pole (south seeking [SS]) (4). The geomagnetic field is inclined downward from horizontal in the Northern Hemisphere and upward in the Southern Hemisphere, with the inclination magnitude increasing from the equator to the poles. Therefore, magnetotaxis might guide cells in each hemisphere downward to less-oxygenated regions of aquatic habitats, where they would presumably stop swimming until conditions change (1). A recent study reported the coexistence of both NS and SS magnetotactic bacteria in the Northern Hemisphere, which conflicts with the prevalent model of the adaptive value of magnetotaxis (14).Under laboratory conditions, magnetotactic bacteria form microaerophilic bands of cells in oxygen-gradient medium. In fact, magnetotaxis and aerotaxis work together in these bacteria, and the behavior observed has been referred to as “magnetoaerotaxis.” Two different magnetoaerotactic mechanisms, termed polar and axial, are found in different bacterial species (6). The magnetotactic bacteria, principally the magnetotactic cocci, that swim persistently in one direction along the magnetic field (NS or SS) are polar magnetoaerotactic. Magnetotactic bacteria, especially the freshwater spirilla, that swim in either direction along the magnetic field lines with frequent, spontaneous reversals of swimming direction without turning around are axial magnetoaerotactic. For polar magnetotactic bacteria, the magnetic field provides an axis and a direction for motility, whereas for axial magnetotactic bacteria, the magnetic field provides only an axis of motility. The two mechanisms can best be seen in flattened capillary tubes containing suspensions of cells in reduced medium in a magnetic field oriented parallel to the capillary. An oxygen gradient forms along the tube, beginning at the ends of the capillary, with one oriented parallel and the other antiparallel to the magnetic field (1). Band formation by axial magnetoaerotactic cells, such as Magnetospirillum magnetotacticum cells, occurs at both ends of the capillary. Rotation of the magnetic field by 180° after the formation of the bands causes the cells in both bands to rotate 180°, but the bands remain intact. In contrast, band formation by polar magnetoaerotactic cells, such as the marine cocci, occurs only at the end of the capillary for which the magnetic field and the oxygen concentration gradient are oriented opposite to each other. Rotation of the magnetic field by 180° after the formation of the band causes the cells in the band to rotate 180° and swim away, resulting in the dispersal of the band (1). In this study, we developed a magnetospectrophotometry (MSP) assay that provides an alternative method for the quantitative and versatile characterization of the two magnetotactic mechanisms. Using this assay, we demonstrated the effect of artificial magnetic fields on the generation of homogeneous NS or SS magnetotactic bacterial populations.     

固体平板磁泳分离细菌新方法的研究

氧化亚铁硫杆菌（Acidithiobacillus ferrooxidans）能够在胞内形成电子致密的磁性颗粒,它的这种特性使利用氧化亚铁硫杆菌合成生物纳米磁性材料成为了可能。本课题组为了筛选出合成磁性颗粒能力强的菌株,对原有的液体磁泳进行了改进,采用了新的固体平板磁泳方法来筛选纯化目的菌株。经过磁泳分离后,细菌中含磁性颗粒的细胞比例由原始菌群的30％上升到90％,胞内含有的磁颗粒数目也由1~2颗增加至2~5颗,筛选得到的细菌在人工磁场下会进行趋磁运动。实验结果表明,氧化亚铁硫杆菌具有较弱的趋磁性,在人工磁场下会进行趋磁运动,但仅在地磁场作用下不能定向运动,利用固体平板磁泳筛选纯化含有磁性颗粒的氧化亚铁硫杆菌的方法是切实可行的,磁泳分离技术的进一步完善和改进为传统的微生物菌种分离提供了新的途径,为研究纯氧化亚铁硫杆菌菌株胞内磁性颗粒的形成条件及机理提供了前提条件,也为今后从浸矿细菌中分离筛选更多的含有磁性颗粒的菌株打下基础。     

Quantitative Modeling and Optimization of Magnetic Tweezers

Magnetic tweezers are a powerful tool to manipulate single DNA or RNA molecules and to study nucleic acid-protein interactions in real time. Here, we have modeled the magnetic fields of permanent magnets in magnetic tweezers and computed the forces exerted on superparamagnetic beads from first principles. For simple, symmetric geometries the magnetic fields can be calculated semianalytically using the Biot-Savart law. For complicated geometries and in the presence of an iron yoke, we employ a finite-element three-dimensional PDE solver to numerically solve the magnetostatic problem. The theoretical predictions are in quantitative agreement with direct Hall-probe measurements of the magnetic field and with measurements of the force exerted on DNA-tethered beads. Using these predictive theories, we systematically explore the effects of magnet alignment, magnet spacing, magnet size, and of adding an iron yoke to the magnets on the forces that can be exerted on tethered particles. We find that the optimal configuration for maximal stretching forces is a vertically aligned pair of magnets, with a minimal gap between the magnets and minimal flow cell thickness. Following these principles, we present a configuration that allows one to apply ≥40 pN stretching forces on ≈1-μm tethered beads.     

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.     

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．