Showing papers on "Magnetotactic bacteria published in 2011"

Chains of magnetosomes extracted from AMB-1 magnetotactic bacteria for application in alternative magnetic field cancer therapy.

Abstract: Chains of magnetosomes extracted from AMB-1 magnetotactic bacteria are shown to be highly efficient for cancer therapy when they are exposed to an alternative magnetic field. When a suspension containing MDA-MB-231 breast cancer cells was incubated in the presence of various amounts of extracted chains of magnetosomes, the viability of these cells remained high in the absence of an alternative magnetic field. By contrast, when this suspension was exposed to an alternative magnetic field of frequency 183 kHz and field strengths of 20, 40, or 60 mT, up to 100% of these cells were destroyed. The antitumoral activity of the extracted chains of magnetosomes is demonstrated further by showing that they can be used to fully eradicate a tumor xenografted under the skin of a mouse. For that, a suspension containing ∼1 mg of extracted chains of magnetosomes was administered within the tumor and the mouse was exposed to three heat cycles of 20 min, during which the tumor temperature was raised to ∼43 °C. We also dem...

Functional Analysis of the Magnetosome Island in Magnetospirillum gryphiswaldense: The mamAB Operon Is Sufficient for Magnetite Biomineralization

Abstract: Bacterial magnetosomes are membrane-enveloped, nanometer-sized crystals of magnetite, which serve for magnetotactic navigation. All genes implicated in the synthesis of these organelles are located in a conserved genomic magnetosome island (MAI). We performed a comprehensive bioinformatic, proteomic and genetic analysis of the MAI in Magnetospirillum gryphiswaldense. By the construction of large deletion mutants we demonstrate that the entire region is dispensable for growth, and the majority of MAI genes have no detectable function in magnetosome formation and could be eliminated without any effect. Only <25% of the region comprising four major operons could be associated with magnetite biomineralization, which correlated with high expression of these genes and their conservation among magnetotactic bacteria. Whereas only deletion of the mamAB operon resulted in the complete loss of magnetic particles, deletion of the conserved mms6, mamGFDC, and mamXY operons led to severe defects in morphology, size and organization of magnetite crystals. However, strains in which these operons were eliminated together retained the ability to synthesize small irregular crystallites, and weakly aligned in magnetic fields. This demonstrates that whereas the mamGFDC, mms6 and mamXY operons have crucial and partially overlapping functions for the formation of functional magnetosomes, the mamAB operon is the only region of the MAI, which is necessary and sufficient for magnetite biomineralization. Our data further reduce the known minimal gene set required for magnetosome formation and will be useful for future genome engineering approaches.

A Cultured Greigite-Producing Magnetotactic Bacterium in a Novel Group of Sulfate-Reducing Bacteria

Abstract: Magnetotactic bacteria contain magnetosomes—intracellular, membrane-bounded, magnetic nanocrystals of magnetite (Fe3O4) or greigite (Fe3S4)—that cause the bacteria to swim along geomagnetic field lines. We isolated a greigite-producing magnetotactic bacterium from a brackish spring in Death Valley National Park, California, USA, strain BW-1, that is able to biomineralize greigite and magnetite depending on culture conditions. A phylogenetic comparison of BW-1 and similar uncultured greigite- and/or magnetite-producing magnetotactic bacteria from freshwater to hypersaline habitats shows that these organisms represent a previously unknown group of sulfate-reducing bacteria in the Deltaproteobacteria. Genomic analysis of BW-1 reveals the presence of two different magnetosome gene clusters, suggesting that one may be responsible for greigite biomineralization and the other for magnetite.

Conservation of proteobacterial magnetosome genes and structures in an uncultivated member of the deep-branching Nitrospira phylum

Abstract: Magnetotactic bacteria (MTB) are a phylogenetically diverse group which uses intracellular membrane-enclosed magnetite crystals called magnetosomes for navigation in their aquatic habitats. Although synthesis of these prokaryotic organelles is of broad interdisciplinary interest, its genetic analysis has been restricted to a few closely related members of the Proteobacteria, in which essential functions required for magnetosome formation are encoded within a large genomic magnetosome island. However, because of the lack of cultivated representatives from other phyla, it is unknown whether the evolutionary origin of magnetotaxis is monophyletic, and it has been questioned whether homologous mechanisms and structures are present in unrelated MTB. Here, we present the analysis of the uncultivated “Candidatus Magnetobacterium bavaricum” from the deep branching Nitrospira phylum by combining micromanipulation and whole genome amplification (WGA) with metagenomics. Target-specific sequences obtained by WGA of cells, which were magnetically collected and individually sorted from sediment samples, were used for PCR screening of metagenomic libraries. This led to the identification of a genomic cluster containing several putative magnetosome genes with homology to those in Proteobacteria. A variety of advanced electron microscopic imaging tools revealed a complex cell envelope and an intricate magnetosome architecture. The presence of magnetosome membranes as well as cytoskeletal magnetosome filaments suggests a similar mechanism of magnetosome formation in “Cand. M. bavaricum” as in Proteobacteria. Altogether, our findings suggest a monophyletic origin of magnetotaxis, and relevant genes were likely transferred horizontally between Proteobacteria and representatives of the Nitrospira phylum.

Microbial synthesis of iron-based nanomaterials—A review

Abstract: Nanoparticles are the materials having dimensions of the order of 100 nm or less. They exhibit a high surface/volume ratio leading to different properties far different from those of the bulk materials. The development of uniform nanoparticles has been intensively pursued because of their technological and fundamental scientific importance. A number of chemical methods are available and are extensively used, but these are often energy intensive and employ toxic chemicals. An alternative approach for the synthesis of uniform nanoparticles is the biological route that occurs at ambient temperature, pressure and at neutral pH. The main aim of this review is to enlist and compare various methods of synthesis of iron-based nanoparticles with emphasis on the biological method. Biologically induced and controlled mineralization mechanisms are the two modes through which the micro-organisms synthesize iron oxide nanoparticles. In biologically induced mineralization (BIM) mode, the environmental factors like pH, pO2, pCO2, redox potential, temperature etc govern the synthesis of iron oxide nanoparticles. In contrast, biologically controlled mineralization (BCM) process initiates the micro-organism itself to control the synthesis. BIM can be observed in the Fe(III) reducing bacterial species of Shewanella, Geobacter, Thermoanaerobacter, and sulphate reducing bacterial species of Archaeoglobus fulgidus, Desulfuromonas acetoxidans, whereas BCM mode can be observed in the magnetotactic bacteria (MTB) like Magnetospirillum magnetotacticum, M. gryphiswaldense and sulphate-reducing magnetic bacteria (Desulfovibrio magneticus). Magnetite crystals formed by Fe(III)-reducing bacteria are epicellular, poorly crystalline, irregular in shapes, having a size range of 10–50 nm super-paramagnetic particles, with a saturation magnetization value ranging from 75–77 emu/g and are not aligned in chains. Magnetite crystals produced by MTB have uniform species-specific morphologies and sizes, which are mostly unknown from inorganic systems. The unusual characteristics of magnetosome particles have attracted a great interdisciplinary interest and inspired numerous ideas for their biotechnological applications. The nanoparticles synthesized through biological method are uniform with size ranging from 5 to 100 nm, which can potentially be used for various applications.

Common ancestry of iron oxide- and iron-sulfide-based biomineralization in magnetotactic bacteria

Abstract: Magnetosomes are prokaryotic organelles produced by magnetotactic bacteria that consist of nanometer-sized magnetite (Fe3O4) or/and greigite (Fe3S4) magnetic crystals enveloped by a lipid bilayer membrane. In magnetite-producing magnetotactic bacteria, proteins present in the magnetosome membrane modulate biomineralization of the magnetite crystal. In these microorganisms, genes that encode for magnetosome membrane proteins as well as genes involved in the construction of the magnetite magnetosome chain, the mam and mms genes, are organized within a genomic island. However, partially because there are presently no greigite-producing magnetotactic bacteria in pure culture, little is known regarding the greigite biomineralization process in these organisms including whether similar genes are involved in the process. Here using culture-independent techniques, we now show that mam genes involved in the production of magnetite magnetosomes are also present in greigite-producing magnetotactic bacteria. This finding suggest that the biomineralization of magnetite and greigite did not have evolve independently (that is, magnetotaxis is polyphyletic) as once suggested. Instead, results presented here are consistent with a model in which the ability to biomineralize magnetosomes and the possession of the mam genes was acquired by bacteria from a common ancestor, that is, the magnetotactic trait is monophyletic.

The HtrA/DegP family protease MamE is a bifunctional protein with roles in magnetosome protein localization and magnetite biomineralization.

Abstract: Summary Magnetotactic bacteria contain nanometre-sized, membrane-bound organelles, called magnetosomes, which are tasked with the biomineralization of small crystals of the iron oxide magnetite allowing the organism to use geomagnetic field lines for navigation. A key player in this process is the HtrA/DegP family protease MamE. In its absence, Magnetospirillum magneticum str AMB-1 is able to form magnetosome membranes but not magnetite crystals, a defect previously linked to the mislocalization of magnetosome proteins. In this work we use a directed genetic approach to find that MamE, and another predicted magnetosome-associated protease, MamO, likely function as proteases in vivo. However, as opposed to the complete loss of mamE where no biomineralization is observed, the protease-deficient variant of this protein still supports the initiation and formation of small, 20 nm-sized crystals of magnetite, too small to hold a permanent magnetic dipole moment. This analysis also reveals that MamE is a bifunctional protein with a protease-independent role in magnetosome protein localization and a protease-dependent role in maturation of small magnetite crystals. Together, these results imply the existence of a previously unrecognized ‘checkpoint’ in biomineralization where MamE moderates the completion of magnetite formation and thus committal to magneto-aerotaxis as the organism's dominant mode of navigating the environment.

Magnetosome chains are recruited to cellular division sites and split by asymmetric septation.

Abstract: Magnetotactic bacteria navigate along magnetic field lines usingwell-ordered chains of membrane-enclosed magnetic crystals, referred toas magnetosomes, which have emerged as model to investigate organellebiogenesis in prokaryotic systems. To become divided and segregatedfaithfully during cytokinesis, the magnetosome chain has to be properlypositioned, cleaved and separated against intrachain magnetostaticforces. Here we demonstrate that magnetotactic bacteria use dedicatedmechanisms to control the position and division of the magnetosomechain, thus maintaining magnetic orientation throughout divisionalcycle. Using electron and time-lapse microscopy of synchronized cells ofMagnetospirillum gryphiswaldense, we confirm that magnetosome chainsundergo a dynamic pole-to-midcell translocation during cytokinesis.Nascent chains were recruited to division sites also indivision-inhibited cells, but not in a mamK mutant, indicating an activemechanism depending upon the actin-like cytoskeletal magnetosomefilament. Cryo-electron tomography revealed that both the magnetosomechain and the magnetosome filament are spilt into halves by asymmetricseptation and unidirectional indentation, which we interpret in terms ofa specific adaptation required to overcome the magnetostaticinteractions between separating daughter chains. Our study demonstratesthat magnetosome division and segregation is co-ordinated withcytokinesis and resembles partitioning mechanisms of other organellesand macromolecular complexes in bacteria.

Isolation of obligately alkaliphilic magnetotactic bacteria from extremely alkaline environments.

Abstract: SummaryLarge numbers of magnetotactic bacteria were dis-covered in mud and water samples collected from anumber of highly alkaline aquatic environments withpH values of ~ 9.5. These bacteria were helical in mor-phology and biomineralized chains of bullet-shapedcrystals of magnetite and were present in all thehighly alkaline sites sampled. Three strains from dif-ferent sites were isolated and cultured and grew opti-mally at pH 9.0–9.5 but not at 8.0 and below,demonstrating that these organisms truly requirehighly alkaline conditions and are not simplysurviving/growing in neutral pH micro-niches intheir natural habitats. All strains grew anaerobicallythrough the reduction of sulfate as a terminal electronacceptor and phylogenetic analysis, based on 16SrRNA gene sequences, as well as some physiologicalfeatures, showed that they could represent strains of Desulfonatronum thiodismutans , a known alkaliphilicbacterium that does not biomineralize magneto-somes. Our results show that some magnetotacticbacteria can be considered extremophilic and greatlyextend the known ecology of magnetotactic bacteriaand the conditions under which they can biomineral-ize magnetite. Moreover, our results show that thistype of magnetotactic bacterium is common in highlyalkaline environments. Our findings also greatlyinfluence the interpretation of the presence ofnanometer-sized magnetite crystals, so-called mag-netofossils, in highly alkaline environments.Introduction

Magnetic nanochains: a review

Abstract: One-dimensional (1D) chain-like structures are of special significance because of their interparticle magnetic interactions and potential applications in various fields, such as micromechanical sensors. This paper attempts to review the field of research into magnetic chains including monatomic chains and nanoparticle chains. The synthesis methods used mostly belong to one of the following categories: magnetosome chains in magnetotactic bacteria, zero-field self-assembly, magnetic field induced (MFI) assembly, template-directed synthesis, and gas phase synthesis. The potential applications of nanoparticle chains, mainly in the field of magnetic recording media, sensor, biomedicine and magnetic-field tunable photonic crystal are discussed.

Structural purity of magnetite nanoparticles in magnetotactic bacteria

Abstract: Magnetosome biomineralization and chain formation in magnetotactic bacteria are two processes that are highly controlled at the cellular level in order to form cellular magnetic dipoles. However, even if the magnetosome chains are well characterized, controversial results about the microstructure of magnetosomes were obtained and its possible influence in the formation of the magnetic dipole is to be specified. For the first time, the microstructure of intracellular magnetosomes was investigated using high-resolution synchrotron X-ray diffraction. Significant differences in the lattice parameter were found between intracellular magnetosomes from cultured magnetotactic bacteria and isolated ones. Through comparison with abiotic control materials of similar size, we show that this difference can be associated with different oxidation states and that the biogenic nanomagnetite is stoichiometric, i.e. structurally pure whereas isolated magnetosomes are slightly oxidized. The hierarchical structuring of the magnetosome chain thus starts with the formation of structurally pure magnetite nanoparticles that in turn might influence the magnetic property of the magnetosome chains.

Single-step production of a recyclable nanobiocatalyst for organophosphate pesticides biodegradation using functionalized bacterial magnetosomes.

Abstract: Enzymes are versatile catalysts in laboratories and on an industrial scale; improving their immobilization would be beneficial to broadening their applicability and ensuring their (re)use. Lipid-coated nano-magnets produced by magnetotactic bacteria are suitable for a universally applicable single-step method of enzyme immobilization. By genetically functionalizing the membrane surrounding these magnetite particles with a phosphohydrolase, we engineered an easy-to-purify, robust and recyclable biocatalyst to degrade ethyl-paraoxon, a commonly used pesticide. For this, we genetically fused the opd gene from Flavobacterium sp. ATCC 27551 encoding a paraoxonase to mamC, an abundant protein of the magnetosome membrane in Magnetospirillum magneticum AMB-1. The MamC protein acts as an anchor for the paraoxonase to the magnetosome surface, thus producing magnetic nanoparticles displaying phosphohydrolase activity. Magnetosomes functionalized with Opd were easily recovered from genetically modified AMB-1 cells: after cellular disruption with a French press, the magnetic nanoparticles are purified using a commercially available magnetic separation system. The catalytic properties of the immobilized Opd were measured on ethyl-paraoxon hydrolysis: they are comparable with the purified enzyme, with Km (and kcat) values of 58 µM (and 178 s−1) and 43 µM (and 314 s−1) for the immobilized and purified enzyme respectively. The Opd, a metalloenzyme requiring a zinc cofactor, is thus properly matured in AMB-1. The recycling of the functionalized magnetosomes was investigated and their catalytic activity proved to be stable over repeated use for pesticide degradation. In this study, we demonstrate the easy production of functionalized magnetic nanoparticles with suitably genetically modified magnetotactic bacteria that are efficient as a reusable nanobiocatalyst for pesticides bioremediation in contaminated effluents.

Bioinspired Magnetite Mineralization of Peptide−Amphiphile Nanofibers

Abstract: Peptide−amphiphile nanofibers displaying iron-binding sequences were used as templates to control the growth and organization of magnetite (Fe3O4) nanocrystals into linear arrays, mimicking aspects of the linear arrangement of magnetite crystals along a filamentous structure in magnetotactic bacteria.

Multicellular photo-magnetotactic bacteria.

Abstract: Multicellular magnetotactic bacteria (MMB) are unique microorganisms typically comprised of 10-40 bacterial cells arranged around a central acellular compartment. Their life cycle has no known unicellular stage and division occurs by separation of a single MMB aggregate into two identical offspring. In this study, South-seeking multicellular magnetotactic bacteria (ssMMB) were enriched from a New England salt marsh. When exposed to light, ssMMB reversed their magnetotactic behaviour to become North-seeking. The exposure time needed to generate the reversal response varied with light wavelength and intensity. Extensive exposure to light appeared to be lethal. This is the first report of a Northern hemisphere MMB displaying South-seeking behaviour and the first time a MMB is found to exhibit photo-magnetotaxis. We suggest that this mechanism enables ssMMB to optimize their location with regard to chemical gradients and light intensities, and propose a model to explain the peculiar balance between photo- and magnetotaxis.

A Bacterial Backbone: Magnetosomes in Magnetotactic Bacteria

Abstract: Magnetosomes are intracellular, tens of nanometer-sized, membrane-bounded crystals of the magnetic minerals magnetite (Fe3O4) and greigite (Fe3S4) synthesized by a diverse group of prokaryotes termed the magnetotactic bacteria. These unusual microorganisms biomineralize magnetosomes via a biologically controlled biomineralization process where the composition, size and morphology of the mineral crystals are under fine chemical, biochemical and genetic controls. Magnetosomes are most often arranged as a chain within the cell and they provide a permanent magnetic dipole moment to the cell causing it to passively align along magnetic field lines like a compass needle. Magnetotaxis is the result of this passive alignment while the cell swims. The magnetotactic bacteria presumably utilize magnetotactic in conjunction with chemotaxis (e.g., aerotaxis) to locate and maintain an optimal position in vertical chemical and/or redox gradients in natural habitats. The locus of biomineralization of magnetosome crystals is the magnetosome membrane vesicle which contains proteins that are unique to it that are not found in other parts of the cell. The roles of some of these magnetosome membrane proteins in the biomineralization process and the construction of the magnetosome chain have been determined while the roles of most have not. The genes that encode for magnetosome membrane proteins are located in clusters in a magnetosome gene island in many magnetotactic bacteria that also contain a number of mobile elements suggesting the island can be transferred to different bacteria via horizontal gene transfer. Magnetosome crystals possess novel magnetic properties that have been exploited in numerous applications and are important in biotechnology.

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

Abstract: 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.

Magnetic anisotropy and Verwey transition of magnetosome chains in Magnetospirillum gryphiswaldense

Abstract: SUMMARY Magnetotactic bacteria (MTB) are characterized by cellular magnetic dipoles formed by the 1-D assembly of magnetite and/or greigite particles aligned along their magnetic easy axes. This alignment creates strong interaction-induced shape anisotropy. Ferromagnetic resonance (FMR) spectroscopy is applied to study the changes in anisotropy of the MTB Magnetospirillum gryphiswaldense between room temperature and 10 K. The Verwey transition is found at about 100 K. The characteristic FMR signal of the cellular dipole at room temperature vanishes upon cooling to the isotropic point at Ti≈ 130 K, where the magnetocrystalline anisotropy constant K1 becomes zero. Monitoring of the FMR response of intact MTB as a function of temperature is taken to discuss theoretically the reduction of the interaction-induced shape anisotropy in magnetofossils because of diagenetic processes. It is concluded that there is a similarity in the FMR response between magnetofossils at room temperature and intact MTB near Ti. This is because the critical effect of the magnetocrystalline anisotropy constant K1 and of the alignment of magnetic easy axes on the cellular dipole. Low-temperature FMR results of intact MTB can thus be used as a guideline for detecting magnetofossils in geological environments.

Magnetite biomineralization in bacteria.

Abstract: Magnetotactic bacteria are able to biomineralize magnetic crystals in intracellular organelles, so-called “magnetosomes.” These particles exhibit species- and strain-specific size and morphology. They are of great interest for biomimetic nanotechnological and biotechnological research due to their fine-tuned magnetic properties and because they challenge our understanding of the classical principles of crystallization. Magnetotactic bacteria use these highly optimized particles, which form chains within the bacterial cells, as a magnetic field actuator, enabling them to navigate. In this chapter, we discuss the current biological and chemical knowledge of magnetite biomineralization in these bacteria. We highlight the extraordinary properties of magnetosomes and some resulting potential applications.

Iron Uptake Kinetics and Magnetosome Formation by Magnetospirillum gryphiswaldense as a Function of pH, Temperature and Dissolved Iron Availability

Abstract: The dynamics of iron uptake and magnetosome formation by the magnetotactic bacteria (MTB) Magnetospirillum gryphiswaldense was investigated at a broad range of pH, temperature and iron availability to evaluate the role of MTB in the iron biogeochemical cycle. Except at pH 5.0, all incubations have shown significant bacterial growth. However, magnetosome formation was limited at pH 8.0 and 9.0 as well as at 4°C, 10°C and 35°C. At optimal conditions (i.e., pH 7 and 28°C), the uptake rates of dissolved Fe(III) as a function of initial Fe concentration can be described by a Michaelis-Menten-type kinetic model with a maximum iron uptake rate, Vmax ,, of 11 × 10−12 μmoles cell−1 h−1 and an affinity constant, Ks of 26 μM Fe. High resolution imaging of magnetosomes synthesized at the different pH values, revealed a large range of morphologies and sizes, which illustrate the impact of environmental conditions on the formation of magnetite crystals by MTB.

Intracellular magneto-spatial organization of magnetic organelles inside intact bacterial cells.

Abstract: Magnetotactic bacteria naturally produce magnetosomes, i.e., biological membrane bound nanomagnets, at ambient conditions. It is important to understand simultaneously the possible size variations and the magnetic behavior of nano-magnets inside intact bacterial cells for both applicational purposes as well as to enhance the basic understanding of biomineralization leading to intracellular nano-magnet synthesis. In this work, we utilize High-resolution Transmission Electron Microscopy and Near-field Scanning Optical Microscopy based measurements on intact non-fixed single cells to rigorously and quantitatively understand the intra-cellular magneto-spatial distribution of nano-magnets synthesized by Magnetospirillum gryphiswaldense. We demonstrate that it is possible to measure the relative magnetic moments along the intracellular magnetosomal chains for intact and non-fixed bacterial cells. Using our in vivo measurements on several single cells, we report that magnetic behavior of intracellular nano-magnets synthesized by magnetotactic bacteria depend on their relative location in the magnetosomal chains. Our work opens promising avenues in the direction of measuring the magnetic behavior of nano-magnets inside living systems by utilizing an operationally straight-forward approach.

How iron is transported into magnetosomes.

Abstract: Magnetotactic bacteria are microaerophilic organisms found in sediments or stratified water columns at the oxic-anoxic transition zone or the anoxic regions below. They use magnetite-filled membrane vesicles, magnetosomes, to passively align with, and actively swim along, the geomagnetic field lines in a magneto-aerotactic search for the ideal concentration of molecular oxygen. Such an efficient chemotaxis needs magnetosomes that contain nearly perfect magnetite crystals. These magnetosomes originate as invaginations of the inner membrane and the empty vesicles are aligned in a chain by an actin-like protein. Subsequently, the vesicles are filled with iron, which then is converted to magnetite crystals. Until now it was unclear how such a process might be accomplished. In this issue, Uebe et al., 2011 unveil a part of this complicated bio-mineralization process. In Magnetospirillum gryphiswaldense, MamM and MamB, two members of the cation diffusion facilitator (CDF) transport protein family, are required for magnetite formation. MamM increases the stability of MamB by forming a heterodimer. The MamBM heterodimer strongly influences the biomineralization process by controlling the size and the shape of the crystals, and even the nature of the formed iron mineral. Thus, these two CDF proteins not only transport iron, but they also control the magnetite biomineralization.

Magnetospirillum magneticum AMB-1 peroxiredoxins contribute to the aerotolerance and genetic stability of the genomic magnetosome island

Abstract: The magnetotactic bacterium Magnetospirillum magneticum AMB-1 can grow at variable oxygen concentrations, although the intracellular magnetic structures, magnetosomes, are only synthesized under microaerobic or anaerobic conditions. Three members of the peroxiredoxin family were identified in M. magneticum AMB-1. All purified recombinant proteins displayed thiol-dependent peroxidase activities. Allelic replacement mutagenesis revealed that, although the absence of the three peroxidase genes had no effect on either the growth or the formation of magnetosome under anaerobic conditions, the growth of mutants was compromised in an aerobic culture. Moreover, an accelerated loss in the genomic ‘magnetosome island’ (MAI) was observed in the null mutants cultured in the presence of oxygen. Taken together, these data suggest that the thiol-peroxidases identified act as key antioxidants in magnetotactic bacteria and, as a result, contribute to maintaining their capacity to synthesize magnetosome by shielding the genetic stability of the genomic MAI in adaptation to constant physiological change and stress.

Snapping magnetosome chains by asymmetric cell division in magnetotactic bacteria.

Abstract: The mechanism by which prokaryotic cells organize and segregate their intracellular organelles during cell division has recently been the subject of substantial interest. Unlike other microorganisms, magnetotactic bacteria (MTB) form internal magnets (known as magnetosome chain) for magnetic orientation, and thus face an additional challenge of dividing and equipartitioning this magnetic receptor to their daughter cells. Although MTB have been investigated more than four decades, it is only recently that the basic mechanism of how MTB divide and segregate their magnetic organelles has been addressed. In this issue of Molecular Microbiology, the cell cycle of the model magnetotactic bacterium, Magnetospirillum gryphiswaldense is characterized by Katzmann and co-workers. The authors have found that M. gryphiswaldense undergoes an asymmetric cell division along two planes. A novel wedge-like type of cellular constriction is observed before separation of daughter cells and magnetosome chains, which is assumed to help cell cope with the magnetic force within the magnetosome chain. The data shows that the magnetosome chain becomes actively recruited to the cellular division site, in agreement with the previous suggestions described by Staniland et al. (2010), and the actin-like protein MamK is likely involved in this fast polar-to-midcell translocalization. With the use of cryo-electron tomography, an arc-shaped Z ring is observed near the division site, which is assumed to trigger the asymmetric septation of cell and magnetosome chain.

Characterization of natural microcosms of estuarine magnetotactic bacteria

Abstract: To date, no complete study of magnetotactic bacteria’s (MTB) natural microcosms in estuarine or tropical environments has been reported. Besides, almost all the studies around magnetotactic bacteria have been based on fresh waters away from the Equator. In this work, we focused the experimental region at the Equator and present a comprehensive mineralogical and physicochemical characterization of two estuarine bacterial microcosms. The results show that mineral lixiviation in the sediments may be an important factor in the solubilization of elements required by magnetotactic bacteria. Specifically, we show that clinochlore, phlogopite, nontronite, and halloysite could be among the main minerals that lixiviate iron to the estuarine microcosms. We conclude that nitrate concentration in the water should not be as low as those that have been reported for other authors to achieve optimal bacteria growth. It is confirmed that magnetotactic bacteria do not need large amounts of dissolved iron to grow or to synthesize magnetosomes.