Showing papers on "Magnetotactic bacteria published in 2018"

Applications of Magnetotactic Bacteria, Magnetosomes and Magnetosome Crystals in Biotechnology and Nanotechnology: Mini-Review.

Abstract: Magnetotactic bacteria (MTB) biomineralize magnetosomes, which are defined as intracellular nanocrystals of the magnetic minerals magnetite (Fe3O4) or greigite (Fe3S4) enveloped by a phospholipid bilayer membrane The synthesis of magnetosomes is controlled by a specific set of genes that encode proteins, some of which are exclusively found in the magnetosome membrane in the cell Over the past several decades, interest in nanoscale technology (nanotechnology) and biotechnology has increased significantly due to the development and establishment of new commercial, medical and scientific processes and applications that utilize nanomaterials, some of which are biologically derived One excellent example of a biological nanomaterial that is showing great promise for use in a large number of commercial and medical applications are bacterial magnetite magnetosomes Unlike chemically-synthesized magnetite nanoparticles, magnetosome magnetite crystals are stable single-magnetic domains and are thus permanently magnetic at ambient temperature, are of high chemical purity, and display a narrow size range and consistent crystal morphology These physical/chemical features are important in their use in biotechnological and other applications Applications utilizing magnetite-producing MTB, magnetite magnetosomes and/or magnetosome magnetite crystals include and/or involve bioremediation, cell separation, DNA/antigen recovery or detection, drug delivery, enzyme immobilization, magnetic hyperthermia and contrast enhancement of magnetic resonance imaging Metric analysis using Scopus and Web of Science databases from 2003 to 2018 showed that applied research involving magnetite from MTB in some form has been focused mainly in biomedical applications, particularly in magnetic hyperthermia and drug delivery

Genomic expansion of magnetotactic bacteria reveals an early common origin of magnetotaxis with lineage-specific evolution.

Abstract: The origin and evolution of magnetoreception, which in diverse prokaryotes and protozoa is known as magnetotaxis and enables these microorganisms to detect Earth's magnetic field for orientation and navigation, is not well understood in evolutionary biology. The only known prokaryotes capable of sensing the geomagnetic field are magnetotactic bacteria (MTB), motile microorganisms that biomineralize intracellular, membrane-bounded magnetic single-domain crystals of either magnetite (Fe3O4) or greigite (Fe3S4) called magnetosomes. Magnetosomes are responsible for magnetotaxis in MTB. Here we report the first large-scale metagenomic survey of MTB from both northern and southern hemispheres combined with 28 genomes from uncultivated MTB. These genomes expand greatly the coverage of MTB in the Proteobacteria, Nitrospirae, and Omnitrophica phyla, and provide the first genomic evidence of MTB belonging to the Zetaproteobacteria and "Candidatus Lambdaproteobacteria" classes. The gene content and organization of magnetosome gene clusters, which are physically grouped genes that encode proteins for magnetosome biosynthesis and organization, are more conserved within phylogenetically similar groups than between different taxonomic lineages. Moreover, the phylogenies of core magnetosome proteins form monophyletic clades. Together, these results suggest a common ancient origin of iron-based (Fe3O4 and Fe3S4) magnetotaxis in the domain Bacteria that underwent lineage-specific evolution, shedding new light on the origin and evolution of biomineralization and magnetotaxis, and expanding significantly the phylogenomic representation of MTB.

Acidithiobacillus ferrooxidans and its potential application

Abstract: The widely distributed Acidithiobacillus ferrooxidans (A. ferrooxidans) lives in extremely acidic conditions by fixing CO2 and nitrogen, and by obtaining energy from Fe2+ oxidation with either downhill or uphill electron transfer pathway and from reduced sulfur oxidation. A. ferrooxidans exists as different genomovars and its genome size is 2.89-4.18 Mb. The chemotactic movement of A. ferrooxidans is regulated by quorum sensing. A. ferrooxidans shows weak magnetotaxis due to formation of 15-70 nm magnetite magnetosomes with surface functional groups. The room- and low-temperature magnetic features of A. ferrooxidans are different from other magnetotactic bacteria. A. ferrooxidans has potential for removing sulfur from solids and gases, metals recycling from metal-bearing ores, electric wastes and sludge, biochemical production synthesizing, and metal workpiece machining.

Magnetosomes Extracted from Magnetospirillum gryphiswaldense as Theranostic Agents in an Experimental Model of Glioblastoma

Abstract: Magnetic fluid hyperthermia (MFH) with chemically synthesized nanoparticles is currently used in clinical trials as it destroys tumor cells with an extremely localized deposition of thermal energy In this paper, we investigated an MFH protocol based on magnetic nanoparticles naturally produced by magnetotactic bacteria: magnetosomes The efficacy of such protocol is tested in a xenograft model of glioblastoma Mice receive a single intratumoral injection of magnetosomes, and they are exposed three times in a week to an alternating magnetic field with concurrent temperature measurements MRI is used to visualize the nanoparticles and to monitor tumor size before and after the treatment Statistically significant inhibition of the tumor growth is detected in subjects exposed to the alternating magnetic field compared to control groups Moreover, thanks to magnetosomes high transversal relaxivity, their effective delivery to the tumor tissue is monitored by MRI It is apparent that the efficacy of this protocol is limited by inhomogeneous delivery of magnetosomes to tumor tissue These results suggest that naturally synthesized magnetosomes could be effectively considered as theranostic agent candidates for hyperthermia based on iron oxide nanoparticles

Genomic study of a novel magnetotactic Alphaproteobacteria uncovers the multiple ancestry of magnetotaxis

Abstract: Ecological and evolutionary processes involved in magnetotactic bacteria (MTB) adaptation to their environment have been a matter of debate for many years. Ongoing efforts for their characterization are progressively contributing to understand these processes, including the genetic and molecular mechanisms responsible for biomineralization. Despite numerous culture-independent MTB characterizations, essentially within the Proteobacteria phylum, only few species have been isolated in culture because of their complex growth conditions. Here, we report a newly cultivated magnetotactic, microaerophilic and chemoorganoheterotrophic bacterium isolated from the Mediterranean Sea in Marseille, France: Candidatus Terasakiella magnetica strain PR-1 that belongs to an Alphaproteobacteria genus with no magnetotactic relative. By comparing the morphology and the whole genome shotgun sequence of this MTB with those of closer relatives, we brought further evidence that the apparent vertical ancestry of magnetosome genes suggested by previous studies within Alphaproteobacteria hides a more complex evolutionary history involving horizontal gene transfers and/or duplication events before and after the emergence of Magnetospirillum, Magnetovibrio and Magnetospira genera. A genome-scale comparative genomics analysis identified several additional candidate functions and genes that could be specifically associated to MTB lifestyle in this class of bacteria.

Removal of Cr (III) and Ni (II) from tannery effluent using calcium carbonate coated bacterial magnetosomes.

Abstract: Heavy metal contamination of surface water bodies and ground water has been a major problem around the world. Calcium-based adsorbents are effective but cannot be separated easily after the treatment. Magnetosomes are biogenic magnetite synthesised as highly ordered chain-like structures by magnetotactic bacteria. In this study, we have prepared magnetically controlled calcite microcrystals using magnetosomes for the adsorption experiment. The ability of magnetic calcite as adsorbent was investigated for the removal of Cr (III) and Ni (II) ions from synthetic solution. Critical parameters, such as the effect of pH, temperature, contact time, initial ion concentration, and adsorbent dose, were optimised in comparison with calcite, magnetosomes, and activated carbon for maximum metal ion removal. The study showed that equilibrium was established in 1 h for both Cr (III) and Ni (II) at a pH of 6.0 and 8.0, respectively. The adsorption process follows pseudo-second-order reaction kinetics, along with Langmuir and Freundlich adsorption isotherms. The thermodynamics of adsorption of both metal ions on magnetic calcite showed that the adsorption was spontaneous and endothermic in nature. Magnetically controlled calcite crystals successfully removed Cr (III) and Ni (II) from collected tannery effluent and separated from the solution by applying magnetic field. Maximum removal of chromium and nickel (94 and 84%) by magnetic calcite is similar to calcite crystals but higher than magnetosomes and activated carbon. The results indicated that magnetic calcite could be used as an alternative adsorbent for removing heavy metals from tannery effluent. Bacterial magnetosomes coated with calcite can adsorb heavy metals from industrial waste while being easily recoverable with a magnet. The tanning industry produces large volumes of wastewater contaminated with heavy metals, and these toxins require removal prior to waste discharge into the environment. Calcium carbonate adsorbs heavy metals with high efficiency, but is difficult to recover from treated water, lowering water quality. A team led by Krish Suthindhiran at VIT University in India combine environmentally-innocent magnetosomes—magnetic iron-based minerals enveloped by lipid membranes and produced by magnetotactic bacteria—with a calcium carbonate coating to achieve materials that not only remove Cr(III) and Ni(II) from tannery effluents with efficiencies of up to 94% and 84%, but are easily separated post-treatment by application of a magnetic field.

Generation of multifunctional magnetic nanoparticles with amplified catalytic activities by genetic expression of enzyme arrays on bacterial magnetosomes

Abstract: Due to their highly regulated biosynthesis, magnetosomes biomineralized by magnetotactic bacteria represent natural magnetic nanoparticles with unique physical and chemical properties. They consist of a magnetite core that is surrounded by a biological membrane and are therefore reminiscent to magnetic “core–shell” nanoparticles. Their usability in many nanotechnological and biomedical applications would be further improved by the introduction of additional catalytic and imaging modalities. Here, a new in vivo strategy is explored for magnetosome display of foreign polypeptides with maximized protein-to-particle ratios. Arrays of up to five monomers of the model enzyme glucuronidase GusA plus the additional fluorophore mEGFP are genetically fused as single large hybrid proteins to highly expressed magnetosome protein anchors. In total, about 190 GusA monomers are covalently attached to individual particles. Assuming layers of GusA rows surrounding the particles, the monomers would thus cover up to 90% of the magnetosome surface. The approach generates nanoparticles that exhibit magnetism, fluorescence, and stable catalytic activities, which are stepwise increased with the number of GusA monomers. In summary, multicopy expression of arrayed foreign proteins represents a powerful methodology for the biosynthesis of tailored biohybrid magnetic nanoparticles with several genetically encoded and tunable functionalities.

Magnetotaxis Enables Magnetotactic Bacteria to Navigate in Flow.

Abstract: Magnetotactic bacteria (MTB) play an important role in Earth's biogeochemical cycles by transporting minerals in aquatic ecosystems, and have shown promise for controlled transport of microscale objects in flow conditions. However, how MTB traverse complex flow environments is not clear. Here, using microfluidics and high-speed imaging, it is revealed that magnetotaxis enables directed motion of Magnetospirillum magneticum over long distances in flow velocities ranging from 2 to 1260 µm s-1 , corresponding to shear rates ranging from 0.2 to 142 s-1 -a range relevant to both aquatic environments and biomedical applications. The ability of MTB to overcome a current is influenced by the flow, the magnetic field, and their relative orientation. MTB can overcome 2.3-fold higher flow velocities when directed to swim perpendicular to the flow as compared to upstream, as the latter orientation induces higher drag. The results indicate a threshold drag of 9.5 pN, corresponding to a flow velocity of 550 µm s-1 , where magnetotaxis enables MTB to overcome counterdirectional flow. These findings bring new insights into the interactions of MTB with complex flow environments relevant to aquatic ecosystems, while suggesting opportunities for in vivo applications of MTB in microbiorobotics and targeted drug delivery.

A Comparative Study of Receptor-Targeted Magnetosome and HSA-Coated Iron Oxide Nanoparticles as MRI Contrast-Enhancing Agent in Animal Cancer Model

Abstract: Magnetosomes are specialized organelles arranged in intracellular chains in magnetotactic bacteria. The superparamagnetic property of these magnetite crystals provides potential applications as contrast-enhancing agents for magnetic resonance imaging. In this study, we compared two different nanoparticles that are bacterial magnetosome and HSA-coated iron oxide nanoparticles for targeting breast cancer. Both magnetosomes and HSA-coated iron oxide nanoparticles were chemically conjugated to fluorescent-labeled anti-EGFR antibodies. Antibody-conjugated nanoparticles were able to bind the MDA-MB-231 cell line, as assessed by flow cytometry. To compare the cytotoxic effect of nanoparticles, MTT assay was used, and according to the results, HSA-coated iron oxide nanoparticles were less cytotoxic to breast cancer cells than magnetosomes. Magnetosomes were bound with higher rate to breast cancer cells than HSA-coated iron oxide nanoparticles. While 250 μg/ml of magnetosomes was bound 92 ± 0.2%, 250 μg/ml of HSA-coated iron oxide nanoparticles was bound with a rate of 65 ± 5%. In vivo efficiencies of these nanoparticles on breast cancer generated in nude mice were assessed by MRI imaging. Anti-EGFR-modified nanoparticles provide higher resolution images than unmodified nanoparticles. Also, magnetosome with anti-EGFR produced darker image of the tumor tissue in T2-weighted MRI than HSA-coated iron oxide nanoparticles with anti-EGFR. In vivo MR imaging in a mouse breast cancer model shows effective intratumoral distribution of both nanoparticles in the tumor tissue. However, magnetosome demonstrated higher distribution than HSA-coated iron oxide nanoparticles according to fluorescence microscopy evaluation. According to the results of in vitro and in vivo study results, magnetosomes are promising for targeting and therapy applications of the breast cancer cells.

Magnetic Study of Co-Doped Magnetosome Chains

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

Morphological and cellular diversity of magnetotactic bacteria: A review

Abstract: Magnetotactic bacteria (MTB) are getting much attention in the recent years due to the biomineralization in their magnetosomes (MS). MS are unique organelles that are bio-mineralized due to MTB. MS contains nanosized crystal minerals of magnetite or greigite covered by bilayer lipid membrane, which are originated from cytoplasmic membrane (CM). MS are organized as an ordered chain into the cell which acts as a miniature compass needle. Furthermore, the biodiversity of MTB and their distribution is principally linked with the characteristics and growths of the MS. MTB are often considered as a part of the bacterial biomass from all of the aquatic environments. There have been a lot of genes that control the functions of MTB by accumulating as clusters of genomes such as magnetosomes genomic island (MAI). Therefore, in the present review, the function of the genes and proteins has been highlighted, which are mainly associated with the construction and formation of MS. In addition, the biodiversity, morphology and cell biology of MTB is discussed in greater detail to understand the formation of MS crystals by MTB.

Magnetic-field induced rotation of magnetosome chains in silicified magnetotactic bacteria.

Abstract: Understanding the biological processes enabling magnetotactic bacteria to maintain oriented chains of magnetic iron-bearing nanoparticles called magnetosomes is a major challenge. The study aimed to constrain the role of an external applied magnetic field on the alignment of magnetosome chains in Magnetospirillum magneticum AMB-1 magnetotactic bacteria immobilized within a hydrated silica matrix. A deviation of the chain orientation was evidenced, without significant impact on cell viability, which was preserved after the field was turned-off. Transmission electron microscopy showed that the crystallographic orientation of the nanoparticles within the chains were preserved. Off-axis electron holography evidenced that the change in magnetosome orientation was accompanied by a shift from parallel to anti-parallel interactions between individual nanocrystals. The field-induced destructuration of the chain occurs according to two possible mechanisms: (i) each magnetosome responds individually and reorients in the magnetic field direction and/or (ii) short magnetosome chains deviate in the magnetic field direction. This work enlightens the strong dynamic character of the magnetosome assembly and widens the potentialities of magnetotactic bacteria in bionanotechnology.

Understanding the Biomineralization Role of Magnetite-Interacting Components (MICs) From Magnetotactic Bacteria.

Abstract: Biomineralization is a process that takes place in all domains of life and which usually helps organisms to harden soft tissues by creating inorganic structures that facilitate their biological functions. It was shown that biominerals are under tight biological control via proteins that are involved in nucleation initiation and/or which act as structural skeletons. Magnetotactic bacteria (MTB) use iron biomineralization to create nano-magnetic particles in a specialized organelle, the magnetosome, to align to the geomagnetic field. A specific set of magnetite-associated proteins (MAPs) is involved in regulating magnetite nucleation, size, and shape. These MAPs are all predicted to contain specific 17 to 22 residue-long sequences involved in magnetite formation. To understand the mechanism of magnetite formation, we focused on three different MAPs, MamC, Mms6 and Mms7, and studied the predicted iron-binding sequences. Using nuclear magnetic resonance (NMR), we differentiated the recognition mode of each MAP based on ion specificity, affinity, and binding residues. The significance of critical residues in each peptide was evaluated by mutation followed by an iron co-precipitation assay. Among the peptides, MamC showed weak ion binding but created the most significant effect in enhancing magnetite particle size, indicating the potency in controlling magnetite particle shape and size. Alternatively, Mms6 and Mms7 had strong binding affinities but less effect in modulating magnetite particle size, representing their major role potentially in initiating nucleation by increasing local metal concentration. Overall, our results explain how different MAPs affect magnetite synthesis, interact with Fe2+ ions and which residues are important for the MAPs functions.

Characterization of marine ferromanganese crust from the Pacific using residues of selective chemical leaching: identification of fossil magnetotactic bacteria with FE-SEM and rock magnetic methods

Abstract: Hydrogenetic ferromanganese crusts (hereafter referred to as “crusts”) on Pacific seamounts are formed by the precipitation of iron–manganese oxides from seawater on volcanic and biogenic substrate rocks. As crusts grow continuously and have very slow growth rates of between 1 and 10 mm/m.y., they can potentially be used as records of the Neogene paleoceanographic and paleoclimatic conditions. Crusts can be considered as compressed sediment cores containing biogenic, volcanogenic, and terrestrial particles that include eolian dusts and the partly weathered products of substrate acquired during its growth. In this study, selective leaching experiments were conducted on a sample of ferromanganese crust, which had been obtained from the Federated States of Micronesia at a water depth of 2262 m. Chemical leaching experiments were conducted using oxalic acid buffered with ammonium oxalate on the crushed crust samples, which is an optimization of previously proposed sequential leaching procedures. The applied method was found to be effective in separating the major mineral phases of crusts from associated metallic components, thereby providing concentration of the residual fraction for use in analysis following the leaching experiment. Using this method, polygenetic particles were extracted from the crust and identified using optical and electron microscopes. They were found to be of various origins and included volcanogenic, biogenic, terrestrial, and extraterrestrial material. In addition, well-sorted prism-shaped chained magnetic particles were observed in residual fractions. Rock magnetic experiments support the idea that the magnetic particles are magnetites and originated from fossil magnetotactic bacteria. The fossil magnetotactic bacteria might have been living on the crust at the time of crust formation. Alternatively, fossil magnetotactic bacteria could have been transported by deep sea currents from the sediment where magnetotactic bacteria originated.

Synthesis of Magnetic Nanoparticles Stabilized by Magnetite-Binding Protein for Targeted Delivery to Cancer Cells.

Abstract: A new method for obtaining biomodified magnetite nanoparticles for targeted delivery to cells was developed. The method is based on the use of the C-terminal fragment of the Mms6 protein, which is involved in the magnetite biomineralization during the synthesis of magnetosomes in magnetotactic bacteria Magnetospirillum magneticum AMB-1, and the barnase*barstar high-affinity protein pair. The Mms6 protein fragment is required for stabilizing magnetite, and the barnase*barstar pair mediates the interaction between nanoparticles and the component for modification. The efficiency of this method was confirmed in the synthesis of magnetite nanoparticles recognizing the HER2/neu tumor marker and in the selective labeling of HER2/neu with these nanoparticles on the surface of cancer cells.

Culture‐independent characterization of a novel magnetotactic member affiliated to the Beta class of the Proteobacteria phylum from an acidic lagoon

Abstract: Magnetotactic bacteria (MTB) comprise a group of motile microorganisms common in most mesothermal aquatic habitats with pH values around neutrality. However, during the last two decades, a number of MTB from extreme environments have been characterized including: cultured alkaliphilic strains belonging to the Deltaproteobacteria class of the Proteobacteria phylum; uncultured moderately thermophilic strains belonging to the Nitrospirae phylum; cultured and uncultured moderately halophilic or strongly halotolerant bacteria affiliated with the Deltaproteobacteria and Gammaproteobacteria classes and an uncultured psychrophilic species belonging to the Alphaproteobacteria class. Here, we used culture-independent techniques to characterize MTB from an acidic freshwater lagoon in Brazil (pH ∼ 4.4). MTB morphotypes found in this acidic lagoon included cocci, rods, spirilla and vibrioid cells. Magnetite (Fe3 O4 ) was the only mineral identified in magnetosomes of these MTB while magnetite magnetosome crystal morphologies within the different MTB cells included cuboctahedral (present in spirilla), elongated prismatic (present in cocci and vibrios) and bullet-shaped (present in rod-shaped cells). Intracellular pH measurements using fluorescent dyes showed that the cytoplasmic pH was close to neutral in most MTB cells and acidic in some intracellular granules. Based on 16S rRNA gene phylogenetic analyses, some of the retrieved gene sequences belonged to the genus Herbaspirillum within the Betaproteobacteria class of the Proteobacteria phylum. Fluorescent in situ hybridization using a Herbaspirillum-specific probe hybridized with vibrioid MTB in magnetically-enriched samples. Transmission electron microscopy of the Herbaspirillum-like MTB revealed the presence of many intracellular granules and a single chain of elongated prismatic magnetite magnetosomes. Diverse populations of MTB have not seemed to have been described in detail in an acid environment. In addition, this is the first report of an MTB phylogenetically affiliated with Betaproteobacteria class.

Magnetotactic Bacteria: Magnetism Beyond Magnetosomes

Abstract: Magnetotactic bacteria are a group of organisms deeply studied in the last years due to their interesting magnetic behavior and potential applications in nanometrology, hyperthermia, and biosensor devices. One intrinsic common characteristic is the presence, inside the bacteria, of magnetic nanoparticles called magnetosomes. The role of magnetosomes as bacterial tools to orient the bacteria and find new habitats is universally accepted, but the way they develop still is not fully understood. A strain of Magnetospirillum magnetotacticum was grown and investigated at the nanoscale using transmission electron microscopy and atomic/magnetic force microscopy techniques. Magnetosomes were observed as well as long filaments with magnetic response that could be associated to the actin-like filaments being crucial to allow the nanoparticles orientation and magnetosomes formation. To the best of our knowledge, this paper is the first to visualize these reproducible long-range size magnetic crystalline structures.

Enhanced Tubulation of Liposome Containing Cardiolipin by MamY Protein from Magnetotactic Bacteria

Abstract: Lipid tubules are of particular interest for many potential applications in nanotechnology. Among various lipid tubule fabrication techniques, the morphological regulation of membrane structure by proteins mimicking biological processes may provide the chances to form lipid tubes with highly tuned structures. Magnetotactic bacteria synthesize magnetosomes (a unique prokaryotic organelle comprising a magnetite crystal within a lipid envelope). MamY protein is previously identified as the magnetosome protein responsible for magnetosome vesicle formation and stabilization. Furthermore, MamY is shown in vitro liposome tubulation activity. In this study, the interaction of MamY and phospholipids is investigated by using a lipids-immobilized membrane strip and a peptide array. Here, the binding of MamY to the anionic phospholipid, cardiolipin, is found and enhanced liposome tubulation efficiency. The authors propose the interaction is responsible for recruiting and locating cardiolipin to elongate liposome in vitro. The authors also suggest a similar mechanism for the invagination site in magnetosomes vesicle formation, where the lipid itself contributes further to increasing the curvature. These findings are highly important to develop an effective biomimetic synthesis technique of lipid tubules and to elucidate the unique prokaryotic organelle formation in magnetotactic bacteria.

Migration of magnetotactic bacteria in porous media.

Abstract: Magnetotactic bacteria (MTB) migrate in complex porous sediments where fluid flow is ubiquitous. Here, we demonstrate that magnetotaxis enables MTB to migrate effectively through porous micromodels. Directed MTB can circumvent curved obstacles by traveling along the boundaries and pass flat obstacles by repeatedly switching between forward and backward runs. Magnetotaxis enables directed motion of MTB through heterogeneous porous media, overcoming tortuous flow fields with local velocities as high as 250 μm s−1. Our findings bring new insights into the migration behaviour of MTB in their natural habitats and their potential in vivo applications as microbiorobots.

Accumulation and Dissolution of Magnetite Crystals in a Magnetically Responsive Ciliate

Abstract: Magnetotactic bacteria (MTB) represent a group of microorganisms that are widespread in aquatic habitats and thrive at oxic-anoxic interfaces. They are able to scavenge high concentrations of iron thanks to the biomineralization of magnetic crystals in their unique organelles, the so-called magnetosome chains. Although their biodiversity has been intensively studied, their ecology and impact on iron cycling remain largely unexplored. Predation by protozoa was suggested as one of the ecological processes that could be involved in the release of iron back into the ecosystem. Magnetic protozoa were previously observed in aquatic environments, but their diversity and the fate of particulate iron during grazing are poorly documented. In this study, we report the morphological and molecular characterizations of a magnetically responsive MTB-grazing protozoan able to ingest high quantities of MTB. This protozoan is tentatively identified as $Uronema\ marinum$, a ciliate known to be a predator of bacteria. Using light and electron microscopy, we investigated in detail the vacuoles in which the lysis of phagocytized prokaryotes occurs. We carried out high-resolution observations of aligned magnetosome chains and ongoing dissolution of crystals. Particulate iron in the ciliate represented approximately 0.01% of its total volume. We show the ubiquity of this interaction in other types of environments and describe different grazing strategies. These data contribute to the mounting evidence that the interactions between MTB and protozoa might play a significant role in iron turnover in microaerophilic habitats.

Magnetotactic bacteria used to generate electricity based on Faraday's law of electromagnetic induction

Abstract: Table S1. Voltage measured from the uninoculated medium, MTB culture and purified magnetosomes respectively.

Fingerprints of partial oxidation of biogenic magnetite from cultivated and natural marine magnetotactic bacteria using synchrotron radiation.

Abstract: Magnetotactic bacteria are a multi-phyletic group of bacteria that synthesize membrane-bound magnetic minerals. Understanding the preservation of these minerals in various environments (e.g., with varying oxygen concentrations and iron supply) is important for understanding their role as carriers of primary magnetizations in sediments and sedimentary rocks. Here we present X-ray near edge structure (XANES) spectra for Fe in magnetotactic bacteria samples from recent sediments to assess surface oxidation and crystal structure changes in bacterial magnetite during early burial. Our results are compared with a XANES spectrum of cultivated Magnetofaba australis samples, and with magnetic properties, and indicate that oxidation of magnetite to maghemite increases with depth in the sediment due to longer exposure to molecular oxygen. These results are relevant to understanding magnetic signatures carried by magnetofossils in oxic sediments and sedimentary rocks of different ages.

Reduction of Hg(II) to Hg(0) by biogenic magnetite from two magnetotactic bacteria

Abstract: Understanding the biogeochemical cycle of the highly toxic element mercury (Hg) is necessary to predict its fate and transport. In this study, we determined that biogenic magnetite isolated from Ma...

Effect of applied magnetic fields on motility and magnetotaxis in the uncultured magnetotactic multicellular prokaryote 'Candidatus Magnetoglobus multicellularis'.

Abstract: Magnetotactic bacteria are found in the chemocline of aquatic environments worldwide. They produce nanoparticles of magnetic minerals arranged in chains in the cytoplasm, which enable these microorganisms to align to magnetic fields while swimming propelled by flagella. Magnetotactic bacteria are diverse phylogenetically and morphologically, including cocci, rods, vibria, spirilla and also multicellular forms, known as magnetotactic multicellular prokaryotes (MMPs). We used video-microscopy to study the motility of the uncultured MMP 'Candidatus Magnetoglobus multicellularis' under applied magnetic fields ranging from 0.9 to 32 Oersted (Oe). The bidimensional projections of the tridimensional trajectories where interpreted as plane projections of cylindrical helices and fitted as sinusoidal curves. The results showed that 'Ca. M. multicellularis' do not orient efficiently to low magnetic fields, reaching an efficiency of about 0.65 at 0.9-1.5 Oe, which are four to six times the local magnetic field. Good efficiency (0.95) is accomplished for magnetic fields ≥10 Oe. For comparison, unicellular magnetotactic microorganisms reach such efficiency at the local magnetic field. Considering that the magnetic moment of 'Ca. M. multicellularis' is sufficient for efficient alignment at the Earth's magnetic field, we suggest that misalignments are due to flagella movements, which could be driven by photo-, chemo- and/or other types of taxis.