Magnetosome magnetite biomineralization in a flagellated protist: evidence for an early evolutionary origin for magnetoreception in eukaryotes

The most well-recognized magnetoreception behaviour is that of the magnetotactic bacteria (MTB), which synthesize membrane-bounded magnetic nanocrystals called magnetosomes via a biologically controlled process. The magnetic minerals identified in prokaryotic magnetosomes are magnetite (Fe₃O₄) and greigite (Fe₃S₄). Magnetosome crystals, regardless of composition, have consistent, species-specific morphologies and single-domain size range. Because of these features, magnetosome magnetite crystals possess specific properties in comparison to abiotic, chemically synthesized magnetite. Despite numerous discoveries regarding MTB phylogeny over the last decades, this diversity is still considered underestimated. Characterization of magnetotactic microorganisms is important as it might provide insights into the origin and establishment of magnetoreception in general, including eukaryotes. Here, we describe the magnetotactic behaviour and characterize the magnetosomes from a flagellated protist using culture-independent methods. Results strongly suggest that, unlike previously described magnetotactic protists, this flagellate is capable of biomineralizing its own anisotropic magnetite magnetosomes, which are aligned in complex aggregations of multiple chains within the cell. This organism has a similar response to magnetic field inversions as MTB. Therefore, this eukaryotic species might represent an early origin of magnetoreception based on magnetite biomineralization. It should add to the definition of parameters and criteria to classify biogenic magnetite in the fossil record.     

Molecular analysis of a subcellular compartment: the magnetosome membrane in<Emphasis Type="Italic"> Magnetospirillum gryphiswaldense</Emphasis>

The ability of magnetotactic bacteria (MTB) to orient and migrate along magnetic field lines is based on magnetosomes, which are membrane-enclosed intracellular crystals of a magnetic iron mineral. Magnetosome biomineralization is achieved by a process involving control over the accumulation of iron and deposition of the magnetic particle, which has a specific morphology, within a vesicle provided by the magnetosome membrane. In Magnetospirillum gryphiswaldense, the magnetosome membrane has a distinct biochemical composition and comprises a complex and specific subset of magnetosome membrane proteins (MMPs). Classes of MMPs include those with presumed function in magnetosome-directed uptake and binding of iron, nucleation of crystal growth, and the assembly of magnetosome membrane multiprotein complexes. Other MMPs comprise protein families of so far unknown function, which apparently are conserved between all other MTB. The mam and mms genes encode most of the MMPs and are clustered within several operons, which are part of a large, unstable genomic region constituting a putative magnetosome island. Current research is directed towards the biochemical and genetic analysis of MMP functions in magnetite biomineralization as well as their expression and localization during growth.Abbreviations MM Magnetosome membrane - MMP Magnetosome membrane protein - MTB Magnetotactic bacteria     

Magnetotactic bacteria,magnetosomes and their application

Magnetotactic bacteria (MTB) are a diverse group of microorganisms with the ability to orient and migrate along geomagnetic field lines. This unique feat is based on specific intracellular organelles, the magnetosomes, which, in most MTB, comprise nanometer-sized, membrane bound crystals of magnetic iron minerals and organized into chains via a dedicated cytoskeleton. Because of the special properties of the magnetosomes, MTB are of great interest for paleomagnetism, environmental magnetism, biomarkers in rocks, magnetic materials and biomineralization in organisms, and bacterial magnetites have been exploited for a variety of applications in modern biological and medical sciences. In this paper, we describe general characteristics of MTB and their magnetic mineral inclusions, but focus mainly on the magnetosome formation and the magnetisms of MTB and bacterial magnetosomes, as well as on the significances and applications of MTB and their intracellular magnetic mineral crystals.     

Effect of Polyethylene Glycol on the Formation of Magnetic Nanoparticles Synthesized by Magnetospirillum magnetotacticum MS-1

Magnetotactic bacteria (MTB) synthesize intracellular magnetic nanocrystals called magnetosomes, which are composed of either magnetite (Fe₃O₄) or greigite (Fe₃S₄) and covered with lipid membranes. The production of magnetosomes is achieved by the biomineralization process with strict control over the formation of magnetosome membrane vesicles, uptake and transport of iron ions, and synthesis of mature crystals. These magnetosomes have high potential for both biotechnological and nanotechnological applications, but it is still extremely difficult to grow MTB and produce a large amount of magnetosomes under the conventional cultural conditions. Here, we investigate as a first attempt the effect of polyethylene glycol (PEG) added to the culture medium on the increase in the yield of magnetosomes formed in Magnetospirillum magnetotacticum MS-1. We find that the yield of the formation of magnetosomes can be increased up to approximately 130 % by adding PEG200 to the culture medium. We also measure the magnetization of the magnetosomes and find that the magnetosomes possess soft ferromagnetic characteristics and the saturation mass magnetization is increased by 7 %.     

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

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 (Fe₃O₄) 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.     

Spatial localizations of Mam22 and Mam12 in the magnetosomes of Magnetospirillum magnetotacticum

Magnetospirillum magnetotacticum possesses intracellular magnetite particles with a chain-like structure, termed magnetosomes. The bacterium expresses 22-kDa and 12-kDa magnetosome-associated proteins, termed Mam22 (MamA) and Mam12 (MamC), respectively. In this study, we investigated the structure of the purified magnetosomes with transmission electron microscopic techniques and found that the magnetosomes consisted of four compartments, i.e., magnetite crystal, magnetosomal membrane, interparticle connection, and magnetosomal matrix. Furthermore, we determined the precise localizations of Mam22 and Mam12 using immunogold staining of the purified magnetosomes and ultrathin sections of the bacterial cells. Interestingly, most Mam22 existed in the magnetosomal matrix, whereas Mam12 was strictly localized in the magnetosomal membrane. Moreover, the recombinant Mam22 was attached to the magnetosomal matrix of the Mam22-deficient magnetosomes prepared by alkaline treatment, such as 0.1 M Caps-NaOH buffer (pH 11.0). The spatial localization of the magnetosome-associated proteins in the magnetosomal chain provides useful information to elucidate the functional roles of these proteins.     

Genetics and cell biology of magnetosome formation in magnetotactic bacteria

The ability of magnetotactic bacteria (MTB) to orient in magnetic fields is based on the synthesis of magnetosomes, which are unique prokaryotic organelles comprising membrane-enveloped, nano-sized crystals of a magnetic iron mineral that are aligned in well-ordered intracellular chains. Magnetosome crystals have species-specific morphologies, sizes, and arrangements. The magnetosome membrane, which originates from the cytoplasmic membrane by invagination, represents a distinct subcellular compartment and has a unique biochemical composition. The roughly 20 magnetosome-specific proteins have functions in vesicle formation, magnetosomal iron transport, and the control of crystallization and intracellular arrangement of magnetite particles. The assembly of magnetosome chains is under genetic control and involves the action of an acidic protein that links magnetosomes to a novel cytoskeletal structure, presumably formed by a specific actin-like protein. A total of 28 conserved genes present in various magnetic bacteria were identified to be specifically associated with the magnetotactic phenotype, most of which are located in the genomic magnetosome island. The unique properties of magnetosomes attracted broad interdisciplinary interest, and MTB have recently emerged as a model to study prokaryotic organelle formation and evolution.     

Magnetotactic bacteria: nanodrivers of the future

Magnetotactic bacteria (MTB) represent a heterogeneous group of Gram-negative aquatic prokaryotes with a broad range of morphological types, including vibrioid, coccoid, rod and spirillum. MTBs possess the virtuosity to passively align and actively swim along the magnetic field. Magnetosomes are the trademark nano-ranged intracellular structures of MTB, which comprise magnetic iron-bearing inorganic crystals enveloped by an organic membrane, and are dedicated organelles for their magnetotactic lifestyle. Magnetosomes endue high and even dispersion in aqueous solutions compared with artificial magnetites, claiming them as paragon nanomaterials. MTB and magnetosomes offer high technological potential in modern science, technology and medicines. This review focuses on the applicability of MTB and magnetosomes in various areas of modern benefits.     

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．     

Diverse phylogeny and morphology of magnetite biomineralized by magnetotactic cocci

Magnetotactic bacteria (MTB) are diverse prokaryotes that produce magnetic nanocrystals within intracellular membranes (magnetosomes). Here, we present a large-scale analysis of diversity and magnetosome biomineralization in modern magnetotactic cocci, which are the most abundant MTB morphotypes in nature. Nineteen novel magnetotactic cocci species are identified phylogenetically and structurally at the single-cell level. Phylogenetic analysis demonstrates that the cocci cluster into an independent branch from other Alphaproteobacteria MTB, that is, within the Etaproteobacteria class in the Proteobacteria phylum. Statistical analysis reveals species-specific biomineralization of magnetosomal magnetite morphologies. This further confirms that magnetosome biomineralization is controlled strictly by the MTB cell and differs among species or strains. The post-mortem remains of MTB are often preserved as magnetofossils within sediments or sedimentary rocks, yet paleobiological and geological interpretation of their fossil record remains challenging. Our results indicate that magnetofossil morphology could be a promising proxy for retrieving paleobiological information about ancient MTB.     

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

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

Effects of pulsed magnetic field on the formation of magnetosomes in the Magnetospirillum sp. strain AMB‐1

Magnetotactic bacteria are a diverse group of microorganisms which possess one or more chains of magnetosomes and are endowed with the ability to use geomagnetic fields for direction sensing, thus providing a simple and excellent model for the study of magnetite‐based magnetoreception. In this study, a 50 Hz, 2 mT pulsed magnetic field (PMF) was applied to study the effects on the formation of magnetosomes in Magnetospirillum sp. strain AMB‐1. The results showed that the cellular magnetism (R_mag) of AMB‐1 culture significantly increased while the growth of cells remained unaffected after exposure. The number of magnetic particles per cell was enhanced by about 15% and slightly increased ratios of magnetic particles of superparamagnetic property (size <20 nm) and mature magnetosomes (size >50 nm) were observed after exposure to PMF. In addition, the intracellular iron accumulation slightly increased after PMF exposure. Therefore, it was concluded that 50 Hz, 2 mT PMF enhances the formation of magnetosomes in Magnetospirillum sp. strain AMB‐1. Our results suggested that lower strength of PMF has no significant effects on the bacterial cell morphologies but could affect crystallization process of magnetosomes to some extent. Bioelectromagnetics 31:246–251, 2010. © 2009 Wiley‐Liss, Inc.     

Comparative genomic analysis of magnetotactic bacteria from the Deltaproteobacteria provides new insights into magnetite and greigite magnetosome genes required for magnetotaxis

Magnetotactic bacteria (MTB) represent a group of diverse motile prokaryotes that biomineralize magnetosomes, the organelles responsible for magnetotaxis. Magnetosomes consist of intracellular, membrane‐bounded, tens‐of‐nanometre‐sized crystals of the magnetic minerals magnetite (Fe₃O₄) or greigite (Fe₃S₄) and are usually organized as a chain within the cell acting like a compass needle. Most information regarding the biomineralization processes involved in magnetosome formation comes from studies involving Alphaproteobacteria species which biomineralize cuboctahedral and elongated prismatic crystals of magnetite. Many magnetosome genes, the mam genes, identified in these organisms are conserved in all known MTB. Here we present a comparative genomic analysis of magnetotactic Deltaproteobacteria that synthesize bullet‐shaped crystals of magnetite and/or greigite. We show that in addition to mam genes, there is a conserved set of genes, designated mad genes, specific to the magnetotactic Deltaproteobacteria, some also being present in Candidatus Magnetobacterium bavaricum of the Nitrospirae phylum, but absent in the magnetotactic Alphaproteobacteria. Our results suggest that the number of genes associated with magnetotaxis in magnetotactic Deltaproteobacteria is larger than previously thought. We also demonstrate that the minimum set of mam genes necessary for magnetosome formation in Magnetospirillum is also conserved in magnetite‐producing, magnetotactic Deltaproteobacteria. Some putative novel functions of mad genes are discussed.     

趋磁细菌及磁小体在肿瘤治疗中的应用

提高抗肿瘤药物的靶向性是肿瘤治疗、降低药物副作用的重要手段。在肿瘤组织内部由于癌细胞的快速增殖致使其形成低氧区,低氧区会对多种肿瘤治疗方案产生耐受。趋磁细菌 (Magnetotactic bacteria, MTB) 是一类能在细胞内产生外包生物膜、纳米尺寸、单磁畴磁铁矿 (Fe3O4) 或硫铁矿 (Fe3S4) 晶体颗粒-磁小体的微生物的统称。在磁场的作用下,趋磁细菌可凭借鞭毛运动至厌氧区。趋磁细菌在动物体内毒性较低且生物相容性良好,其磁小体与人工合成的磁性纳米材料相比优势显著。文中在介绍趋磁细菌及其磁小体生物学特点、理化性能的基础上,综述了趋磁细菌作为载体偶联药物进入肿瘤内部,并通过感受低氧信号定位于肿瘤低氧区,以及趋磁细菌竞争肿瘤细胞铁源的研究进展,总结了磁小体运载化疗药物、抗体、DNA疫苗靶向结合肿瘤的研究进展,分析了趋磁细菌及磁小体肿瘤治疗中面临的问题,并对趋磁细菌和磁小体在肿瘤治疗中的应用进行了展望。     

Uncultivated Magnetotactic Cocci from Yuandadu Park in Beijing,China

In the present study, we investigated a group of uncultivated magnetotactic cocci, which was magnetically isolated from a freshwater pond in Beijing, China. Light and transmission electron microscopy showed that these cocci ranged from 1.5 to 2.5 μm and contained two to four chains of magnetite magnetosomes, which sometimes were partially disorganized. Overall, the size of the disorganized magnetosomes was significantly smaller than that arranged in chains. All characterized magnetosome crystals were elongated (shape factor = 0.64) and fall into the single-domain size range (30 to 115 nm). Comparative 16S rRNA gene sequence analysis and fluorescence in situ hybridization showed that the enriched bacteria were a virtually homogeneous population and represented a novel lineage in the Alphaproteobacteria. The closest cultivated relative was magnetotactic coccoid strain MC-1 (88% sequence identity). First-order reversal curve diagrams revealed that these cocci had relatively strong magnetic interactions compared to the single-chain magnetotactic bacteria. Low-temperature magnetic measurements showed that the Verwey transition of them was ∼108 K, confirming magnetite magnetosomes, and the delta ratio δ_FC/δ_ZFC was >2. Based on the structure, phylogenetic position and magnetic properties, the enriched magnetotactic cocci of Alphaproteobacteria are provisionally named as “Candidatus Magnetococcus yuandaducum.”Magnetotactic bacteria (MTB) can mineralize intracellular nanosized iron oxides or sulfides called magnetosomes, which in most MTB are normally single-domain (SD) magnetite with a narrow range of grain sizes from 30 to 120 nm (3). The chain configuration of magnetosomes renders MTB able to navigate the oxic-anoxic interface in chemically stratified environments by swimming along the Earth''s magnetic field (13). Diverse MTB, including coccoid, spirillar, rod-shaped bacteria and multicellular magnetotactic prokaryotes with one, two, or more chains of magnetosomes, thrive in a broad range of aquatic environments, which sometimes even are dominant strains of the microbial biomass in sediment (10, 47). Based on their phylogeny, all currently known MTB can be divided into two taxonomic groups: Proteobacteria and Nitrospira phyla (2).When MTB die, the magnetosomes can be preserved in sediment as fossil magnetosomes (or magnetofossils) (6). Fossil magnetosomes have been found in lacustrine and deep-sea sediments (6, 35, 43, 51), which are stable carriers of natural remanence and may play substantial contributions to the bulk magnetization of sediments due to their SD sizes (6, 19, 26, 32, 33). Moreover, since most known MTB are microaerophilic or anaerobic and are concentrated in the oxic-anoxic transition zone, the presence and characteristics of MTB species in vertically stratified sediments can be used as a potential paleoenvironmental proxy (19, 44, 45). However, how to identify bacterial magnetite or greigite (Fe₃S₄) in sediments is still challenging. Recently, characterizing the magnetic properties of MTB has attracted increasing interests because magnetic techniques are fast and effective in distinguishing bacterial crystals from abiogenic magnetic minerals in sediments (11, 26, 27, 33, 38).In spite of their wide distribution and abundance in aquatic environments, most MTB are intractable, and so far only a few of them, e.g., Magnetospirillum gryphiswaldense strain MSR-1 (41), M. magnetotacticum strain AMB-1 (17), M. magnetotacticum strain MS-1 (4), and magnetotactic coccoid strain MC-1 (24), can be grown in pure culture. Until recently, most insights into the molecular characterizations and magnetic properties of MTB have been based on pure cultures, which have a single magnetosome chain per cell (10, 11, 19, 20, 25, 27, 28, 37, 38, 42, 52). However, knowledge of uncultivated MTB, especially strains with multiple chains of magnetosomes that are commonly encountered in natural environments, remains limited. In the present study, we investigated a population of uncultivated magnetotactic cocci with multiple magnetosome chains, which were abundant in the pond in Yuan Dynasty Capital City Wall Relics Park (Yuandadu Park) in Beijing, China, in order to characterize their morphological features, phylogenetic positions, and magnetic properties and finally to classify them in a provisional Candidatus taxon.     

Isolation and characterization of <Emphasis Type="Italic">Magnetospirillum</Emphasis> from saline lagoon

Magnetotactic bacteria (MTB) are aquatic prokaryotes that orient themselves to earth’s magnetic field with the help of intracellular organelle magnetosomes. Although many species of MTB have been identified, the isolation of MTB is a challenging task due to the lack of systematic isolation procedure and/or commercial media. In this study, we are reporting the isolation of magnetotactic spirillum from the Pulicat lagoon, India using a systematic and selective procedure. Sampling site was chosen on the basis of physicochemical properties of the ecosystem and the catalysed reporter deposition fluorescence in situ hybridization (CARD–FISH) analysis of sediment samples. In the current study, a combination of techniques including ‘capillary racetrack’ Purification and gradient cultivation resulted in the isolation of magnetotactic spirilla from aquatic sediments. Based on the 16S rRNA gene sequence analysis, the strain was identified as Magnetospirillum and was designated as Magnetospirillum sp. VITRJS1. The genes responsible for magnetosome formation (mamA, B, E, F, K, M, O, P, Q, T) were successfully detected using PCR amplification. The presence of cbbM gene confirmed that the isolate is chemolithoautotroph and utilises reduced sulphur as an electron source. Furthermore, magnetosomes extracted from VITRJS1 found to be cubo-octahedral in shape and 45 nm in size. Our results indicate that the systematic procedure using sediment analysis, CARD–FISH, and a combination of isolation methods enables the selective and rapid isolation of MTB from aquatic sediment sample.     

Large-scale production of magnetosomes by chemostat culture of <Emphasis Type="Italic">Magnetospirillum gryphiswaldense</Emphasis> at high cell density

Background  

Magnetotactic bacteria have long intrigued researchers because they synthesize intracellular nano-scale (40-100 nm) magnetic particles composed of Fe₃O₄, termed magnetosomes. Current research focuses on the molecular mechanisms of bacterial magnetosome formation and its practical applications in biotechnology and medicine. Practical applications of magnetosomes are based on their ferrimagnetism, nanoscale size, narrow size distribution, dispersal ability, and membrane-bound structure. However, the applications of magnetosomes have not yet been developed commercially, mainly because magnetotactic bacteria are difficult to cultivate and consistent, high yields of magnetosomes have not yet been achieved.     

Monophyletic origin of magnetotaxis and the first magnetosomes

Horizontal gene transfer (HGT), the transfer of genetic material other than by descent, is thought to have played significant roles in the evolution and distribution of genes in prokaryotes. These include those responsible for the ability of motile, aquatic magnetotactic bacteria (MTB) to align and swim along magnetic field lines and the biomineralization of magnetosomes that are responsible for this behaviour. There is some genomic evidence that HGT might be responsible for the distribution of magnetosome genes in different phylogenetic groups of bacteria. For example, in the genomes of a number of MTB, magnetosome genes are present as clusters within a larger structure known as the magnetosome genomic island surrounded by mobile elements such as insertion sequences and transposases as well as tRNA genes. Despite this, there is no strong direct proof of HGT between these organisms. Here we show that a phylogenetic tree based on magnetosome protein amino acid sequences from a number of MTB was congruent with the tree based on the organisms' 16S rRNA gene sequences. This shows that evolution and divergence of these proteins and the 16S rRNA gene occurred similarly. This suggests that magnetotaxis originated monophyletically in the Proteobacteria phylum and implies that the common ancestor of all Proteobacteria was magnetotactic.     

Mass collection of magnetotactic bacteria from the permanently stratified ferruginous Lake Pavin,France

Obtaining high biomass yields of specific microorganisms for culture-independent approaches is a challenge faced by scientists studying organism's recalcitrant to laboratory conditions and culture. This difficulty is highly decreased when studying magnetotactic bacteria (MTB) since their unique behaviour allows their enrichment and purification from other microorganisms present in aquatic environments. Here, we use Lake Pavin, a permanently stratified lake in the French Massif Central, as a natural laboratory to optimize collection and concentration of MTB that thrive in the water column and sediments. A method is presented to separate MTB from highly abundant abiotic magnetic particles in the sediment of this crater lake. For the water column, different sampling approaches are compared such as in situ collection using a Niskin bottle and online pumping. By monitoring several physicochemical parameters of the water column, we identify the ecological niche where MTB live. Then, by focusing our sampling at the peak of MTB abundance, we show that the online pumping system is the most efficient for fast recovering of large volumes of water at a high spatial resolution, which is necessary considering the sharp physicochemical gradients observed in the water column. Taking advantage of aerotactic and magnetic MTB properties, we present an efficient method for MTB concentration from large volumes of water. Our methodology represents a first step for further multidisciplinary investigations of the diversity, metagenomic and ecology of MTB populations in Lake Pavin and elsewhere, as well as chemical and isotopic analyses of their magnetosomes.     

An orphaned Mce‐associated membrane protein of Mycobacterium tuberculosis is a virulence factor that stabilizes Mce transporters

Mycobacterium tuberculosis proteins that are exported out of the bacterial cytoplasm are ideally positioned to be virulence factors; however, the functions of individual exported proteins remain largely unknown. Previous studies identified Rv0199 as an exported membrane protein of unknown function. Here, we characterized the role of Rv0199 in M. tuberculosis virulence using an aerosol model of murine infection. Rv0199 appears to be a member of a Mce‐associated membrane (Mam) protein family leading us to rename it OmamA, for orphaned Mam protein A. Consistent with a role in Mce transport, we showed OmamA is required for cholesterol import, which is a Mce4‐dependent process. We further demonstrated a function for OmamA in stabilizing protein components of the Mce1 transporter complex. These results indicate a function of OmamA in multiple Mce transporters and one that may be analogous to the role of VirB8 in stabilizing Type IV secretion systems, as structural similarities between Mam proteins and VirB8 proteins are predicted by the Phyre 2 program. In this study, we provide functional information about OmamA and shed light on the function of Mam family proteins in Mce transporters.