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Magnetotactic bacteria are widely represented microorganisms that have the ability to synthesize magnetosomes. The magnetotactic cocci of the order Magnetococcales are the most frequently identified, but their classification remains unclear due to the low number of cultivated representatives. This paper reports the analysis of an uncultivated magnetotactic coccus UR-1 collected from the Uda River (in eastern Siberia). Genome analyses of this bacterium and comparison to the available Magnetococcales genomes identified a novel species called "Ca. Magnetaquicoccus inordinatus," and a delineated candidate family "Ca. Magnetaquicoccaceae" within the order Magnetococcales is proposed. We used average amino acid identity values Magnetococcales. Analyses of the genome sequence of UR-1 revealed a potential ability for a chemolithoautotrophic lifestyle, with the oxidation of a reduced sulfur compound and carbon assimilation by rTCA. A nearly complete magnetosome genome island, containing a set of mam and mms genes, was also identified. Further comparative analyses of the magnetosome genes showed vertical inheritance as well as horizontal gene transfer as the evolutionary drivers of magnetosome biomineralization genes in strains of the order Magnetococcales.

Magnetotactic bacteria biomineralize intracellular magnetic nanocrystals surrounded by a lipid bilayer called magnetosomes. Due to their unique characteristics, magnetite magnetosomes are promising tools in Biomedicine. However, the uptake, persistence, and accumulation of magnetosomes within mammalian cells have not been well studied. Here, the endocytic pathway of magnetite magnetosomes and their effects on human cervix epithelial (HeLa) cells were studied by electron microscopy and high spatial resolution nano-analysis techniques. Transmission electron microscopy of HeLa cells after incubation with purified magnetosomes showed the presence of magnetic nanoparticles inside or outside endosomes within the cell, which suggests different modes of internalization, and that these structures persisted beyond 120 h after internalization. High-resolution transmission electron microscopy and electron energy loss spectra of internalized magnetosome crystals showed no structural or chemical changes in these structures. Although crystal morphology was preserved, iron oxide crystalline particles of approximately 5 nm near internalized magnetosomes suggests that minor degradation of the original mineral structures might occur. Cytotoxicity and microscopy analysis showed that magnetosomes did not result in any apparent effect on HeLa cells viability or morphology. Based on our results, magnetosomes have significant biocompatibility with mammalian cells and thus have great potential in medical, biotechnological applications.

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.

Acanthamoeba castellanii (Ac) are ubiquitously distributed in nature, and by contaminating medical devices such as heart valves and contact lenses, they cause a broad range of clinical presentations to humans. Although several molecules have been described to play a role in Ac pathogenesis, including parasite host-tissue invasion and escaping of host-defense, little information is available on their mechanisms of secretion. Herein, we describe the molecular components secreted by Ac, under different protein availability conditions to simulate host niches. Ac extracellular vesicles (EVs) were morphologically and biochemically characterized. Dynamic light scattering analysis of Ac EVs identified polydisperse populations, which correlated to electron microscopy measurements. High-performance thin liquid chromatography of Ac EVs identified phospholipids, steryl-esters, sterol and free-fatty acid, the last two also characterized by GC-MS. Secretome composition (EVs and EVs-free supernatants) was also determined and proteins biological functions classified. In peptone-yeast-glucose (PYG) medium, a total of 179 proteins were identified (21 common proteins, 89 exclusive of EVs and 69 in EVs-free supernatant). In glucose alone, 205 proteins were identified (134 in EVs, 14 common and 57 proteins in EVs-free supernatant). From those, stress response, oxidative and protein and amino acid metabolism proteins prevailed. Qualitative differences were observed on carbohydrate metabolism enzymes from Krebs cycle and pentose phosphate shunt. Serine proteases and metalloproteinases predominated. Analysis of the cytotoxicity of Ac EVs (upon uptake) and EVs-free supernatant to epithelial and glioblastoma cells revealed a dose-dependent effect. Therefore, the Ac secretome differs depending on nutrient conditions, and is also likely to vary during infection.

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, for the most part, are free-living, motile, unicellular prokaryotes that inhabit almost all marine and freshwater environments. One notable exception to the unicellular mode, however, are the magnetotactic multicellular prokaryotes. These morphologically unique prokaryotes (e.g., Candidatus Magnetoglobus multicellularis) are motile aggregates of 20-40 genetically identical, Gram-negative cells organised as a sphere (or ovoid in shape) and only motile as a unit. No specific close physical association between magnetotactic bacteria and non-magnetotactic microorganisms has ever been reported. Here, using culture-independent approaches, we show an unusual association between the spherical magnetotactic multicellular prokaryote Ca. Magnetoglobus multicellularis and Pseudoalteromonas species in environmental sediment and water samples collected from the Araruama Lagoon in Brazil. Cells of Pseudoalteromonas species were observed to be physically attached to the surface and, notably, even in the intercellular space of these spherical magnetotactic multicellular prokaryotes. An attempt to correlate the frequency of association between Pseudoalteromonas and magnetotactic multicellular prokaryotes with sediment depth was made but only a slight decrease in the number of Pseudoalteromonas cells per magnetotactic multicellular prokaryote was observed with increasing depth. Similar observations were made with magnetotactic multicellular prokaryotes from another Brazilian Lagoon (Rodrigo de Freitas) and the putative symbiont/parasite was detected. Although our results suggest some sort of specificity in the relationship between these prokaryotes, the precise nature of this association remains unclear.

Scanning transmission X-ray microscopy at the Fe 2p (L2,3), O1s, C1s, and S2p edges was used to study greigite magnetosomes and other cellular content of a magnetotactic bacterium known as a multicellular magnetotactic prokaryote (MMP). X-ray absorption spectrum (XAS) and X-ray magnetic circular dichroism (XMCD) spectra of greigite (Fe3S4) nanoparticles, synthesized via a hydrothermal method, were measured. Although XAS of the synthetic greigite nanoparticles and biotic magnetosome crystals in MMPs are slightly different due to partial oxidation of the MMP greigite, the XMCD spectra of the two materials are in good agreement. The Fe 2p XAS and XMCD spectra of Fe3S4 are quite different from those of its oxygen analog, magnetite (Fe3O4), suggesting Fe3S4 has a different electronic and magnetic structure than Fe3O4 despite having the same crystal structure. Sulfate and sulfide species were also identified in MMPs, both of which are likely involved in sulfur metabolism.

Magnetotactic multicellular prokaryotes (MMPs) consist of unique microorganisms formed by genetically identical Gram-negative bacterial that live as a single individual capable of producing magnetic nano-particles called magnetosomes. Two distinct morphotypes of MMPs are known: spherical MMPs (sMMPs) and ellipsoidal MMPs (eMMPs). sMMPs have been extensively characterized, but less information exists for eMMPs. Here, we report the ultrastructure and organization as well as gene clusters responsible for magnetosome and flagella biosynthesis in the magnetite magnetosome producer eMMP Candidatus Magnetananas rongchenensis. Transmission electron microscopy and focused ion beam scanning electron microscopy (FIB-SEM) 3D reconstruction reveal that cells with a conspicuous core-periphery polarity were organized around a central space. Magnetosomes were organized in multiple chains aligned along the periphery of each cell. In the partially sequenced genome, magnetite-related mamAB gene and mad gene clusters were identified. Two cell morphologies were detected: irregular elliptical conical 'frustum-like' (IECF) cells and H-shaped cells. IECF cells merge to form H-shaped cells indicating a more complex structure and possibly a distinct evolutionary position of eMMPs when compared with sMMPs considering multicellularity.

The hologenome theory of evolution (HTE), which is under fierce debate, presupposes that parts of the microbiome are transmitted from one generation to the next [vertical transmission (VT)], which may also influence the evolution of the holobiont. Even though bacteria have previously been described in early life stages of corals, these early life stages (larvae) could have been inoculated in the water and not inside the parental colony (through gametes) carrying the parental microbiome. How Symbiodinium is transmitted to offspring is also not clear, as only one study has described this mechanism in spawners. All other studies refer to incubators. To explore the VT hypothesis and the key components being transferred, colonies of the broadcast spawner species Mussismilia hispida were kept in nurseries until spawning. Gamete bundles, larvae and adult corals were analyzed to identify their associated microbiota with respect to composition and location. Symbiodinium and bacteria were detected by sequencing in gametes and coral planula larvae. However, no cells were detected using microscopy at the gamete stage, which could be related to the absence of those cells inside the oocytes/dispersed in the mucus or to a low resolution of our approach. A preliminary survey of Symbiodinium diversity indicated that parental colonies harbored Symbiodinium clades B, C and G, whereas only clade B was found in oocytes and planula larvae [5 days after fertilization (a.f.)]. The core bacterial populations found in the bundles, planula larvae and parental colonies were identified as members of the genera Burkholderia, Pseudomonas, Acinetobacter, Ralstonia, Inquilinus and Bacillus, suggesting that these populations could be vertically transferred through the mucus. The collective data suggest that spawner corals, such as M. hispida, can transmit Symbiodinium cells and the bacterial core to their offspring by a coral gamete (and that this gamete, with its bacterial load, is released into the water), supporting the HTE. However, more data are required to indicate the stability of the transmitted populations to indicate whether the holobiont can be considered a unit of natural selection or a symbiotic assemblage of independently evolving organisms.

BackgroundMagnetotactic bacteria (MTB) are a unique group of prokaryotes that have a potentially high impact on global geochemical cycling of significant primary elements because of their metabolic plasticity and the ability to biomineralize iron-rich magnetic particles called magnetosomes. Understanding the genetic composition of the few cultivated MTB along with the unique morphological features of this group of bacteria may provide an important framework for discerning their potential biogeochemical roles in natural environments.ResultsGenomic and ultrastructural analyses were combined to characterize the cultivated magnetotactic coccus Magnetofaba australis strain IT-1. Cells of this species synthesize a single chain of elongated, cuboctahedral magnetite (Fe3O4) magnetosomes that cause them to align along magnetic field lines while they swim being propelled by two bundles of flagella at velocities up to 300 μm s-1. High-speed microscopy imaging showed the cells move in a straight line rather than in the helical trajectory described for other magnetotactic cocci. Specific genes within the genome of Mf. australis strain IT-1 suggest the strain is capable of nitrogen fixation, sulfur reduction and oxidation, synthesis of intracellular polyphosphate granules and transporting iron with low and high affinity. Mf. australis strain IT-1 and Magnetococcus marinus strain MC-1 are closely related phylogenetically although similarity values between their homologous proteins are not very high.ConclusionMf. australis strain IT-1 inhabits a constantly changing environment and its complete genome sequence reveals a great metabolic plasticity to deal with these changes. Aside from its chemoautotrophic and chemoheterotrophic metabolism, genomic data indicate the cells are capable of nitrogen fixation, possess high and low affinity iron transporters, and might be capable of reducing and oxidizing a number of sulfur compounds. The relatively large number of genes encoding transporters as well as chemotaxis receptors in the genome of Mf. australis strain IT-1 combined with its rapid swimming velocities, indicate that cells respond rapidly to environmental changes.

UnlabelledMagnetotactic bacteria (MTB) comprise a phylogenetically diverse group of prokaryotes capable of orienting and navigating along magnetic field lines. Under oxic conditions, MTB in natural environments in the Northern Hemisphere generally display north-seeking (NS) polarity, swimming parallel to the Earth's magnetic field lines, while those in the Southern Hemisphere generally swim antiparallel to magnetic field lines (south-seeking [SS] polarity). Here, we report a population of an uncultured, monotrichously flagellated, and vibrioid MTB collected from a brackish lagoon in Brazil in the Southern Hemisphere that consistently exhibits NS polarity. Cells of this organism were mainly located below the oxic-anoxic interface (OAI), suggesting it is capable of some type of anaerobic metabolism. Magnetosome crystalline habit and composition were consistent with elongated prismatic magnetite (Fe3O4) particles. Phylogenetic analysis based on 16S rRNA gene sequencing indicated that this organism belongs to a distinct clade of the Gammaproteobacteria class. The presence of NS MTB in the Southern Hemisphere and the previously reported finding of SS MTB in the Northern Hemisphere reinforce the idea that magnetotaxis is more complex than we currently understand and may be modulated by factors other than O2 concentration and redox gradients in sediments and water columns.ImportanceMagnetotaxis is a navigational mechanism used by magnetotactic bacteria to move along geomagnetic field lines and find an optimal position in chemically stratified sediments. For that, magnetotactic bacteria swim parallel to the geomagnetic field lines under oxic conditions in the Northern Hemisphere, whereas those in the Southern Hemisphere swim antiparallel to magnetic field lines. A population of uncultured vibrioid magnetotactic bacteria was discovered in a brackish lagoon in the Southern Hemisphere that consistently swim northward, i.e., the opposite of the overwhelming majority of other Southern Hemisphere magnetotactic bacteria. This finding supports the idea that magnetotaxis is more complex than previously thought.

Magnetotactic bacteria (MTB) are a heterogeneous group of ubiquitous aquatic microorganisms capable of biomineralizing nano-sized, membrane-bound, magnetic iron-rich mineral particles called magnetosomes. MTB are found in chemically-stratified aquatic sediments and/or water columns with a wide range of salinities, moderate to high temperatures, and pH varying from neutral to strongly alkaline. MTB from very cold environments have not been investigated to any great degree and here we characterize MTB from the low temperature Antarctic maritime region. Sediment samples were collected at nine sampling sites within Admiralty Bay, King George Island (62°23'S 58°27'W) from 2009 to 2013. Samples from five sites contained MTB and those from two of these sites contained large number of magnetotactic cocci that were studied using electron microscopy and molecular techniques. The magnetotactic cocci contained magnetosomes either arranged as two or four chains or as a disorganized cluster. The crystalline habit and composition of all magnetosomes analyzed with high-resolution transmission electron microscopy and energy dispersive X-ray microanalysis were consistent with elongated prismatic crystals of magnetite (Fe3 O4 ). The retrieved 16S rRNA gene sequences from magnetically-enriched magnetotactic cocci clustered into three distinct groups affiliated with the Alphaproteobacteria class of the Proteobacteria. Novel sequences of each phylogenetic cluster were confirmed using fluorescent in situ hybridization. Metagenomic data analysis of magnetically-enriched magnetotactic cocci revealed the presence of mam genes and MTB-specific hypothetical protein coding genes. Sequence homology and phylogenetic analysis indicated that predicted proteins are related to those of cultivated alphaproteobacterial MTB. The consistent and continuous low temperature of the sediment where the magnetotactic cocci are present (always below 1°C) suggests that these MTB from maritime Antarctica are psychrophiles. Moreover, similar morphotypes and 16S gene sequences were retrieved from samples collected from different sites from maritime Antarctica for several years suggesting that these new strains of MTB are indigenous members of Antarctic microbiota.

Candidatus Magnetoglobus multicellularis is an uncultured magnetotactic multicellular prokaryote composed of 17-40 Gram-negative cells that are capable of synthesizing organelles known as magnetosomes. The magnetosomes of Ca. M. multicellularis are composed of greigite and are organized in chains that are responsible for the microorganism's orientation along magnetic field lines. The characteristics of the microorganism, including its multicellular life cycle, magnetic field orientation, and swimming behavior, and the lack of viability of individual cells detached from the whole assembly, are considered strong evidence for the existence of a unique multicellular life cycle among prokaryotes. It has been proposed that the position of each cell within the aggregate is fundamental for the maintenance of its distinctive morphology and magnetic field orientation. However, the cellular organization of the whole organism has never been studied in detail. Here, we investigated the magnetosome organization within a cell, its distribution within the microorganism, and the intercellular relationships that might be responsible for maintaining the cells in the proper position within the microorganism, which is essential for determining the magnetic properties of Ca. M. multicellularis during its life cycle. The results indicate that cellular interactions are essential for the determination of individual cell shape and the magnetic properties of the organism and are likely directly associated with the morphological changes that occur during the multicellular life cycle of this species.

The growth and magnetosome production of the marine magnetotactic vibrio Magnetovibrio blakemorei strain MV-1 were optimized through a statistics-based experimental factorial design. In the optimized growth medium, maximum magnetite yields of 64.3 mg/liter in batch cultures and 26 mg/liter in a bioreactor were obtained.