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.

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. Magnetotactic bacteria produce intracellular linear chains of nano-sized magnetic organelles called magnetosomes (1). Each magnetosome consists of a magnetite (Fe 3 O 4 ) or greigite (Fe 3 S 4 ) crystal enveloped by a lipid bilayer membrane that contains magnetosome-specific proteins, some of which are responsible for the biomineralization process (1). The biomineralization of magnetosomes is a highly controlled process regulated at the gene level that results in high-purity, single-magnetic-domain particles with defined crystallographic properties and narrow size distributions (1). Because of their unique characteristics, magnetosomes have a great potential for biotechnological applications. In fact, magnetosomes have been used in the immobilization of biological molecules such as enzymes, antibodies, and nucleic acids (2). Moreover, the production of functionalized magnetosomes by the expression of different proteins on or in the magnetosome membrane is among the most promising immobilization approaches for biotechnological use (3) because it combines biologically active macromolecules with the relatively smooth surface of the nano-sized magnetic crystal of the magnetosome. In contrast, the use of nonbiological magnetic carriers with membranes and proteins is a challenge that remains to be met satisfactorily (4).Thus far, biotechnological studies involving magnetosomes have been focused on a very limited number of strains of the genus Magnetospirillum, mostly Magnetospirillum gryphiswaldense strain MSR-1 and Magnetospirillum magneticum strain AMB-1 (5, 6), both of which biomineralize cuboctahedral crystals of magnetite. Little information exists on other cultivated magnetotactic strains like the magnetotactic vibrio Magnetovibrio blakemorei strain MV-1, which produces chains of elongated prismatic magnetosomes (7). Size and shape are important parameters when designing nanoparticles in numerous biomedical applications like drug delivery because the dimensional properties (aspect ratio) of the nanoparticles affect fluid dynamics, retention times, and internalization by cells (8). The magnetosomes of M. blakemorei are a promising alternative in biotechnology applications such as the immobilization of macromolecules because (i) they have an aspect ratio different from that of cuboctahedral magnetosomes, (ii) they intrinsically contain more magnetite than cuboctahedral magnetosomes because of their size (length, ϳ50 nm; width, ϳ40 nm; 40 nm for Magnetospirillum magnetosomes), and (iii) they have a larger surface volume available for immobilization. Thus, M. blakemorei should be an excellent candidate for the development of high-yield cultivation strategies aimed at potential applications of elongat...

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.

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 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 <55–56% and <64–65% as thresholds for the separation of families and genera, respectively, within the order 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.

20Magnetotactic bacteria (MTB) belong to different taxonomic groups according to 16S 21 rRNA or whole-genome phylogeny. Magnetotactic representatives of the class 22 Alphaproteobacteria and the order Magnetococcales are the most frequently isolated 23 MTB in environmental samples. This bias is due in part to limitations of currently 24 2 available methods to isolate MTB. Here we describe a new approach for isolation of 25 MTB cells that does not depend on cell motility and will allow collecting bacteria 26 both south-and north-seeking movement. We also designed a specific primer system 27 for the gene encoding the MamK protein that effectively detects diverse MTB 28 phylogenetic groups in any sample type. The combination of these two approaches 29 allowed the identification of a novel MTB belonging to the family Syntrophaceae of 30 the class Deltaproteobacteria. Moreover, we found that Nitrospirae bacteria 31 predominated in the MTB fraction of a sample taken from Lake Beloe Bordukovskoe 32 near Moscow, Russia. We describe the novel dominant Nitrospirae bacterium 33 'Candidatus Magnetomonas plexicatena' and propose its taxonomic name. 34 IMPORTANCE 35Among magnetotactic bacteria (MTB), the members of phyla Proteobacteria, 36 Nitrospirae and 'Ca. Omnitrophica' have been studied extensively using the existing 37 approaches. However, in recent years, analyses of the metagenomic databases have 38 revealed the presence of MTB in phylogenetic groups, which had not been previously 39 detected using standard approaches. This finding indicates that the biodiversity of 40 MTB is much broader than is currently known. The difficulty of identifying MTB 41 based on comparative analysis of 16S rRNA genes lies in the existence of closely 42 related species of non-magnetotactic bacteria. Moreover, there is an absence of 16S 43 rRNA MTB sequences from such taxonomic groups as 'Latescibacteria' and 44 Planctomycetes. In addition, the standard methods of separating MTB can benefit 45 bacteria with high motility. Developing novel strategies for investigation offers great 46 3 promise towards identifying MTB groups. We have proposed new approach to 47 separate MTB cells from environmental samples and have also proposed a specific 48 primer system for the MTB identification. 49 INTRODUCTION 50 Prokaryotes having directed active movement that is guided by geomagnetism are 51 collectively called magnetotactic bacteria (MTB) (1). The term MTB has no 52 taxonomic meaning such that it representatives are physiologically, morphologically 53 and phylogenetically different and share only the ability to synthesize special 54 organelles called magnetosomes. Magnetosomes consist of nanosized magnetite 55 (Fe 3 O 4 ) (2) or greigite (Fe 3 S 4 ) (3-5) crystals surrounded by a lipid bilayer membrane 56having proteins specific to the organelle (6, 7). Magnetosomes frequently assemble 57 into chains inside the cell (8). MTB evolved the ability to conduct a special type of 58 movement called magnetotaxis, which is based on orientation relative...

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.