A Method for Producing Highly Pure Magnetosomes in Large Quantity for Medical Applications Using Magnetospirillum gryphiswaldense MSR-1 Magnetotactic Bacteria Amplified in Minimal Growth Media

Purification, Characterization, and Assessment of Anticancer Activity of Iron Oxide Nanoparticles Biosynthesized by Novel Thermophilic Bacillus tequilensis ASFS1‏

Magnetic nanoparticles (MNPs), particularly iron oxide nanoparticles (IONPs), are a fascinating group of nanoparticles that have been considerably investigated for biomedical applications because of their superparamagnetic properties, biodegradable nature, and biocompatibility. A novel Gram-positive moderately thermophilic bacterial strain, namely Bacillus tequilensis ASFS.1, was isolated and identified. This strain is capable of producing superparamagnetic Fe3O4 nanoparticles and exhibiting magnetotaxis behavior. This strain swimming behavior was investigated under static and dynamic environments, where it behaved very much similar to the magnetotaxis in magnetotactic bacteria. This study is the first report of a bacterium from the Bacillaceae family that has the potential to intracellular biosynthesis of IONPs. MNPs were separated by a magnetic and reproducible method which was designed for the first time for this study. In addition, UV-visible spectrophotometer, Fourier-transform infrared spectroscopy, vibrating sample magnetometer, field emission scanning electron microscopy (FESEM), X-ray diffraction, and thermal gravimetric analysis were utilized to characterize the bio-fabricated magnetite nanoparticles. Analysis of the particle size distribution pattern of the biogenic MNPs by FESEM imaging revealed the size range of 10-100 nm with the size range of 10-40 nm MNPs being the most frequent particles. VSM analysis demonstrated that biogenic MNPs displayed superparamagnetic properties with a high saturation magnetization value of 184 emu/g. After 24 h treatment of 3T3, U87, A549, MCF-7, and HT-29 cell lines with the biogenic MNPs, IC50 values were measured to be 339, 641, 582, 149, and 184 μg mL-1, respectively. This study presents the novel strain ASFS.1 capable of magnetotaxis by the aid of its magnetite nanoparticles and paving information on isolation, characterization, and in vitro cytotoxicity of its MNPs. The MNPs showed promising potential for biomedical applications, obviously subject to additional studies.

Medium optimization to improve growth and iron uptake by Bacillus tequilensis ASFS1 using fractional factorial designs

Many notable applications have been described for magnetic nanoparticles in delivery of diverse drugs and bioactive compounds into cells, magnetofection for the treatment of cancer, photodynamic therapy, photothermal therapy, and magnetic particle imaging (MPI). In response to the growing demand for magnetic nanoparticles for drug delivery or biomedical imaging applications, more effective and eco-friendly methodologies are required for large-scale biosynthesis of this nanoparticles. The major challenge in the large-scale biomedical application of magnetic nanoparticles lies in its low efficiency and optimization of nanoparticle production can address this issue. In the current study, a prediction model is suggested by the fractional factorial designs. The present study aims to optimize culture media components for improved growth and iron uptake of this strain. The result of optimization for iron uptake by the strain ASFS1 is to increase the production of magnetic nanoparticles by this strain for biomedical applications in the future. In the present study, design of experiment method was used to probe the effects of some key medium components (yeast extract, tryptone, FeSO4, Na2-EDTA, and FeCl3) on Fe content in biomass and dried biomass of strain ASFS1. A 25-1 fractional factorial design showed that Na2-EDTA, FeCl3, yeast extract-tryptone interaction, and FeSO4-Na2-EDTA interaction were the most parameters on Fe content in biomass within the experimented levels (p < 0.05), while yeast extract, FeCl3, and yeast extract-tryptone interaction were the most significant factors within the experimented levels (p < 0.05) to effect on dried biomass of strain ASFS1. The optimum culture media components for the magnetic nanoparticles production by strain ASFS1 was reported to be 7.95 g L-1 of yeast extract, 5 g L-1 of tryptone, 75 μg mL-1 of FeSO4, 192.3 μg mL-1 of Na2-EDTA and 150 μg mL-1 of FeCl3 which was theoretically able to produce Fe content in biomass (158 μg mL-1) and dried biomass (2.59 mg mL-1) based on the obtained for medium optimization. Using these culture media components an experimental maximum Fe content in biomass (139 ± 13 μg mL-1) and dried biomass (2.2 ± 0.2 mg mL-1) was obtained, confirming the efficiency of the used method.

Elucidating the Bioinspired Synthesis Process of Magnetosomes-Like Fe3O4 Nanoparticles

Iron oxide nanoparticles are a kind of important biomedical nanomaterials. Although their industrial-scale production can be realized by the conventional coprecipitation method, the controllability of their size and morphology remains a huge challenge. In this study, a kind of synthetic polypeptide Mms6-28 which mimics the magnetosome protein Mms6 is used for the bioinspired synthesis of Fe3O4 nanoparticles (NPs). Magnetosomes-like Fe3O4 NPs with uniform size, cubooctahedral shape, and smooth crystal surfaces are synthesized via a partial oxidation process. The Mms6-28 polypeptides play an important role by binding with iron ions and forming nucleation templates and are also preferably attached to the [100] and [111] crystal planes to induce the formation of uniform cubooctahedral Fe3O4 NPs. The continuous release and oxidation of Fe2+ from pre-formed Fe2+-rich precursors within the Mms6-28-based template make the reaction much controllable. The study affords new insights into the bioinspired- and bio-synthesis mechanism of magnetosomes.

Set-up of a pharmaceutical cell bank of Magnetospirillum gryphiswaldense MSR1 magnetotactic bacteria producing highly pure magnetosomes

We report the successful fabrication of a pharmaceutical cellular bank (PCB) containing magnetotactic bacteria (MTB), which belong to the Magnetospirillum gryphiswaldense MSR1 species. To produce such PCB, we amplified MTB in a minimal growth medium essentially devoid of other heavy metals than iron and of CMR (Carcinogenic, mutagenic and reprotoxic) products. The PCB enabled to acclimate MTB to such minimal growth conditions and then to produce highly pure magnetosomes composed of more than 99.9% of iron. The qualification of the bank as a PCB relies first on a preserved identity of the MTB compared with the original strain, second on genetic bacterial stability observed over 100 generations or under cryo-preservation for 16 months, third on a high level of purity highlighted by an absence of contaminating microorganisms in the PCB. Furthermore, the PCB was prepared under high-cell load conditions (9.108 cells/mL), allowing large-scale bacterial amplification and magnetosome production. In the future, the PCB could therefore be considered for commercial as well as research orientated applications in nanomedicine. We describe for the first-time conditions for setting-up an effective pharmaceutical cellular bank preserving over time the ability of certain specific cells, i.e. Magnetospirillum gryphiswaldense MSR1 MTB, to produce nano-minerals, i.e. magnetosomes, within a pharmaceutical setting.

Biomedical applications of magnetosomes: State of the art and perspectives

Magnetosomes, synthesized by magnetotactic bacteria (MTB), have been used in nano- and biotechnological applications, owing to their unique properties such as superparamagnetism, uniform size distribution, excellent bioavailability, and easily modifiable functional groups. In this review, we first discuss the mechanisms of magnetosome formation and describe various modification methods. Subsequently, we focus on presenting the biomedical advancements of bacterial magnetosomes in biomedical imaging, drug delivery, anticancer therapy, biosensor. Finally, we discuss future applications and challenges. This review summarizes the application of magnetosomes in the biomedical field, highlighting the latest advancements and exploring the future development of magnetosomes.

Evaluation of cell disruption technologies on magnetosome chain length and aggregation behaviour from Magnetospirillum gryphiswaldense MSR-1

Magnetosomes are biologically-derived magnetic nanoparticles (MNPs) naturally produced by magnetotactic bacteria (MTB). Due to their distinctive characteristics, such as narrow size distribution and high biocompatibility, magnetosomes represent an attractive alternative to existing commercially-available chemically-synthesized MNPs. However, to extract magnetosomes from the bacteria, a cell disruption step is required. In this study, a systematic comparison between three disruption techniques (enzymatic treatment, probe sonication and high-pressure homogenization) was carried out to study their effect on the chain length, integrity and aggregation state of magnetosomes isolated from Magnetospirillum gryphiswaldense MSR-1 cells. Experimental results revealed that all three methodologies show high cell disruption yields (>89%). Transmission electron microscopy (TEM), dynamic light scattering (DLS) and, for the first time, nano-flow cytometry (nFCM) were employed to characterize magnetosome preparations after purification. TEM and DLS showed that high-pressure homogenization resulted in optimal conservation of chain integrity, whereas enzymatic treatment caused higher chain cleavage. The data obtained suggest that nFCM is best suited to characterize single membrane-wrapped magnetosomes, which can be particularly useful for applications that require the use of individual magnetosomes. Magnetosomes were also successfully labelled (>90%) with the fluorescent CellMask™ Deep Red membrane stain and analysed by nFCM, demonstrating the promising capacity of this technique as a rapid analytical tool for magnetosome quality assurance. The results of this work contribute to the future development of a robust magnetosome production platform.

Non-pyrogenic highly pure magnetosomes for efficient hyperthermia treatment of prostate cancer

We report the fabrication of highly pure magnetosomes that are synthesized by magnetotactic bacteria (MTB) using pharmaceutically compatible growth media, i.e., without compounds of animal origin (yeast extracts), carcinogenic, mutagenic, or toxic for reproduction (CMR) products, and other heavy metals than iron. To enable magnetosome medical applications, these growth media are reduced and amended compared with media commonly used to grow these bacteria. Furthermore, magnetosomes are made non-pyrogenic by being extracted from these micro-organisms and heated above 400 °C to remove and denature bacterial organic material and produce inorganic magnetosome minerals. To be stabilized, these minerals are further coated with citric acid to yield M-CA, leading to fully reconstructed chains of magnetosomes. The heating properties and anti-tumor activity of highly pure M-CA are then studied by bringing M-CA into contact with PC3-Luc tumor cells and by exposing such assembly to an alternating magnetic field (AMF) of 42 mT and 195 kHz during 30 min. While in the absence of AMF, M-CA are observed to be non-cytotoxic, they result in a 35% decrease in cell viability following AMF application. The treatment efficacy can be associated with a specific absorption rate (SAR) value of M-CA, which is relatively high in cellular environment, i.e., SAR_cell = 253 ± 11 W/g_Fe, while being lower than the M-CA SAR value measured in water, i.e., SAR_water = 1025 ± 194 W/g_Fe, highlighting that a reduction in the Brownian contribution to the SAR value in cellular environment does not prevent efficient tumor cell destruction with these nanoparticles. KEY POINTS : • Highly pure magnetosomes were produced in pharmaceutically compatible growth media • Non-pyrogenic and stable magnetosomes were prepared for human injection • Magnetosomes efficiently destroyed prostate tumor cells in magnetic hyperthermia.

Large-Scale Cultivation of Magnetotactic Bacteria and the Optimism for Sustainable and Cheap Approaches in Nanotechnology

Magnetotactic bacteria (MTB), a diverse group of marine and freshwater microorganisms, have attracted the scientific community's attention since their discovery. These bacteria biomineralize ferrimagnetic nanocrystals, the magnetosomes, or biological magnetic nanoparticles (BMNs), in a single or multiple chain(s) within the cell. As a result, cells experience an optimized magnetic dipolar moment responsible for a passive alignment along the lines of the geomagnetic field. Advances in MTB cultivation and BMN isolation have contributed to the expansion of the biotechnological potential of MTB in recent decades. Several studies with mass-cultured MTB expanded the possibilities of using purified nanocrystals and whole cells in nano- and biotechnology. Freshwater MTB were primarily investigated in scaling up processes for the production of BMNs. However, marine MTB have the potential to overcome freshwater species applications due to the putative high efficiency of their BMNs in capturing molecules. Regarding the use of MTB or BMNs in different approaches, the application of BMNs in biomedicine remains the focus of most studies, but their application is not restricted to this field. In recent years, environment monitoring and recovery, engineering applications, wastewater treatment, and industrial processes have benefited from MTB-based biotechnologies. This review explores the advances in MTB large-scale cultivation and the consequent development of innovative tools or processes.

Defining Local Chemical Conditions in Magnetosomes of Magnetotactic Bacteria

Defining chemical properties of intracellular organelles is necessary to determine their function(s) as well as understand and mimic the reactions they host. However, the small size of bacterial and archaeal microorganisms often prevents defining local intracellular chemical conditions in a similar way to what has been established for eukaryotic organelles. This work proposes to use magnetite (Fe₃O₄) nanocrystals contained in magnetosome organelles of magnetotactic bacteria as reporters of elemental composition, pH, and redox potential of a hypothetical environment at the site of formation of intracellular magnetite. This methodology requires combining recent single-cell mass spectrometry measurements together with elemental composition of magnetite in trace and minor elements. It enables a quantitative characterization of chemical disequilibria of 30 chemical elements between the intracellular and external media of magnetotactic bacteria, revealing strong transfers of elements with active influx or efflux processes that translate into elemental accumulation (Mo, Se, and Sn) or depletion (Sr and Bi) in the bacterial internal medium of up to seven orders of magnitude relative to the extracellular medium. Using this concept, we show that chemical conditions in magnetosomes are compatible with a pH of 7.5-9.5 and a redox potential of -0.25 to -0.6 V.

Parallel Manipulation and Flexible Assembly of Micro-Spiral via Optoelectronic Tweezers

Micro-spiral has a wide range of applications in smart materials, such as drug delivery, deformable materials, and micro-scale electronic devices by utilizing the manipulation of electric fields, magnetic fields, and flow fields. However, it is incredibly challenging to achieve a massively parallel manipulation of the micro-spiral to form a particular microstructure in these conventional methods. Here, a simple method is reported for assembling micro-spirals into various microstructures via optoelectronic tweezers (OETs), which can accurately manipulate the micro-/bio-particles by projecting light patterns. The manipulation force of micro-spiral is analyzed and simulated first by the finite element simulation. When the micro-spiral lies at the bottom of the microfluidic chip, it can be translated or rotated toward the target position by applying control forces simultaneously at multiple locations on the long axis of the micro-spiral. Through the OET manipulation, the length of the micro-spiral chain can reach 806.45 μm. Moreover, the different parallel manipulation modes are achieved by utilizing multiple light spots. The results show that the micro-spirulina can be manipulated by a real-time local light pattern and be flexibly assembled into design microstructures by OETs, such as a T-shape circuit, link lever, and micro-coil pairs of devices. This assembly method using OETs has promising potential in fabricating innovative materials and microdevices for practical engineering applications.

Advances with metal oxide-based nanoparticles as MDR metastatic breast cancer therapeutics and diagnostics

Metal oxide nanoparticles have attracted increased attention due to their emerging applications in cancer detection and therapy. This study envisioned to highlight the great potential of metal oxide NPs due to their interesting properties including high payload, response to magnetic field, affluence of surface modification to overcome biological barriers, and biocompatibility. Mammogram, ultrasound, X-ray computed tomography (CT), MRI, positron emission tomography (PET), optical or fluorescence imaging are used for breast imaging. Drug-loaded metal oxide nanoparticle delivered to the breast cancer cells leads to higher drug uptake. Thus, enhanced the cytotoxicity to target cells compared to free drug. The drug loaded metal oxide nanoparticle formulations hold great promise to enhance efficacy of breast cancer therapy including multidrug resistant (MDR) and metastatic breast cancers. Various metal oxides including magnetic metal oxides and magnetosomes are of current interests to explore cancer drug delivery and diagnostic efficacy especially for metastatic breast cancer. Metal oxide-based nanocarrier formulations are promising for their usage in drug delivery and release to breast cancer cells, cancer diagnosis and their clinical translations.

Biomarker targeted therapy approaches for TNBC using metal oxide-based NPs are highly effective and promising.

Why Does Not Nanotechnology Go Green? Bioprocess Simulation and Economics for Bacterial-Origin Magnetite Nanoparticles

Nanotechnological developments, including fabrication and use of magnetic nanomaterials, are growing at a fast pace. Magnetic nanoparticles are exciting tools for use in healthcare, biological sensors, and environmental remediation. Due to better control over final-product characteristics and cleaner production, biogenic nanomagnets are preferable over synthetic ones for technological use. In this sense, the technical requirements and economic factors for setting up industrial production of magnetotactic bacteria (MTB)-derived nanomagnets were studied in the present work. Magnetite fabrication costs in a single-stage fed-batch and a semicontinuous process were US$ 10,372 and US$ 11,169 per kilogram, respectively. Depending on the variations of the production process, the minimum selling price for biogenic nanomagnets ranged between US$ 21 and US$ 120 per gram. Because these prices are consistently below commercial values for synthetic nanoparticles, we suggest that microbial production is competitive and constitutes an attractive alternative for a greener manufacturing of magnetic nanoparticles nanotools with versatile applicability.

Bioinspired Magnetic Nanochains for Medicine

Superparamagnetic iron oxide nanoparticles (SPIONs) have been widely used for medicine, both in therapy and diagnosis. Their guided assembly into anisotropic structures, such as nanochains, has recently opened new research avenues; for instance, targeted drug delivery. Interestingly, magnetic nanochains do occur in nature, and they are thought to be involved in the navigation and geographic orientation of a variety of animals and bacteria, although many open questions on their formation and functioning remain. In this review, we will analyze what is known about the natural formation of magnetic nanochains, as well as the synthetic protocols to produce them in the laboratory, to conclude with an overview of medical applications and an outlook on future opportunities in this exciting research field.

High-Yield Production, Characterization, and Functionalization of Recombinant Magnetosomes in the Synthetic Bacterium Rhodospirillum rubrum "magneticum"

Recently, the photosynthetic Rhodospirillum rubrum has been endowed with the ability of magnetosome biosynthesis by transfer and expression of biosynthetic gene clusters from the magnetotactic bacterium Magnetospirillum gryphiswaldense. However, the growth conditions for efficient magnetite biomineralization in the synthetic R. rubrum "magneticum", as well as the particles themselves (i.e., structure and composition), have so far not been fully characterized. In this study, different cultivation strategies, particularly the influence of temperature and light intensity, are systematically investigated to achieve optimal magnetosome biosynthesis. Reduced temperatures ≤16 °C and gradual increase in light intensities favor magnetite biomineralization at high rates, suggesting that magnetosome formation might utilize cellular processes, cofactors, and/or pathways that are linked to photosynthetic growth. Magnetosome yields of up to 13.6 mg magnetite per liter cell culture are obtained upon photoheterotrophic large-scale cultivation. Furthermore, it is shown that even more complex, i.e., oligomeric, catalytically active functional moieties like enzyme proteins can be efficiently expressed on the magnetosome surface, thereby enabling the in vivo functionalization by genetic engineering. In summary, it is demonstrated that the synthetic R. rubrum "magneticum" is a suitable host for high-yield magnetosome biosynthesis and the sustainable production of genetically engineered, bioconjugated magnetosomes.

Evaluation of an oxygen-dependent self-inducible surfactin synthesis in B. subtilis by substitution of native promoter PsrfA by anaerobically active PnarG and PnasD

A novel approach targeting self-inducible surfactin synthesis under oxygen-limited conditions is presented. Because both the nitrate (NarGHI) and nitrite (NasDE) reductase are highly expressed during anaerobic growth of B. subtilis, the native promoter PsrfA of the surfactin operon in strain B. subtilis JABs24 was replaced by promoters PnarG and PnasD to induce surfactin synthesis anaerobically. Shake flask cultivations with varying oxygen availabilities indicated no significant differences in native PsrfA expression. As hypothesized, activity of PnarG and PnasD increased with lower oxygen levels and surfactin was not produced by PsrfA::PnarG as well as PsrfA::PnasD mutant strains under conditions with highest oxygen availability. PnarG showed expressions similar to PsrfA at lowest oxygen availability, while maximum value of PnasD was more than 5.5-fold higher. Although the promoter exchange PsrfA::PnarG resulted in a decreased surfactin titer at lowest oxygen availability, the strain carrying PsrfA::PnasD reached a 1.4-fold increased surfactin concentration with 696 mg/L and revealed an exceptional high overall YP/X of 1.007 g/g. This value also surpassed the YP/X of the reference strain JABs24 at highest and moderate oxygen availability. Bioreactor cultivations illustrated that significant cell lysis occurred when the process of "anaerobization" was performed too fast. However, processes with a constantly low agitation and aeration rate showed promising potential for process improvement, especially by employing the strain carrying PsrfA::PnasD promoter exchange. Additionally, replacement of other native promoters by nitrite reductase promoter PnasD represents a promising tool for anaerobic-inducible bioprocesses in Bacillus.

Dissimilatory Iron-Reducing Microorganisms Are Present and Active in the Sediments of the Doce River and Tributaries Impacted by Iron Mine Tailings from the Collapsed Fundão Dam (Mariana, MG, Brazil)

On 5 November 2015, a large tailing deposit failed in Brazil, releasing an estimated 32.6 to 62 million m3 of iron mining tailings into the environment. Tailings from the Fundão Dam flowed down through the Gualaxo do Norte and Carmo riverbeds and floodplains and reached the Doce River. Since then, bottom sediments have become enriched in Fe(III) oxyhydroxides. Dissimilatory iron-reducing microorganisms (DIRMs) are anaerobes able to couple organic matter oxidation to Fe(III) reduction, producing CO2 and Fe(II), which can precipitate as magnetite (FeO·Fe2O3) and other Fe(II) minerals. In this work, we investigated the presence of DIRMs in affected and non-affected bottom sediments of the Gualaxo do Norte and Doce Rivers. The increase in Fe(II) concentrations in culture media over time indicated the presence of Fe(III)-reducing microorganisms in all sediments tested, which could reduce Fe(III) from both tailings and amorphous ferric oxyhydroxide. Half of our enrichment cultures converted amorphous Fe(III) oxyhydroxide into magnetite, which was characterized by X-ray diffraction, transmission electron microscopy, and magnetic measurements. The conversion of solid Fe(III) phases to soluble Fe(II) and/or magnetite is characteristic of DIRM cultures. The presence of DIRMs in the sediments of the Doce River and tributaries points to the possibility of reductive dissolution of goethite (α-FeOOH) and/or hematite (α-Fe2O3) from sediments, along with the consumption of organics, release of trace elements, and impairment of water quality.

An automated oxystat fermentation regime for microoxic cultivation of Magnetospirillum gryphiswaldense

BACKGROUND

Magnetosomes produced by magnetotactic bacteria represent magnetic nanoparticles with unprecedented characteristics. However, their use in many biotechnological applications has so far been hampered by their challenging bioproduction at larger scales.

RESULTS

Here, we developed an oxystat batch fermentation regime for microoxic cultivation of Magnetospirillum gryphiswaldense in a 3 L bioreactor. An automated cascade regulation enabled highly reproducible growth over a wide range of precisely controlled oxygen concentrations (1-95% of air saturation). In addition, consumption of lactate as the carbon source and nitrate as alternative electron acceptor were monitored during cultivation. While nitrate became growth limiting during anaerobic growth, lactate was the growth limiting factor during microoxic cultivation. Analysis of microoxic magnetosome biomineralization by cellular iron content, magnetic response, transmission electron microscopy and small-angle X-ray scattering revealed magnetosomal magnetite crystals were highly uniform in size and shape.

CONCLUSION

The fermentation regime established in this study facilitates stable oxygen control during culturing of Magnetospirillum gryphiswaldense. Further scale-up seems feasible by combining the stable oxygen control with feeding strategies employed in previous studies. Results of this study will facilitate the highly reproducible laboratory-scale bioproduction of magnetosomes for a diverse range of future applications in the fields of biotechnology and biomedicine.

Magnetotactic Bacteria Accumulate a Large Pool of Iron Distinct from Their Magnetite Crystals

Magnetotactic bacteria (MTB) produce iron-based intracellular magnetic crystals. They represent a model system for studying iron homeostasis and biomineralization in microorganisms. MTB sequester a large amount of iron in their crystals and have thus been proposed to significantly impact the iron biogeochemical cycle. Several studies proposed that MTB could also accumulate iron in a reservoir distinct from their crystals. Here, we present a chemical and magnetic methodology for quantifying the iron pools in the magnetotactic strain AMB-1. Results showed that most iron is not contained in crystals. We then adapted protocols for the fluorescent Fe(II) detection in bacteria and showed that iron could be detected outside crystals using fluorescence assays. This work suggests a more complex picture for iron homeostasis in MTB than previously thought. Because iron speciation controls its fate in the environment, our results also provide important insights into the geochemical impact of MTB.

Magnetotactic bacteria (MTB) are ubiquitous aquatic microorganisms that form intracellular nanoparticles of magnetite (Fe₃O₄) or greigite (Fe₃S₄) in a genetically controlled manner. Magnetite and greigite synthesis requires MTB to transport a large amount of iron from the environment. Most intracellular iron was proposed to be contained within the crystals. However, recent mass spectrometry studies suggest that MTB may contain a large amount of iron that is not precipitated in crystals. Here, we attempted to resolve these discrepancies by performing chemical and magnetic assays to quantify the different iron pools in the magnetite-forming strain Magnetospirillum magneticum AMB-1, as well as in mutant strains showing defects in crystal precipitation, cultivated at various iron concentrations. All results show that magnetite represents at most 30% of the total intracellular iron under our experimental conditions and even less in the mutant strains. We further examined the iron speciation and subcellular localization in AMB-1 using the fluorescent indicator FIP-1, which was designed for the detection of labile Fe(II). Staining with this probe suggests that unmineralized reduced iron is found in the cytoplasm and associated with magnetosomes. Our results demonstrate that, under our experimental conditions, AMB-1 is able to accumulate a large pool of iron distinct from magnetite. Finally, we discuss the biochemical and geochemical implications of these results.

IMPORTANCE Magnetotactic bacteria (MTB) produce iron-based intracellular magnetic crystals. They represent a model system for studying iron homeostasis and biomineralization in microorganisms. MTB sequester a large amount of iron in their crystals and have thus been proposed to significantly impact the iron biogeochemical cycle. Several studies proposed that MTB could also accumulate iron in a reservoir distinct from their crystals. Here, we present a chemical and magnetic methodology for quantifying the iron pools in the magnetotactic strain AMB-1. Results showed that most iron is not contained in crystals. We then adapted protocols for the fluorescent Fe(II) detection in bacteria and showed that iron could be detected outside crystals using fluorescence assays. This work suggests a more complex picture for iron homeostasis in MTB than previously thought. Because iron speciation controls its fate in the environment, our results also provide important insights into the geochemical impact of MTB.

Improved methods for mass production of magnetosomes and applications: a review

Magnetotactic bacteria have the unique ability to synthesize magnetosomes (nano-sized magnetite or greigite crystals arranged in chain-like structures) in a variety of shapes and sizes. The chain alignment of magnetosomes enables magnetotactic bacteria to sense and orient themselves along geomagnetic fields. There is steadily increasing demand for magnetosomes in the areas of biotechnology, biomedicine, and environmental protection. Practical difficulties in cultivating magnetotactic bacteria and achieving consistent, high-yield magnetosome production under artificial environmental conditions have presented an obstacle to successful development of magnetosome applications in commercial areas. Here, we review information on magnetosome biosynthesis and strategies for enhancement of bacterial cell growth and magnetosome formation, and implications for improvement of magnetosome yield on a laboratory scale and mass-production (commercial or industrial) scale.

Intrinsically Magnetic Cells: A Review on Their Natural Occurrence and Synthetic Generation

The magnetization of non-magnetic cells has great potential to aid various processes in medicine, but also in bioprocess engineering. Current approaches to magnetize cells with magnetic nanoparticles (MNPs) require cellular uptake or adsorption through in vitro manipulation of cells. A relatively new field of research is "magnetogenetics" which focuses on in vivo production and accumulation of magnetic material. Natural intrinsically magnetic cells (IMCs) produce intracellular, MNPs, and are called magnetotactic bacteria (MTB). In recent years, researchers have unraveled function and structure of numerous proteins from MTB. Furthermore, protein engineering studies on such MTB proteins and other potentially magnetic proteins, like ferritins, highlight that in vivo magnetization of non-magnetic hosts is a thriving field of research. This review summarizes current knowledge on recombinant IMC generation and highlights future steps that can be taken to succeed in transforming non-magnetic cells to IMCs.

Bio-synthesized iron oxide nanoparticles for cancer treatment

Various living organisms, such as bacteria, plants, and animals can synthesize iron oxide nanoparticles (IONP). The mechanism of nanoparticle (NP) formation is usually described as relying on the reduction of ferric/ferrous iron ions into crystallized nanoparticulate iron that is surrounded by an organic stabilizing layer. The properties of these NP are characterized by a composition made of different types of iron oxide whose most stable and purest one appears to be maghemite, by a size predominantly comprised between 5 and 380 nm, by a crystalline core, by a surface charge which depends on the nature of the material coating the iron oxide, and by certain other properties such as a sterility, stability, production in mass, absence of aggregation, that have apparently only been studied in details for IONP synthesized by magnetotactic bacteria, called magnetosomes. In the majority of studies, bio-synthesized IONP are described as being biocompatible and as not inducing cytotoxicity towards healthy cells. Anti-tumor activity of bio-synthesized IONP has mainly been demonstrated in vitro, where this type of NP displayed cytotoxicity towards certain tumor cells, e.g. through the anti-tumor activity of IONP coating or through IONP anti-oxidizing property. Concerning in vivo anti-tumor activity, it was essentially highlighted for magnetosomes administered in different types of glioblastoma tumors (U87-Luc and GL-261), which were exposed to a series of alternating magnetic field applications, resulting in mild hyperthermia treatments at typical temperatures of 41-45 °C, leading to the full disappearance of these tumors without any observable side effects.

Iron-biomineralizing organelle in magnetotactic bacteria: function, synthesis and preservation in ancient rock samples

Magnetotactic bacteria (MTB) are ubiquitous aquatic microorganisms that incorporate iron from their environment to synthesize intracellular nanoparticles of magnetite (Fe₃ O₄ ) or greigite (Fe₃ S₄ ) in a genetically controlled manner. Magnetite and greigite magnetic phases allow MTB to swim towards redox transition zones where they thrive. MTB may represent some of the oldest microorganisms capable of synthesizing minerals on Earth and have been proposed to significantly impact the iron biogeochemical cycle by immobilizing soluble iron into crystals that subsequently fossilize in sedimentary rocks. In the present article, we describe the distribution of MTB in the environment and discuss the possible function of the magnetite and greigite nanoparticles. We then provide an overview of the chemical mechanisms leading to iron mineralization in MTB. Finally, we update the methods used for the detection of MTB crystals in sedimentary rocks and present their occurrences in the geological record.

Natural Metallic Nanoparticles for Application in Nano-Oncology

Here, the various types of naturally synthesized metallic nanoparticles, which are essentially composed of Ce, Ag, Au, Pt, Pd, Cu, Ni, Se, Fe, or their oxides, are presented, based on a literature analysis. The synthesis methods used to obtain them most often involve the reduction of metallic ions by biological materials or organisms, i.e., essentially plant extracts, yeasts, fungus, and bacteria. The anti-tumor activity of these nanoparticles has been demonstrated on different cancer lines. They rely on various mechanisms of action, such as the release of chemotherapeutic drugs under a pH variation, nanoparticle excitation by radiation, or apoptotic tumor cell death. Among these natural metallic nanoparticles, one type, which consists of iron oxide nanoparticles produced by magnetotactic bacteria called magnetosomes, has been purified to remove endotoxins and abide by pharmacological regulations. It has been tested in vivo for anti-tumor efficacy. For that, purified and stabilized magnetosomes were injected in intracranial mouse glioblastoma tumors and repeatedly heated under the application of an alternating magnetic field, leading to the full disappearance of these tumors. As a whole, the results presented in the literature form a strong basis for pursuing the efforts towards the use of natural metallic nanoparticles for cancer treatment first pre-clinically and then clinically.

Applications of magnetotactic bacteria and magnetosome for cancer treatment: A review emphasizing on practical and mechanistic aspects

Magnetotactic bacteria (MTB) synthesize iron oxide (Fe₃O₄) nanoparticles (NPs), called magnetosomes, with large sizes leading to a ferrimagnetic behavior and a stable magnetic moment at physiological temperature, a chain structure that prevents NP aggregation and promotes uniform NP distribution, and a mineral core of magnetite/maghemite composition, which can be stabilized by an organic coating. Such properties can favor magnetosome administration to humans under certain optimized non-toxic conditions of fabrication. In this review, I describe the fabrication methods, physico-chemical properties, and the anti-tumor activity of different types of MTB/magnetosome preparations, highlighting the bio-compatibility and excellent anti-tumor activity of purified non-pyrogenic magnetosome minerals stabilized by a synthetic chemical compound.