Hexavalent chromate bioreduction by a magnetotactic bacterium Magnetospirillum gryphiswaldense MSR-1 and the effect of magnetosome synthesis.

Magnetotactic bacteria (MTB) are receiving attention for heavy metal biotreatment due to their potential for biosorption with heavy metals and the capability of the magnetic recovery. In this study, we investigated the characteristics of Cr(VI) bioreduction and biosorption by an MTB isolate, Magnetospirillum gryphiswaldense MSR-1, which has a higher growth rate and wider reflexivity in culture conditions. Our results demonstrated that the MSR-1 strain could remove Cr(VI) up to the concentration of 40 mg L-1 and with an optimal activity at neutral pH conditions. The magnetosome synthesis existed regulatory mechanisms between Cr(VI) reduction and cell division. The addition of 10 mg L-1 Cr(VI) significantly inhibited cell growth, but the magnetosome-deficient strain, B17316, showed an average specific growth rate of 0.062 h-1 at the same dosage. Cr(VI) reduction examined by the heat-inactivated and resting cells demonstrated that the main mechanism for MSR-1 strain to reduce Cr(VI) was chromate reductase and adsorption, and magnetosome synthesis would enhance the chromate reductase activity. Finally, our results elucidated that the chromate reductase distributes diversely in multiple subcellular components of the MSR-1 cells, including extracellular, membrane-associated, and intracellular cytoplasmic activity; and expression of the membrane-associated chromate reductase was increased after the cells were pre-exposed by Cr(VI).

A bacterial cytolinker couples positioning of magnetic organelles to cell shape control.

Magnetotactic bacteria maneuver within the geomagnetic field by means of intracellular magnetic organelles, magnetosomes, which are aligned into a chain and positioned at midcell by a dedicated magnetosome-specific cytoskeleton, the "magnetoskeleton." However, how magnetosome chain organization and resulting magnetotaxis is linked to cell shape has remained elusive. Here, we describe the cytoskeletal determinant CcfM (curvature-inducing coiled-coil filament interacting with the magnetoskeleton), which links the magnetoskeleton to cell morphology regulation in Magnetospirillum gryphiswaldense Membrane-anchored CcfM localizes in a filamentous pattern along regions of inner positive-cell curvature by its coiled-coil motifs, and independent of the magnetoskeleton. CcfM overexpression causes additional circumferential localization patterns, associated with a dramatic increase in cell curvature, and magnetosome chain mislocalization or complete chain disruption. In contrast, deletion of ccfM results in decreased cell curvature, impaired cell division, and predominant formation of shorter, doubled chains of magnetosomes. Pleiotropic effects of CcfM on magnetosome chain organization and cell morphology are supported by the finding that CcfM interacts with the magnetoskeleton-related MamY and the actin-like MamK via distinct motifs, and with the cell shape-related cytoskeleton via MreB. We further demonstrate that CcfM promotes motility and magnetic alignment in structured environments, and thus likely confers a selective advantage in natural habitats of magnetotactic bacteria, such as aquatic sediments. Overall, we unravel the function of a prokaryotic cytoskeletal constituent that is widespread in magnetic and nonmagnetic spirilla-shaped Alphaproteobacteria.

Formation and function of bacterial organelles.

Advances in imaging technologies have revealed that many bacteria possess organelles with a proteomically defined lumen and a macromolecular boundary. Some are bound by a lipid bilayer (such as thylakoids, magnetosomes and anammoxosomes), whereas others are defined by a lipid monolayer (such as lipid bodies), a proteinaceous coat (such as carboxysomes) or have a phase-defined boundary (such as nucleolus-like compartments). These diverse organelles have various metabolic and physiological functions, facilitating adaptation to different environments and driving the evolution of cellular complexity. This Review highlights that, despite the diversity of reported organelles, some unifying concepts underlie their formation, structure and function. Bacteria have fundamental mechanisms of organelle formation, through which conserved processes can form distinct organelles in different species depending on the proteins recruited to the luminal space and the boundary of the organelle. These complex subcellular compartments provide evolutionary advantages as well as enabling metabolic specialization, biogeochemical processes and biotechnological advances. Growing evidence suggests that the presence of organelles is the rule, rather than the exception, in bacterial cells.

Unlocking the Potential of Magnetotactic Bacteria as Magnetic Hyperthermia Agents.

Magnetotactic bacteria are aquatic microorganisms that internally biomineralize chains of magnetic nanoparticles (called magnetosomes) and use them as a compass. Here it is shown that magnetotactic bacteria of the strain Magnetospirillum gryphiswaldense present high potential as magnetic hyperthermia agents for cancer treatment. Their heating efficiency or specific absorption rate is determined using both calorimetric and AC magnetometry methods at different magnetic field amplitudes and frequencies. In addition, the effect of the alignment of the bacteria in the direction of the field during the hyperthermia experiments is also investigated. The experimental results demonstrate that the biological structure of the magnetosome chain of magnetotactic bacteria is perfect to enhance the hyperthermia efficiency. Furthermore, fluorescence and electron microscopy images show that these bacteria can be internalized by human lung carcinoma cells A549, and cytotoxicity studies reveal that they do not affect the viability or growth of the cancer cells. A preliminary in vitro hyperthermia study, working on clinical conditions, reveals that cancer cell proliferation is strongly affected by the hyperthermia treatment, making these bacteria promising candidates for biomedical applications.

Effects of Environmental Conditions on High-Yield Magnetosome Production by Magnetospirillum gryphiswaldense MSR-1

Background: Magnetotactic bacteria are a heterogeneous group of Gram-negative prokaryote cells that produce linear chains of magnetic particles called magnetosomes, intracellular organelles composed of magnetic iron particles. Many important applications have been defined for magnetic nanoparticles in biotechnology, such as cell separation applications, as well as acting as carriers of enzymes, antibodies, or anti-cancer drugs. Since the bacterial growth is difficult and the yield of magnetosome production is low, the application of magnetosome has not been developed on a commercial scale. Methods: Magnetospirillum gryphiswaldense strain MSR-1 was used in a modified current culture medium supplemented by different concentrations of oxygen, iron, carbon, and nitrogen, to increase the yield of magnetosomes. Results: Our improved MSR-1 culture medium increased magnetosome yield, magnetosome number per bacterial cell, magnetic response, and bacterial cell growth yield significantly. The yield of magnetosome increased approximately four times. The optimized culture medium containing 25 mM of Na-pyruvate, 40 mM of NaNO3, 200 µM of ferrous sulfate, and 5-10 ppm of dissolved oxygen (DO) resulted in 186.67 mg of magnetosome per liter of culture medium. The iron uptake increased significantly, and the magnetic response of the bacteria to the magnetic field was higher than threefold as compared to the previously reported procedures. Conclusion: This technique not only decreases the cultivation time but also reduces the production cost. In this modified method, the iron and DO are the major factors affecting the production of magnetosome by M. gryphiswaldense strain MSR-1. However, refining this technique will enable a further yield of magnetosome and cell density.

Investigation of the magnetosome biomineralization in magnetotactic bacteria using graphene liquid cell - transmission electron microscopy.

Understanding the biomineralization pathways in living biological species is a grand challenge owing to the difficulties in monitoring the mineralization process at sub-nanometer scales. Here, we monitored the nucleation and growth of magnetosome nanoparticles in bacteria and in real time using a transmission electron microscope (TEM). To enable biomineralization within the bacteria, we subcultured magnetotactic bacteria grown in iron-depleted medium and then mixed them with iron-rich medium within graphene liquid cells (GLCs) right before imaging the bacteria under the microscope. Using in situ electron energy loss spectroscopy (EELS), the oxidation state of iron in the biomineralized magnetosome was analysed to be magnetite with trace amount of hematite. The increase of mass density of biomineralized magnetosomes as a function of incubation time indicated that the bacteria maintained their functionality during the in situ TEM imaging. Our results underpin that GLCs enables a new platform to observe biomineralization events in living biological species at unprecedented spatial resolution. Understanding the biomineralization processes in living organisms facilitates the design of biomimetic materials, and will enable a paradigm shift in understanding the evolution of biological species.

In Vivo Coating of Bacterial Magnetic Nanoparticles by Magnetosome Expression of Spider Silk-Inspired Peptides.

Magnetosomes are natural magnetic nanoparticles with exceptional properties that are synthesized in magnetotactic bacteria by a highly regulated biomineralization process. Their usability in many applications could be further improved by encapsulation in biocompatible polymers. In this study, we explored the production of spider silk-inspired peptides on magnetosomes of the alphaproteobacterium Magnetospirillum gryphiswaldense. Genetic fusion of different silk sequence-like variants to abundant magnetosome membrane proteins enhanced magnetite biomineralization and caused the formation of a proteinaceous capsule, which increased the colloidal stability of isolated particles. Furthermore, we show that spider silk peptides fused to a magnetosome membrane protein can be used as seeds for silk fibril growth on the magnetosome surface. In summary, we demonstrate that the combination of two different biogenic materials generates a genetically encoded hybrid composite with engineerable new properties and enhanced potential for various applications.

A quantitative assessment of the membrane-integral sub-proteome of a bacterial magnetic organelle.

Magnetotactic bacteria produce chains of complex membrane-bound organelles that direct the biomineralization of magnetic nanoparticles and serve for magnetic field navigation. These magnetosome compartments have recently emerged as a model for studying the subcellular organization of prokaryotic organelles. Previous studies indicated the presence of specific proteins with various functions in magnetosome biosynthesis. However, the exact composition and stoichiometry of the magnetosome subproteome have remained unknown. In order to quantify and unambiguously identify all proteins specifically targeted to the magnetosome membrane of the Alphaproteobacterium Magnetospirillum gryphiswaldense, we analyzed the protein composition of several cellular fractions by semi-quantitative mass spectrometry. We found that nearly all genuine magnetosome membrane-integral proteins belong to a well-defined set of previously identified proteins encoded by gene clusters within a genomic island, indicating a highly controlled protein composition. Magnetosome proteins were present in different quantities with up to 120 copies per particle as estimated by correlating our results with available quantitative Western blot data. This high abundance suggests an unusually crowded protein composition of the membrane and a tight packing with transmembrane domains of integral proteins. Our findings will help to further define the structure of the organelle and contribute to the elucidation of magnetosome biogenesis. BIOLOGICAL SIGNIFICANCE: Magnetosomes are one of the most complex bacterial organelles and consist of membrane-bounded crystals of magnetic minerals. The exact composition and stoichiometry of the associated membrane integral proteins are of major interest for a deeper understanding of prokaryotic organelle assembly; however, previous proteomic studies failed to reveal meaningful estimations due to the lack of precise and quantitative data, and the inherently high degree of accumulated protein contaminants in purified magnetosomes. Using a highly sensitive mass spectrometer, we acquired proteomic data from several cellular fractions of a magnetosome producing magnetotactic bacterium and developed a comparative algorithm to identify all genuine magnetosome membrane-integral proteins and to discriminate them from contaminants. Furthermore, by combining our data with previously published quantitative Western blot data, we were able to model the protein copy number and density within the magnetosome membrane. Our results suggest that the magnetosome membrane is specifically associated with a small subset of integral proteins that are tightly packed within the lipid layer. Our study provides by far the most comprehensive estimation of magnetosomal protein composition and stoichiometry and will help to elucidate the complex process of magnetosome biogenesis.

Off-axis electron holography of bacterial cells and magnetic nanoparticles in liquid.

The mapping of electrostatic potentials and magnetic fields in liquids using electron holography has been considered to be unrealistic. Here, we show that hydrated cells of Magnetospirillum magneticum strain AMB-1 and assemblies of magnetic nanoparticles can be studied using off-axis electron holography in a fluid cell specimen holder within the transmission electron microscope. Considering that the holographic object and reference wave both pass through liquid, the recorded electron holograms show sufficient interference fringe contrast to permit reconstruction of the phase shift of the electron wave and mapping of the magnetic induction from bacterial magnetite nanocrystals. We assess the challenges of performing in situ magnetization reversal experiments using a fluid cell specimen holder, discuss approaches for improving spatial resolution and specimen stability, and outline future perspectives for studying scientific phenomena, ranging from interparticle interactions in liquids and electrical double layers at solid-liquid interfaces to biomineralization and the mapping of electrostatic potentials associated with protein aggregation and folding.

Influence of the bacterial growth phase on the magnetic properties of magnetosomes synthesized by Magnetospirillum gryphiswaldense.

BACKGROUND: The magnetosome biosynthesis is a genetically controlled process but the physical properties of the magnetosomes can be slightly tuned by modifying the bacterial growth conditions. METHODS: We designed two time-resolved experiments in which iron-starved bacteria at the mid-logarithmic phase are transferred to Fe-supplemented medium to induce the magnetosomes biogenesis along the exponential growth or at the stationary phase. We used flow cytometry to determine the cell concentration, transmission electron microscopy to image the magnetosomes, DC and AC magnetometry methods for the magnetic characterization, and X-ray absorption spectroscopy to analyze the magnetosome structure. RESULTS: When the magnetosomes synthesis occurs during the exponential growth phase, they reach larger sizes and higher monodispersity, displaying a stoichiometric magnetite structure, as fingerprinted by the well defined Verwey temperature. On the contrary, the magnetosomes synthesized at the stationary phase reach smaller sizes and display a smeared Verwey transition, that suggests that these magnetosomes may deviate slightly from the perfect stoichiometry. CONCLUSIONS: Magnetosomes magnetically closer to stoichiometric magnetite are obtained when bacteria start synthesizing them at the exponential growth phase rather than at the stationary phase. GENERAL SIGNIFICANCE: The growth conditions influence the final properties of the biosynthesized magnetosomes. This article is part of a Special Issue entitled "Recent Advances in Bionanomaterials" Guest Editors: Dr. Marie-Louise Saboungi and Dr. Samuel D. Bader.

Structure of the magnetosome-associated actin-like MamK filament at subnanometer resolution.

Magnetotactic bacteria possess cellular compartments called magnetosomes that sense magnetic fields. Alignment of magnetosomes in the bacterial cell is necessary for their function, and this is achieved through anchoring of magnetosomes to filaments composed of the protein MamK. MamK is an actin homolog that polymerizes upon ATP binding. Here, we report the structure of the MamK filament at ∼6.5 Å, obtained by cryo-Electron Microscopy. This structure confirms our previously reported double-stranded, nonstaggered architecture, and reveals the molecular basis for filament formation. While MamK is closest in sequence to the bacterial actin MreB, the longitudinal contacts along each MamK strand most closely resemble those of eukaryotic actin. In contrast, the cross-strand interface, with a surprisingly limited set of contacts, is novel among actin homologs and gives rise to the nonstaggered architecture.

Magnetosome biogenesis in magnetotactic bacteria.

Magnetotactic bacteria derive their magnetic orientation from magnetosomes, which are unique organelles that contain nanometre-sized crystals of magnetic iron minerals. Although these organelles have evident potential for exciting biotechnological applications, a lack of genetically tractable magnetotactic bacteria had hampered the development of such tools; however, in the past decade, genetic studies using two model Magnetospirillum species have revealed much about the mechanisms of magnetosome biogenesis. In this Review, we highlight these new insights and place the molecular mechanisms of magnetosome biogenesis in the context of the complex cell biology of Magnetospirillum spp. Furthermore, we discuss the diverse properties of magnetosome biogenesis in other species of magnetotactic bacteria and consider the value of genetically 'magnetizing' non-magnetotactic bacteria. Finally, we discuss future prospects for this highly interdisciplinary and rapidly advancing field.

Dynamic Remodeling of the Magnetosome Membrane Is Triggered by the Initiation of Biomineralization.

UNLABELLED: Magnetotactic bacteria produce chains of membrane-bound organelles that direct the biomineralization of magnetic nanoparticles. These magnetosome compartments are a model for studying the biogenesis and subcellular organization of bacterial organelles. Previous studies have suggested that discrete gene products build and assemble magnetosomes in a stepwise fashion. Here, using an inducible system, we show that the stages of magnetosome formation are highly dynamic and interconnected. During de novo formation, magnetosomes first organize into discontinuous chain fragments that are subsequently connected by the bacterial actin-like protein MamK. We also find that magnetosome membranes are not uniform in size and can grow in a biomineralization-dependent manner. In the absence of biomineralization, magnetosome membranes stall at a diameter of ~50 nm. Those that have initiated biomineralization then expand to significantly larger sizes and accommodate mature magnetic particles. We speculate that such a biomineralization-dependent checkpoint for membrane growth establishes the appropriate conditions within the magnetosome to ensure successful nucleation and growth of magnetic particles. IMPORTANCE: Magnetotactic bacteria make magnetic nanoparticles inside membrane-bound organelles called magnetosomes; however, it is unclear how the magnetosome membrane controls the biomineralization that occurs within this bacterial organelle. We placed magnetosome formation under inducible control in Magnetospirillum magneticum AMB-1 and used electron cryo-tomography to capture magnetosomes in their near-native state as they form de novo. An inducible system provided the key evidence that magnetosome membranes grow continuously unless they have not properly initiated biomineralization. Our finding that the size of a bacterial organelle impacts its biochemical function is a fundamental advance that impacts our perception of organelle formation and can inform future attempts aimed at creating designer magnetic particles.

Characterizing and optimizing magnetosome production of Magnetospirillum sp. XM-1 isolated from Xi'an City Moat, China.

Pure culture of magnetotactic bacteria is desirable to understand their physiology, evolution and biomineralization. Here, we report a new strain Magnetospirillum sp. XM-1 that was recently isolated and cultivated from the eutrophic city moat of Xi'an, China. Magnetosome biomineralization, crystallographic and magnetic properties of XM-1 were characterized by using a combination of transmission electron microscopy and rock magnetic methods. Cell growth and magnetite production was optimized by response surface methodology. We found that the Magnetospirillum strain XM-1 is different from the model strain Magnetospirillum magneticum AMB-1 in terms of magnetite magnetosomes, optimal growth temperature and nutrient requirements. Sodium succinate, sodium nitrate and ferric citrate are the three most significant factors associated with the optimization of cell growth and magnetosome production for XM-1.

A comparison of the surface nanostructure from two different types of gram-negative cells: Escherichia coli and Rhodobacter sphaeroides.

Bacteria have been studied using different microscopy methods for many years. Recently, the developments of high-speed atomic force microscopy have opened the doors to study bacteria in new ways due to the fact that it uses much less force on the sample while imaging. This makes the high-speed atomic force microscope an indispensable technique for imaging the surface of living bacterial cells because it allows for the high-resolution visualization of surface proteins in their natural condition without disrupting the cell or the activity of the proteins. Previous work examining living cells of Magnetospirillum magneticum AMB-1 demonstrated that the surface of these bacteria was covered with a net-like structure that is mainly composed of porin molecules. However, it was unclear whether or not this feature was unique to other living bacteria. In this study we used the high-speed atomic force microscope to examine the surface of living cells of Escherichia coli and Rhodobacter sphaeroides to compare their structure with that of M. magneticum. Our research clearly demonstrated that both of these types of cells have an outer surface that is covered in a network of nanometer-sized holes similar to M. magneticum. The diameter of the holes was 8.0±1.5 nm for E. coli and 6.6±1.1 nm for R. sphaeroides. The results in this paper confirm that this type of outer surface structure exists in other types of bacteria and it is not unique to Magnetospirillum.

Correlative electron and fluorescence microscopy of magnetotactic bacteria in liquid: toward in vivo imaging.

Magnetotactic bacteria biomineralize ordered chains of uniform, membrane-bound magnetite or greigite nanocrystals that exhibit nearly perfect crystal structures and species-specific morphologies. Transmission electron microscopy (TEM) is a critical technique for providing information regarding the organization of cellular and magnetite structures in these microorganisms. However, conventional TEM can only be used to image air-dried or vitrified bacteria removed from their natural environment. Here we present a correlative scanning TEM (STEM) and fluorescence microscopy technique for imaging viable cells of Magnetospirillum magneticum strain AMB-1 in liquid using an in situ fluid cell TEM holder. Fluorescently labeled cells were immobilized on microchip window surfaces and visualized in a fluid cell with STEM, followed by correlative fluorescence imaging to verify their membrane integrity. Notably, the post-STEM fluorescence imaging indicated that the bacterial cell wall membrane did not sustain radiation damage during STEM imaging at low electron dose conditions. We investigated the effects of radiation damage and sample preparation on the bacteria viability and found that approximately 50% of the bacterial membranes remained intact after an hour in the fluid cell, decreasing to ~30% after two hours. These results represent a first step toward in vivo studies of magnetite biomineralization in magnetotactic bacteria.

Probing the mechanical properties of magnetosome chains in living magnetotactic bacteria.

The mechanical properties of cytoskeletal networks are intimately involved in determining how forces and cellular processes are generated, directed, and transmitted in living cells. However, determining the mechanical properties of subcellular molecular complexes in vivo has proven to be difficult. Here, we combine in vivo measurements by optical microscopy, X-ray diffraction, and transmission electron microscopy with theoretical modeling to decipher the mechanical properties of the magnetosome chain system encountered in magnetotactic bacteria. We exploit the magnetic properties of the endogenous intracellular nanoparticles to apply a force on the filament-connector pair involved in the backbone formation and stabilization. We show that the magnetosome chain can be broken by the application of external field strength higher than 30 mT and suggest that this originates from the rupture of the magnetosome connector MamJ. In addition, we calculate that the biological determinants can withstand in vivo a force of 25 pN. This quantitative understanding provides insights for the design of functional materials such as actuators and sensors using cellular components.

Co-ordinated functions of Mms proteins define the surface structure of cubo-octahedral magnetite crystals in magnetotactic bacteria.

Magnetotactic bacteria synthesize magnetosomes comprised of membrane-enveloped single crystalline magnetite (Fe3 O4 ). The size and morphology of the nano-sized magnetite crystals (< 100 nm) are highly regulated and bacterial species dependent. However, the control mechanisms of magnetite crystal morphology remain largely unknown. The group of proteins, called Mms (Mms5, Mms6, Mms7, and Mms13), was previously isolated from the surface of cubo-octahedral magnetite crystals in Magnetospirillum magneticum strain AMB-1. Analysis of an mms6 gene deletion mutant suggested that the Mms6 protein plays a major role in the regulation of magnetite crystal size and morphology. In this study, we constructed various mms gene deletion mutants and characterized the magnetite crystals formed by the mutant strains. Comparative analysis showed that all mms genes were involved in the promotion of crystal growth in different manners. The phenotypic characterization of magnetites also suggested that these proteins are involved in controlling the geometries of the crystal surface structures. Thus, the co-ordinated functions of Mms proteins regulate the morphology of the cubo-octahedral magnetite crystals in magnetotactic bacteria.

Ferromagnetic resonance of intact cells and isolated crystals from cultured and uncultured magnetite-producing magnetotactic bacteria.

Most magnetotactic bacteria (MB) produce stable, single-domain magnetite nanocrystals with species-specific size, shape and chain arrangement. In addition, most crystals are elongated along the [111] direction, which is the easy axis of magnetization in magnetite, chemically pure and structurally perfect. These special characteristics allow magnetite crystal chains from MB to be recognized in environmental samples including old sedimentary rocks. Ferromagnetic resonance (FMR) has been proposed as a powerful and practical tool for screening large numbers of samples possibly containing magnetofossils. Indeed, several studies were recently published on FMR of cultured MB, mainly Magnetospirillum gryphiswaldense. In this work, we examined both uncultured magnetotactic cocci and the cultured MB M. gryphiswaldense using transmission electron microscopy (TEM) and FMR from 10 K to room temperature (RT). The TEM data supported the FMR spectral characteristics of our samples. The FMR spectra of both bacteria showed the intrinsic characteristics of magnetite produced by MB, such as extended absorption at the low field region of the spectra and a Verwey transition around 100 K. As previously observed, the spectra of M. gryphiswaldense isolated crystals were more symmetrical than the spectra obtained from whole cells, reflecting the loss of chain arrangement due to the small size and symmetrical shape of the crystals. However, the FMR spectra of magnetic crystals isolated from magnetotactic cocci were very similar to the FMR spectra of whole cells, because the chain arrangement was maintained due to the large size and prismatic shape of the crystals. Our data support the use of FMR spectra to detect magnetotactic bacteria and magnetofossils in samples of present and past environments. Furthermore, the spectra suggest the use of the temperature transition of spectral peak-to-peak intensity to obtain the Verwey temperature for these systems.

A comparison of methods to measure the magnetic moment of magnetotactic bacteria through analysis of their trajectories in external magnetic fields.

Magnetotactic bacteria possess organelles called magnetosomes that confer a magnetic moment on the cells, resulting in their partial alignment with external magnetic fields. Here we show that analysis of the trajectories of cells exposed to an external magnetic field can be used to measure the average magnetic dipole moment of a cell population in at least five different ways. We apply this analysis to movies of Magnetospirillum magneticum AMB-1 cells, and compare the values of the magnetic moment obtained in this way to that obtained by direct measurements of magnetosome dimension from electron micrographs. We find that methods relying on the viscous relaxation of the cell orientation give results comparable to that obtained by magnetosome measurements, whereas methods relying on statistical mechanics assumptions give systematically lower values of the magnetic moment. Since the observed distribution of magnetic moments in the population is not sufficient to explain this discrepancy, our results suggest that non-thermal random noise is present in the system, implying that a magnetotactic bacterial population should not be considered as similar to a paramagnetic material.