Polarity of bacterial magnetotaxis is controlled by aerotaxis through a common sensory pathway

Physiological magnetic field strengths help magnetotactic bacteria navigate in simulated sediments

Bacterial motility is typically studied in bulk solution, while their natural habitats often are complex environments. Here, we produced microfluidic channels that contained sediment-mimicking obstacles to study swimming of magnetotactic bacteria in a near-realistic environment. Magnetotactic bacteria are microorganisms that form chains of nanomagnets and that orient in Earth's magnetic field. The obstacles were produced based on micro-computer tomography reconstructions of bacteria-rich sediment samples. We characterized the swimming of the cells through these channels and found that swimming throughput was highest for physiological magnetic fields. This observation was confirmed by extensive computer simulations using an active Brownian particle model. The simulations indicate that swimming at strong fields is impeded by the trapping of bacteria in 'corners' that require transient swimming against the magnetic field for escape. At weak fields, the direction of swimming is almost random, making the process inefficient as well. We confirmed the trapping effect in our experiments and showed that lowering the field strength allows the bacteria to escape. We hypothesize that over the course of evolution, magnetotactic bacteria have thus evolved to produce magnetic properties that are adapted to the geomagnetic field in order to balance movement and orientation in such crowded environments.

Single-cell magnetotaxis in mucus-mimicking polymeric solutions

Magnetotactic bacteria (MTB) are promising candidates for use as biomicrorobots in biomedical applications due to their motility, self-propulsion, and the ability to direct their navigation with an applied magnetic field. When in the body, the MTB may encounter non-Newtonian fluids such as blood plasma or mucus. However, their motility and the effectiveness of directed navigation in non-Newtonian fluids has yet to be studied on a single-cell level. In this work, we investigate motility of Magnetospirillum magneticum AMB-1 in three concentrations of polyacrylamide (PAM) solution, a mucus-mimicking fluid. The swimming speeds increase from 44.0 ± 13.6 μm/s in 0 mg/mL of PAM to 52.73 ± 15.6 μm/s in 1 mg/mL then decreases to 24.51 ± 11.7 μm/s in 2 mg/mL and 21.23 ± 10.5 μm/s in 3 mg/mL. This trend of a speed increase in low polymer concentrations followed by a decrease in speed as the concentration increases past a threshold concentration is consistent with other studies of motile, flagellated bacteria. Past this threshold concentration of PAM, there is a higher percentage of cells with an overall trajectory angle deviating from the angle of the magnetic field lines. There is also less linearity in the trajectories and an increase in reversals of swimming direction. Altogether, we show that MTB can be directed in polymer concentrations mimicking biological mucus, demonstrating the influence of the medium viscosity on the linearity of their trajectories which alters the effective path that could be predefined in Newtonian fluids when transport is achieved by magnetotaxis.

Swimming polarity inversion in uncultured magnetotactic cocci

Magnetotactic bacteria are microorganisms that produce intracellular magnetic nanoparticles organized in chains, conferring a magnetic moment to the bacterial body that allows it to swim following the geomagnetic field lines. Magnetotactic bacteria usually display two swimming polarities in environmental samples: the South-seeking (SS) polarity and the North-seeking (NS) polarity, characterized by the bacteria swimming antiparallel or parallel to the magnetic field lines, respectively. It has been observed that in the presence of inhomogeneous magnetic fields, NS magnetotactic bacteria can change their swimming polarity to SS or vice versa. The present study analyzes populations of NS cocci obtained from SS cocci isolated in the presence of a magnet. The aim was to study differences in the swimming characteristics and magnetic moment among both populations of cocci. For that, trajectories were recorded and the velocity and angle among the velocity and the applied magnetic field were calculated. In addition, micrographs from both SS and NS cocci were obtained and their magnetosomes were measured to analyze their length, width, aspect ratio and magnetic moment, to finally obtain the magnetic moment for each coccus. The results showed the following properties of NS relative to SS cocci: higher velocities, narrow bacterial magnetic moment distribution, higher dispersion in the distribution of angles among the velocity and the applied magnetic field and lower magnetic field sensibility. Those differences cannot be explained by the simple change in magnetic polarity of the magnetosome chain and can be related to the existence of an active magnetoreceptive process in magnetotactic bacteria.

Exploring the host range for genetic transfer of magnetic organelle biosynthesis

Magnetosomes produced by magnetotactic bacteria have great potential for application in biotechnology and medicine due to their unique physicochemical properties and high biocompatibility. Attempts to transfer the genes for magnetosome biosynthesis into non-magnetic organisms have had mixed results. Here we report on a systematic study to identify key components needed for magnetosome biosynthesis after gene transfer. We transfer magnetosome genes to 25 proteobacterial hosts, generating seven new magnetosome-producing strains. We characterize the recombinant magnetosomes produced by these strains and demonstrate that denitrification and anaerobic photosynthesis are linked to the ability to synthesize magnetosomes upon the gene transfer. In addition, we show that the number of magnetosomes synthesized by a foreign host negatively correlates with the guanine-cytosine content difference between the host and the gene donor. Our findings have profound implications for the generation of magnetized living cells and the potential for transgenic biogenic magnetic nanoparticle production.

Analysis of CheW-like domains provides insights into organization of prokaryotic chemotaxis systems

The ability to control locomotion in a dynamic environment provides a competitive advantage for microorganisms, thus driving the evolution of sophisticated regulatory systems. In total, 19 known categories of chemotaxis systems control motility mediated by flagella or Type IV pili, plus other cellular functions. A key feature that distinguishes chemotaxis systems from generic two-component regulatory systems is separation of receptor and kinase functions into distinct proteins, linked by CheW scaffold proteins. This arrangement allows for formation of varied arrays with remarkable signaling properties. We recently analyzed sequences of CheW-like domains found in CheA kinases and CheW and CheV scaffold proteins. In total, 16 Architectures of CheA, CheW, and CheV proteins contain ~94% of all CheW-like domains and form six Classes with likely functional specializations. We surveyed chemotaxis system categories and proteins containing CheW-like domains in ~1900 prokaryotic species, the most comprehensive analysis to date, revealing new insights. Co-occurrence analyses suggested that many chemotaxis systems occur in non-random combinations within species, implying synergy or antagonism. Furthermore, many Architectures of proteins containing CheW-like domains occurred predominantly with specific categories of chemotaxis systems, suggesting specialized functional interactions. We propose Class 1 (~80%) and Class 6 (~20%) CheW proteins exhibit preferences for distinct chemoreceptor structures. Furthermore, rare (~1%) Class 2 CheW proteins frequently co-occurred with methyl-accepting coiled coil proteins, which contain both receptor and kinase functions and so do not require connection via a CheW scaffold but may benefit from arrays. Last, rare multidomain CheW proteins may interact with different receptors than single-domain CheW proteins.

Silent gene clusters encode magnetic organelle biosynthesis in a non-magnetotactic phototrophic bacterium

Horizontal gene transfer is a powerful source of innovations in prokaryotes that can affect almost any cellular system, including microbial organelles. The formation of magnetosomes, one of the most sophisticated microbial mineral-containing organelles synthesized by magnetotactic bacteria for magnetic navigation in the environment, was also shown to be a horizontally transferrable trait. However, the mechanisms determining the fate of such genes in new hosts are not well understood, since non-adaptive gene acquisitions are typically rapidly lost and become unavailable for observation. This likely explains why gene clusters encoding magnetosome biosynthesis have never been observed in non-magnetotactic bacteria. Here, we report the first discovery of a horizontally inherited dormant gene clusters encoding biosynthesis of magnetosomes in a non-magnetotactic phototrophic bacterium Rhodovastum atsumiense. We show that these clusters were inactivated through transcriptional silencing and antisense RNA regulation, but retain functionality, as several genes were able to complement the orthologous deletions in a remotely related magnetotactic bacterium. The laboratory transfer of foreign magnetosome genes to R. atsumiense was found to endow the strain with magnetosome biosynthesis, but strong negative selection led to rapid loss of this trait upon subcultivation, highlighting the trait instability in this organism. Our results provide insight into the horizontal dissemination of gene clusters encoding complex prokaryotic organelles and illuminate the potential mechanisms of their genomic preservation in a dormant state.

Collective Behaviors of Active Matter Learning from Natural Taxes Across Scales

Taxis orientation is common in microorganisms, and it provides feasible strategies to operate active colloids as small-scale robots. Collective taxes involve numerous units that collectively perform taxis motion, whereby the collective cooperation between individuals enables the group to perform efficiently, adaptively, and robustly. Hence, analyzing and designing collectives is crucial for developing and advancing microswarm toward practical or clinical applications. In this review, natural taxis behaviors are categorized and synthetic microrobotic collectives are discussed as bio-inspired realizations, aiming at closing the gap between taxis strategies of living creatures and those of functional active microswarms. As collective behaviors emerge within a group, the global taxis to external stimuli guides the group to conduct overall tasks, whereas the local taxis between individuals induces synchronization and global patterns. By encoding the local orientations and programming the global stimuli, various paradigms can be introduced for coordinating and controlling such collective microrobots, from the viewpoints of fundamental science and practical applications. Therefore, by discussing the key points and difficulties associated with collective taxes of different paradigms, this review potentially offers insights into mimicking natural collective behaviors and constructing intelligent microrobotic systems for on-demand control and preassigned tasks.

An open-source automated magnetic optical density meter for analysis of suspensions of magnetic cells and particles

We present a spectrophotometer (optical density meter) combined with electromagnets dedicated to the analysis of suspensions of magnetotactic bacteria. The instrument can also be applied to suspensions of other magnetic cells and magnetic particles. We have ensured that our system, called MagOD, can be easily reproduced by providing the source of the 3D prints for the housing, electronic designs, circuit board layouts, and microcontroller software. We compare the performance of our system to existing adapted commercial spectrophotometers. In addition, we demonstrate its use by analyzing the absorbance of magnetotactic bacteria as a function of their orientation with respect to the light path and their speed of reorientation after the field has been rotated by 90°. We continuously monitored the development of a culture of magnetotactic bacteria over a period of 5 days and measured the development of their velocity distribution over a period of one hour. Even though this dedicated spectrophotometer is relatively simple to construct and cost-effective, a range of magnetic field-dependent parameters can be extracted from suspensions of magnetotactic bacteria. Therefore, this instrument will help the magnetotactic research community to understand and apply this intriguing micro-organism.

Direct measurement of the aerotactic response in a bacterial suspension

Aerotaxis is the ability of motile cells to navigate toward oxygen. A key question is the dependence of the aerotactic velocity with the local oxygen concentration c. Here we combine simultaneous bacteria tracking and local oxygen concentration measurements using Ruthenium encapsulated in micelles to characterize the aerotactic response of Burkholderia contaminans, a motile bacterium ubiquitous in the environment. In our experiments, an oxygen gradient is produced by the bacterial respiration in a sealed glass capillary permeable to oxygen at one end, producing a bacterial band traveling toward the oxygen source. We compute the aerotactic response χ(c) both at the population scale, from the drift velocity in the bacterial band, and at the bacterial scale, from the angular modulation of the run times. Both methods are consistent with a power-law χ∝c^{-2}, in good agreement with existing models based on the biochemistry of bacterial membrane receptors.

Interplay of surface interaction and magnetic torque in single-cell motion of magnetotactic bacteria in microfluidic confinement

Swimming microorganisms often experience complex environments in their natural habitat. The same is true for microswimmers in envisioned biomedical applications. The simple aqueous conditions typically studied in the lab differ strongly from those found in these environments and often exclude the effects of small volume confinement or the influence that external fields have on their motion. In this work, we investigate magnetically steerable microswimmers, specifically magnetotactic bacteria, in strong spatial confinement and under the influence of an external magnetic field. We trap single cells in micrometer-sized microfluidic chambers and track and analyze their motion, which shows a variety of different trajectories, depending on the chamber size and the strength of the magnetic field. Combining these experimental observations with simulations using a variant of an active Brownian particle model, we explain the variety of trajectories by the interplay between the wall interactions and the magnetic torque. We also analyze the pronounced cell-to-cell heterogeneity, which makes single-cell tracking essential for an understanding of the motility patterns. In this way, our work establishes a basis for the analysis and prediction of microswimmer motility in more complex environments.

Biogeochemical Niche of Magnetotactic Cocci Capable of Sequestering Large Polyphosphate Inclusions in the Anoxic Layer of the Lake Pavin Water Column

Magnetotactic bacteria (MTB) are microorganisms thriving mostly at oxic–anoxic boundaries of aquatic habitats. MTB are efficient in biomineralising or sequestering diverse elements intracellularly, which makes them potentially important actors in biogeochemical cycles. Lake Pavin is a unique aqueous system populated by a wide diversity of MTB with two communities harbouring the capability to sequester not only iron under the form of magnetosomes but also phosphorus and magnesium under the form of polyphosphates, or calcium carbonates, respectively. MTB thrive in the water column of Lake Pavin over a few metres along strong redox and chemical gradients representing a series of different microenvironments. In this study, we investigate the relative abundance and the vertical stratification of the diverse populations of MTB in relation to environmental parameters, by using a new method coupling a precise sampling for geochemical analyses, MTB morphotype description, and in situ measurement of the physicochemical parameters. We assess the ultrastructure of MTB as a function of depth using light and electron microscopy. We evidence the biogeochemical niche of magnetotactic cocci, capable of sequestering large PolyP inclusions below the oxic–anoxic transition zone. Our results suggest a tight link between the S and P metabolisms of these bacteria and pave the way to better understand the implication of MTB for the P cycle in stratified environmental conditions.

Occurrence of south- and north-seeking multicellular magnetotactic prokaryotes in a coastal lagoon in the South Hemisphere

Magnetotactic bacteria (MTB) response to the magnetic field can be classified into north-seeking (NS) and south-seeking (SS), which usually depends on their inhabiting site in the North and South Hemisphere, respectively. However, uncommon inverted polarity was observed on both hemispheres. Here, we studied magnetotactic multicellular prokaryotes (MMPs) from a coastal lagoon in Brazil collected in April and August 2014. MMPs from the first sampling period presented both magnetotactic behaviors, while MMPs collected in August/2014 were only SS. Phylogenetic analysis based on the 16S rRNA coding gene showed that these organisms belong to the Deltaproteobacteria class. The 16S rRNA gene sequences varied among MMPs regardless of the sampling period, and similarity values were not related to the type of magnetotactic response presented by the microorganisms. Therefore, differences in the magnetotactic behavior might result from the physiological state of MMPs, the availability of resources, or the instability of the chemical gradient in the environment. This is the first report of NS magnetotactic behavior on MMPs from the South Hemisphere.

Towards the Idea of Molecular Brains

How can single cells without nervous systems perform complex behaviours such as habituation, associative learning and decision making, which are considered the hallmark of animals with a brain? Are there molecular systems that underlie cognitive properties equivalent to those of the brain? This review follows the development of the idea of molecular brains from Darwin’s “root brain hypothesis”, through bacterial chemotaxis, to the recent discovery of neuron-like r-protein networks in the ribosome. By combining a structural biology view with a Bayesian brain approach, this review explores the evolutionary labyrinth of information processing systems across scales. Ribosomal protein networks open a window into what were probably the earliest signalling systems to emerge before the radiation of the three kingdoms. While ribosomal networks are characterised by long-lasting interactions between their protein nodes, cell signalling networks are essentially based on transient interactions. As a corollary, while signals propagated in persistent networks may be ephemeral, networks whose interactions are transient constrain signals diffusing into the cytoplasm to be durable in time, such as post-translational modifications of proteins or second messenger synthesis. The duration and nature of the signals, in turn, implies different mechanisms for the integration of multiple signals and decision making. Evolution then reinvented networks with persistent interactions with the development of nervous systems in metazoans. Ribosomal protein networks and simple nervous systems display architectural and functional analogies whose comparison could suggest scale invariance in information processing. At the molecular level, the significant complexification of eukaryotic ribosomal protein networks is associated with a burst in the acquisition of new conserved aromatic amino acids. Knowing that aromatic residues play a critical role in allosteric receptors and channels, this observation suggests a general role of π systems and their interactions with charged amino acids in multiple signal integration and information processing. We think that these findings may provide the molecular basis for designing future computers with organic processors.

Dynamic nanoassembly-based drug delivery system (DNDDS): Learning from nature

Dynamic nanoassembly-based drug delivery system (DNDDS) has evolved from being a mere curiosity to emerging as a promising strategy for high-performance diagnosis and/or therapy of various diseases. However, dynamic nano-bio interaction between DNDDS and biological systems remains poorly understood, which can be critical for precise spatiotemporal and functional control of DNDDS in vivo. To deepen the understanding for fine control over DNDDS, we aim to explore natural systems as the root of inspiration for researchers from various fields. This review highlights ingenious designs, nano-bio interactions, and controllable functionalities of state-of-the-art DNDDS under endogenous or exogenous stimuli, by learning from nature at the molecular, subcellular, and cellular levels. Furthermore, the assembly strategies and response mechanisms of tailor-made DNDDS based on the characteristics of various diseased microenvironments are intensively discussed. Finally, the current challenges and future perspectives of DNDDS are briefly commented.

Stokesian dynamics simulations of a magnetotactic bacterium

The swimming of bacteria provides insight into propulsion and steering under the conditions of low-Reynolds number hydrodynamics. Here we address the magnetically steered swimming of magnetotactic bacteria. We use Stokesian dynamics simulations to study the swimming of single-flagellated magnetotactic bacteria (MTB) in an external magnetic field. Our model MTB consists of a spherical cell body equipped with a magnetic dipole moment and a helical flagellum rotated by a rotary motor. The elasticity of the flagellum as well as magnetic and hydrodynamic interactions is taken into account in this model. We characterized how the swimming velocity is dependent on parameters of the model. We then studied the U-turn motion after a field reversal and found two regimes for weak and strong fields and, correspondingly, two characteristic time scales. In the two regimes, the U-turn time is dominated by the turning of the cell body and its magnetic moment or the turning of the flagellum, respectively. In the regime for weak fields, where turning is dominated by the magnetic relaxation, the U-turn time is approximately in agreement with a theoretical model based on torque balance. In the strong-field regime, strong deformations of the flagellum are observed. We further simulated the swimming of a bacterium with a magnetic moment that is inclined relative to the flagellar axis. This scenario leads to intriguing double helical trajectories that we characterize as functions of the magnetic moment inclination and the magnetic field. For small inclination angles ([Formula: see text]) and typical field strengths, the inclination of the magnetic moment has only a minor effect on the swimming of MTB in an external magnetic field. Large inclination angles result in a strong reduction in the velocity in direction of the magnetic field, consistent with recent observations that bacteria with large inclination angles use a different propulsion mechanism.

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

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

Diversity of bacterial chemosensory systems

Chemosensory system is the most complex, specialized mode of signal transduction in bacteria and archaea. It is composed of several core and auxiliary protein components that are highly organized in order to deliver a fast response to changing environmental conditions. Chemosensory pathways were studied in-depth in a handful of model organisms and experimentally characterized at least to some degree in approximately thirty other species. However, genome-wide analyses have revealed their presence in thousands of sequenced microbial genomes. Both experimental and computational studies uncovered substantial diversity in system design, functional regulation, cellular localization and phyletic distribution of chemosensory pathways. Here, we summarize advances and expose gaps in our current understanding of the diversity of chemosensory systems.

Spatiotemporal Organization of Chemotaxis Pathways in Magnetospirillum gryphiswaldense

Magnetospirillum gryphiswaldense employs iron-rich nanoparticles for magnetic navigation within environmental redox gradients. This behavior termed magneto-aerotaxis was previously shown to rely on the sensory pathway CheOp1, but the precise localization of CheOp1-related chemoreceptor arrays during the cell cycle and its possible interconnection with three other chemotaxis pathways have remained unstudied. Here, we analyzed the localization of chemoreceptor-associated adaptor protein CheW₁ and histidine kinase CheA₁ by superresolution microscopy in a spatiotemporal manner. CheW₁ localized in dynamic clusters that undergo occasional segregation and fusion events at lateral sites of both cell poles. Newly formed smaller clusters originating at midcell before completion of cytokinesis were found to grow in size during the cell cycle. Bipolar CheA₁ localization and formation of aerotactic swim halos were affected depending on the fluorescent protein tag, indicating that CheA₁ localization is important for aerotaxis. Furthermore, polar CheW₁ localization was independent of cheOp2 to cheOp4 but lost in the absence of cheOp1 or cheA₁ Results were corroborated by the detection of a direct protein interaction between CheA₁ and CheW₁ and by the observation that cheOp2- and cheOp3-encoded CheW paralogs localized in spatially distinct smaller clusters at the cell boundary. Although the findings of a minor aerotaxis-related CheOp4 phenotype and weak protein interactions between CheOp1 and CheOp4 by two-hybrid analysis implied that CheW₁ and CheW₄ might be part of the same chemoreceptor array, CheW₄ was localized in spatially distinct polar-lateral arrays independent of CheOp1, suggesting that CheOp1 and CheOp4 are also not connected at the molecular level.IMPORTANCE Magnetotactic bacteria (MTB) use the geomagnetic field for navigation in aquatic redox gradients. However, the highly complex signal transduction networks in these environmental microbes are poorly understood. Here, we analyzed the localization of selected chemotaxis proteins to spatially and temporally resolve chemotaxis array localization in Magnetospirillum gryphiswaldense Our findings suggest that bipolar localization of chemotaxis arrays related to the key signaling pathway CheOp1 is important for aerotaxis and that CheOp1 signaling units assemble independent of the three other chemotaxis pathways present in M. gryphiswaldense Overall, our results provide deeper insights into the complex organization of signaling pathways in MTB and add to the general understanding of environmental bacteria possessing multiple chemotaxis pathways.

Genome-Wide Identification of Essential and Auxiliary Gene Sets for Magnetosome Biosynthesis in Magnetospirillum gryphiswaldense

Magnetospirillum gryphiswaldense is one of the few tractable model magnetotactic bacteria (MTB) for studying magnetosome biomineralization. So far, knowledge on the genetic determinants of this complex process has been mainly gathered using reverse genetics and candidate approaches. In contrast, nontargeted forward genetics studies are lacking, since application of such techniques in MTB has been complicated for a number of technical reasons. Here, we report on the first comprehensive transposon mutagenesis study in MTB, aiming at systematic identification of auxiliary genes necessary to support magnetosome formation in addition to key genes harbored in the magnetosome island (MAI). Our work considerably extends the candidate set of novel subsidiary determinants and shows that the full gene complement underlying magnetosome biosynthesis is larger than assumed. In particular, we were able to define certain cellular pathways as specifically important for magnetosome formation that have not been implicated in this process so far.

Magnetotactic bacteria (MTB) stand out by their ability to manufacture membrane-enclosed magnetic organelles, so-called magnetosomes. Previously, it has been assumed that a genomic region of approximately 100 kbp, the magnetosome island (MAI), harbors all genetic determinants required for this intricate biosynthesis process. Recent evidence, however, argues for the involvement of additional auxiliary genes that have not been identified yet. In the present study, we set out to delineate the full gene complement required for magnetosome production in the alphaproteobacterium Magnetospirillum gryphiswaldense using a systematic genome-wide transposon mutagenesis approach. By an optimized procedure, a Tn5 insertion library of 80,000 clones was generated and screened, yielding close to 200 insertants with mild to severe impairment of magnetosome biosynthesis. Approximately 50% of all Tn5 insertion sites mapped within the MAI, mostly leading to a nonmagnetic phenotype. In contrast, in the majority of weakly magnetic Tn5 insertion mutants, genes outside the MAI were affected, which typically caused lower numbers of magnetite crystals with partly aberrant morphology, occasionally combined with deviant intracellular localization. While some of the Tn5-struck genes outside the MAI belong to pathways that have been linked to magnetosome formation before (e.g., aerobic and anaerobic respiration), the majority of affected genes are involved in so far unsuspected cellular processes, such as sulfate assimilation, oxidative protein folding, and cytochrome c maturation, or are altogether of unknown function. We also found that signal transduction and redox functions are enriched in the set of Tn5 hits outside the MAI, suggesting that such processes are particularly important in support of magnetosome biosynthesis.

IMPORTANCEMagnetospirillum gryphiswaldense is one of the few tractable model magnetotactic bacteria (MTB) for studying magnetosome biomineralization. So far, knowledge on the genetic determinants of this complex process has been mainly gathered using reverse genetics and candidate approaches. In contrast, nontargeted forward genetics studies are lacking, since application of such techniques in MTB has been complicated for a number of technical reasons. Here, we report on the first comprehensive transposon mutagenesis study in MTB, aiming at systematic identification of auxiliary genes necessary to support magnetosome formation in addition to key genes harbored in the magnetosome island (MAI). Our work considerably extends the candidate set of novel subsidiary determinants and shows that the full gene complement underlying magnetosome biosynthesis is larger than assumed. In particular, we were able to define certain cellular pathways as specifically important for magnetosome formation that have not been implicated in this process so far.

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.

A Compass To Boost Navigation: Cell Biology of Bacterial Magnetotaxis

Magnetotactic bacteria are aquatic or sediment-dwelling microorganisms able to take advantage of the Earth's magnetic field for directed motility. The source of this amazing trait is magnetosomes, unique organelles used to synthesize single nanometer-sized crystals of magnetic iron minerals that are queued up to build an intracellular compass. Most of these microorganisms cannot be cultivated under controlled conditions, much less genetically engineered, with only few exceptions. However, two of the genetically amenable Magnetospirillum species have emerged as tractable model organisms to study magnetosome formation and magnetotaxis. Recently, much has been revealed about the process of magnetosome biogenesis and dedicated structures for magnetosome dynamics and positioning, which suggest an unexpected cellular intricacy of these organisms. In this minireview, we summarize new insights and place the molecular mechanisms of magnetosome formation in the context of the complex cell biology of Magnetospirillum spp. First, we provide an overview on magnetosome vesicle synthesis and magnetite biomineralization, followed by a discussion of the perceptions of dynamic organelle positioning and its biological implications, which highlight that magnetotactic bacteria have evolved sophisticated mechanisms to construct, incorporate, and inherit a unique navigational device. Finally, we discuss the impact of magnetotaxis on motility and its interconnection with chemotaxis, showing that magnetotactic bacteria are outstandingly adapted to lifestyle and habitat.

Investigating Optimal Time Step Intervals of Imaging for Data Quality through a Novel Fully-Automated Cell Tracking Approach

Computer-based fully-automated cell tracking is becoming increasingly important in cell biology, since it provides unrivalled capacity and efficiency for the analysis of large datasets. However, automatic cell tracking’s lack of superior pattern recognition and error-handling capability compared to its human manual tracking counterpart inspired decades-long research. Enormous efforts have been made in developing advanced cell tracking packages and software algorithms. Typical research in this field focuses on dealing with existing data and finding a best solution. Here, we investigate a novel approach where the quality of data acquisition could help improve the accuracy of cell tracking algorithms and vice-versa. Generally speaking, when tracking cell movement, the more frequent the images are taken, the more accurate cells are tracked and, yet, issues such as damage to cells due to light intensity, overheating in equipment, as well as the size of the data prevent a constant data streaming. Hence, a trade-off between the frequency at which data images are collected and the accuracy of the cell tracking algorithms needs to be studied. In this paper, we look at the effects of different choices of the time step interval (i.e., the frequency of data acquisition) within the microscope to our existing cell tracking algorithms. We generate several experimental data sets where the true outcomes are known (i.e., the direction of cell migration) by either using an effective chemoattractant or employing no-chemoattractant. We specify a relatively short time step interval (i.e., 30 s) between pictures that are taken at the data generational stage, so that, later on, we may choose some portion of the images to produce datasets with different time step intervals, such as 1 min, 2 min, and so on. We evaluate the accuracy of our cell tracking algorithms to illustrate the effects of these different time step intervals. We establish that there exist certain relationships between the tracking accuracy and the time step interval associated with experimental microscope data acquisition. We perform fully-automatic adaptive cell tracking on multiple datasets, to identify optimal time step intervals for data acquisition, while at the same time demonstrating the performance of the computer cell tracking algorithms.

Single-step transfer of biosynthetic operons endows a non-magnetotactic Magnetospirillum strain from wetland with magnetosome biosynthesis

The magnetotactic lifestyle represents one of the most complex traits found in many bacteria from aquatic environments and depends on magnetic organelles, the magnetosomes. Genetic transfer of magnetosome biosynthesis operons to a non-magnetotactic bacterium has only been reported once so far, but it is unclear whether this may also occur in other recipients. Besides magnetotactic species from freshwater, the genus Magnetospirillum of the Alphaproteobacteria also comprises a number of strains lacking magnetosomes, which are abundant in diverse microbial communities. Their close phylogenetic interrelationships raise the question whether the non-magnetotactic magnetospirilla may have the potential to (re)gain a magnetotactic lifestyle upon acquisition of magnetosome gene clusters. Here, we studied the transfer of magnetosome gene operons into several non-magnetotactic environmental magnetospirilla. Single-step transfer of a compact vector harbouring >30 major magnetosome genes from M. gryphiswaldense induced magnetosome biosynthesis in a Magnetospirillum strain from a constructed wetland. However, the resulting magnetic cellular alignment was insufficient for efficient magnetotaxis under conditions mimicking the weak geomagnetic field. Our work provides insights into possible evolutionary scenarios and potential limitations for the dissemination of magnetotaxis by horizontal gene transfer and expands the range of foreign recipients that can be genetically magnetized.

Magnetic genes: Studying the genetics of biomineralization in magnetotactic bacteria

Many species of bacteria can manufacture materials on a finer scale than those that are synthetically made. These products are often produced within intracellular compartments that bear many hallmarks of eukaryotic organelles. One unique and elegant group of organisms is at the forefront of studies into the mechanisms of organelle formation and biomineralization. Magnetotactic bacteria (MTB) produce organelles called magnetosomes that contain nanocrystals of magnetic material, and understanding the molecular mechanisms behind magnetosome formation and biomineralization is a rich area of study. In this Review, we focus on the genetics behind the formation of magnetosomes and biomineralization. We cover the history of genetic discoveries in MTB and key insights that have been found in recent years and provide a perspective on the future of genetic studies in MTB.

Quantifying the Benefit of a Dedicated "Magnetoskeleton" in Bacterial Magnetotaxis by Live-Cell Motility Tracking and Soft Agar Swimming Assay

The alphaproteobacterium Magnetospirillum gryphiswaldense has the intriguing ability to navigate within magnetic fields, a behavior named magnetotaxis, governed by the formation of magnetosomes, intracellular membrane-enveloped crystals of magnetite. Magnetosomes are aligned in chains along the cell's motility axis by a dedicated multipart cytoskeleton ("magnetoskeleton"); however, precise estimates of its significance for magnetotaxis have not been reported. Here, we estimated the alignment of strains deficient in various magnetoskeletal constituents by live-cell motility tracking within defined magnetic fields ranging from 50 μT (reflecting the geomagnetic field) up to 400 μT. Motility tracking revealed that ΔmamY and ΔmamK strains (which assemble mispositioned and fragmented chains, respectively) are partially impaired in magnetotaxis, with approximately equal contributions of both proteins. This impairment was reflected by a required magnetic field strength of 200 μT to achieve a similar degree of alignment as for the wild-type strain in a 50-μT magnetic field. In contrast, the ΔmamJ strain, which predominantly forms clusters of magnetosomes, was only weakly aligned under any of the tested field conditions and could barely be distinguished from a nonmagnetic mutant. Most findings were corroborated by a soft agar swimming assay to analyze magnetotaxis based on the degree of distortion of swim halos formed in magnetic fields. Motility tracking further revealed that swimming speeds of M. gryphiswaldense are highest within the field strength equaling the geomagnetic field. In conclusion, magnetic properties and intracellular positioning of magnetosomes by a dedicated magnetoskeleton are required and optimized for bacterial magnetotaxis and most efficient locomotion within the geomagnetic field.IMPORTANCE In Magnetospirillum gryphiswaldense, magnetosomes are aligned in quasi-linear chains in a helical cell by a complex cytoskeletal network, including the actin-like MamK and adapter MamJ for magnetosome chain concatenation and segregation and MamY to position magnetosome chains along the shortest cellular axis of motility. Magnetosome chain positioning is assumed to be required for efficient magnetic navigation; however, the significance and contribution of all key constituents have not been quantified within defined and weak magnetic fields reflecting the geomagnetic field. Employing two different motility-based methods to consider the flagellum-mediated propulsion of cells, we depict individual benefits of all magnetoskeletal constituents for magnetotaxis. Whereas lack of mamJ resulted almost in an inability to align cells in weak magnetic fields, an approximately 4-fold-increased magnetic field strength was required to compensate for the loss of mamK or mamY In summary, the magnetoskeleton and optimal positioning of magnetosome chains are required for efficient magnetotaxis.

Chemotaxis in external fields: Simulations for active magnetic biological matter

The movement of microswimmers is often described by active Brownian particle models. Here we introduce a variant of these models with several internal states of the swimmer to describe stochastic strategies for directional swimming such as run and tumble or run and reverse that are used by microorganisms for chemotaxis. The model includes a mechanism to generate a directional bias for chemotaxis and interactions with external fields (e.g., gravity, magnetic field, fluid flow) that impose forces or torques on the swimmer. We show how this modified model can be applied to various scenarios: First, the run and tumble motion of E. coli is used to establish a paradigm for chemotaxis and investigate how it is affected by external forces. Then, we study magneto-aerotaxis in magnetotactic bacteria, which is biased not only by an oxygen gradient towards a preferred concentration, but also by magnetic fields, which exert a torque on an intracellular chain of magnets. We study the competition of magnetic alignment with active reorientation and show that the magnetic orientation can improve chemotaxis and thereby provide an advantage to the bacteria, even at rather large inclination angles of the magnetic field relative to the oxygen gradient, a case reminiscent of what is expected for the bacteria at or close to the equator. The highest gain in chemotactic velocity is obtained for run and tumble with a magnetic field parallel to the gradient, but in general a mechanism for reverse motion is necessary to swim against the magnetic field and a run and reverse strategy is more advantageous in the presence of a magnetic torque. This finding is consistent with observations that the dominant mode of directional changes in magnetotactic bacteria is reversal rather than tumbles. Moreover, it provides guidance for the design of future magnetic biohybrid swimmers.

In this paper, we propose a modified Active Brownian particle model to describe bacterial swimming behavior under the influence of external forces and torques, in particular of a magnetic torque. This type of interaction is particularly important for magnetic biohybrids (i.e. motile bacteria coupled to a synthetic magnetic component) and for magnetotactic bacteria (i.e. bacteria with a natural intracellular magnetic chain), which perform chemotaxis to swim along chemical gradients, but are also directed by an external magnetic field. The model allows us to investigate the benefits and disadvantages of such coupling between two different directionality mechanisms. In particular we show that the magnetic torque can speed chemotaxis up in some conditions, while it can hinder it in other cases. In addition to an understanding of the swimming strategies of naturally magnetotactic organisms, the results may guide the design of future biomedical devices.

Magnetoreception in Microorganisms

Magnetoreception is the sense whereby organisms geolocate and navigate in response to the Earth's magnetic field lines. For decades, magnetotactic bacteria have been the only known magnetoreceptive microorganisms. The magnetotactic behaviour of these aquatic prokaryotes is due to the biomineralization of magnetic crystals. While an old report alleged the existence of microbial algae with similar behaviour, recent discoveries have demonstrated the existence of unicellular eukaryotes able to sense the geomagnetic field, and have revealed different mechanisms and strategies involved in such a sensing. Some ciliates can be magnetically guided after predation of magnetotactic bacteria, while some flagellates acquired this sense through symbiosis with magnetic bacteria. A report has even suggested that some magnetotactic protists could biomineralize magnetic crystals.

Magnetotactic bacteria in a droplet self-assemble into a rotary motor

From intracellular protein trafficking to large-scale motion of animal groups, the physical concepts driving the self-organization of living systems are still largely unraveled. Self-organization of active entities, leading to novel phases and emergent macroscopic properties, recently shed new light on these complex dynamical processes. Here we show that under the application of a constant magnetic field, motile magnetotactic bacteria confined in water-in-oil droplets self-assemble into a rotary motor exerting a torque on the external oil phase. A collective motion in the form of a large-scale vortex, reversable by inverting the field direction, builds up in the droplet with a vorticity perpendicular to the magnetic field. We study this collective organization at different concentrations, magnetic fields and droplet radii and reveal the formation of two torque-generating areas close to the droplet interface. We characterize quantitatively the mechanical energy extractable from this new biological and self-assembled motor.

Living Materials Herald a New Era in Soft Robotics

Living beings have an unsurpassed range of ways to manipulate objects and interact with them. They can make autonomous decisions and can heal themselves. So far, a conventional robot cannot mimic this complexity even remotely. Classical robots are often used to help with lifting and gripping and thus to alleviate the effects of menial tasks. Sensors can render robots responsive, and artificial intelligence aims at enabling autonomous responses. Inanimate soft robots are a step in this direction, but it will only be in combination with living systems that full complexity will be achievable. The field of biohybrid soft robotics provides entirely new concepts to address current challenges, for example the ability to self-heal, enable a soft touch, or to show situational versatility. Therefore, "living materials" are at the heart of this review. Similarly to biological taxonomy, there is a recent effort for taxonomy of biohybrid soft robotics. Here, an expansion is proposed to take into account not only function and origin of biohybrid soft robotic components, but also the materials. This materials taxonomy key demonstrates visually that materials science will drive the development of the field of soft biohybrid robotics.

MamY is a membrane-bound protein that aligns magnetosomes and the motility axis of helical magnetotactic bacteria

To navigate within the geomagnetic field, magnetotactic bacteria synthesize magnetosomes, which are unique organelles consisting of membrane-enveloped magnetite nanocrystals. In magnetotactic spirilla, magnetosomes become actively organized into chains by the filament-forming actin-like MamK and the adaptor protein MamJ, thereby assembling a magnetic dipole much like a compass needle. However, in Magnetospirillum gryphiswaldense, discontinuous chains are still formed in the absence of MamK. Moreover, these fragmented chains persist in a straight conformation indicating undiscovered structural determinants able to accommodate a bar magnet-like magnetoreceptor in a helical bacterium. Here, we identify MamY, a membrane-bound protein that generates a sophisticated mechanical scaffold for magnetosomes. MamY localizes linearly along the positive inner cell curvature (the geodetic cell axis), probably by self-interaction and curvature sensing. In a mamY deletion mutant, magnetosome chains detach from the geodetic axis and fail to accommodate a straight conformation coinciding with reduced cellular magnetic orientation. Codeletion of mamKY completely abolishes chain formation, whereas on synthetic tethering of magnetosomes to MamY, the chain configuration is regained, emphasizing the structural properties of the protein. Our results suggest MamY is membrane-anchored mechanical scaffold that is essential to align the motility axis of magnetotactic spirilla with their magnetic moment vector and to perfectly reconcile magnetoreception with swimming direction.

Ectosymbiotic bacteria at the origin of magnetoreception in a marine protist

Mutualistic symbioses are often a source of evolutionary innovation and drivers of biological diversification¹. Widely distributed in the microbial world, particularly in anoxic settings^2,3, they often rely on metabolic exchanges and syntrophy^2,4. Here, we report a mutualistic symbiosis observed in marine anoxic sediments between excavate protists (Symbiontida, Euglenozoa)⁵ and ectosymbiotic Deltaproteobacteria biomineralizing ferrimagnetic nanoparticles. Light and electron microscopy observations as well as genomic data support a multi-layered mutualism based on collective magnetotactic motility with division of labour and interspecies hydrogen-transfer-based syntrophy⁶. The guided motility of the consortia along the geomagnetic field is allowed by the magnetic moment of the non-motile ectosymbiotic bacteria combined with the protist motor activity, which is a unique example of eukaryotic magnetoreception⁷ acquired by symbiosis. The nearly complete deltaproteobacterial genome assembled from a single consortium contains a full magnetosome gene set⁸, but shows signs of reduction, with the probable loss of flagellar genes. Based on the metabolic gene content, the ectosymbiotic bacteria are anaerobic sulfate-reducing chemolithoautotrophs that likely reduce sulfate with hydrogen produced by hydrogenosome-like organelles⁶ underlying the plasma membrane of the protist. In addition to being necessary hydrogen sinks, ectosymbionts may provide organics to the protist by diffusion and predation, as shown by magnetosome-containing digestive vacuoles. Phylogenetic analyses of 16S and 18S ribosomal RNA genes from magnetotactic consortia in marine sediments across the Northern and Southern hemispheres indicate a host-ectosymbiont specificity and co-evolution. This suggests a historical acquisition of magnetoreception by a euglenozoan ancestor from Deltaproteobacteria followed by subsequent diversification. It also supports the cosmopolitan nature of this type of symbiosis in marine anoxic sediments.

Magnetosomes: biogenic iron nanoparticles produced by environmental bacteria

The scientific community's interest in magnetotactic bacteria has increased substantially in recent decades. These prokaryotes have the particularity of synthesizing nanomagnets, called magnetosomes. The majority of research is based on several scientific questions. Where do magnetotactic bacteria live, what are their characteristics, and why are they magnetic? What are the molecular phenomena of magnetosome biomineralization and what are the physical characteristics of magnetosomes? In addition to scientific curiosity to better understand these stunning organisms, there are biotechnological opportunities to consider. Magnetotactic bacteria, as well as magnetosomes, are used in medical applications, for example cancer treatment, or in environmental ones, for example bioremediation. In this mini-review, we investigated all the aspects mentioned above and summarized the currently available knowledge.

The Polar Organizing Protein PopZ Is Fundamental for Proper Cell Division and Segregation of Cellular Content in Magnetospirillum gryphiswaldense

Magnetotactic bacteria (MTB) share the unique capability of magnetic navigation, one of the most complex behavioral responses found in prokaryotes, by means of magnetosomes, which act as an internal compass. Due to formation of these unique nanoparticles, MTB have emerged as a model to study prokaryotic organelle formation and cytoskeletal organization in conjunction with complex motility systems. Despite the high degree of subcellular organization required in MTB, less is known about cell-cycle-related factors or proteins responsible for spatiotemporal polarity control. Here, we investigate the function of the polar organizer PopZ in the magnetotactic alphaproteobacterium Magnetospirillum gryphiswaldense. Although PopZ is widely distributed among the alphaproteobacteria, its function in MTB belonging to this class has remained unexplored. Our results suggest that in M. gryphiswaldense, PopZ has a key role during cell division and subcellular organization. Furthermore, we show that PopZ localization and function differ from other nonmagnetotactic alphaproteobacterial model organisms.

Magnetotactic bacteria (MTB) are of special scientific interest due to the formation of magnetosomes, intracellular membrane-enveloped magnetite crystals arranged into a linear chain by a dedicated cytoskeleton. Magnetotaxis relies on the formation and proper inheritance of these unique magnetic organelles, both of which need to be coordinated with the segregation of other cellular content such as chromosomes or motility and chemotaxis related structures. Thus, elaborated mechanisms are required in MTB to coordinate and maintain a high level of spatial and temporal subcellular organization during cytokinesis. However, thus far, underlying mechanisms and polarity determinants such as landmark proteins remained obscure in MTB. Here, we analyzed an ortholog of the polar organizing protein Z in the alphaproteobacterium Magnetospirillum gryphiswaldense termed PopZ_Mgr. We show that deletion of the popZ_Mgr gene causes abnormal cell elongation, minicell formation, DNA missegregation, and impairs motility. Overproduction of PopZ_Mgr results in PopZ-rich regions near the poles, which are devoid of larger macromolecules, such as ribosomes, chromosomal DNA, and polyhydroxybutyrate (PHB) granules. Using superresolution microscopy, we show that PopZ_Mgr exhibits a bipolar localization pattern throughout the cell cycle, indicating that the definition of new poles in M. gryphiswaldense occurs immediately upon completion of cytokinesis. Moreover, substitution of PopZ orthologs between M. gryphiswaldense and the related alphaproteobacterium Caulobacter crescentus indicated that PopZ localization depends on host-specific cues and that both orthologs have diverged to an extent that allows only partial reciprocal functional complementation. Altogether, our results indicate that in M. gryphiswaldense, PopZ plays a critical role during cell division and segregation of cellular content.

High-Throughput Microfluidic Sorting of Live Magnetotactic Bacteria

Magnetic nanoparticles (MNPs) are useful for many biomedical applications, but it is challenging to synthetically produce them in large numbers with uniform properties and surface functionalization. Magnetotactic bacteria (MTB) produce magnetosomes with homogenous sizes, shapes, and magnetic properties. Consequently, there is interest in using MTB as biological factories for MNP production. Nonetheless, MTB can only be grown to low yields, and wild-type strains produce low numbers of MNPs/bacterium. There are also limited technologies to facilitate the selection of MTB with different magnetic contents, such as MTB with compromised and enhanced biomineralization ability. Here, we describe a magnetic microfluidic platform combined with transient cold/alkaline treatment to temporarily reduce the rapid flagellar motion of MTB without compromising their long-term proliferation and biomineralization ability for separating MTB on the basis of their magnetic contents. This strategy enables live MTB to be enriched, which, to the best of our knowledge, has not been achieved with another previously described magnetic microfluidic device that makes use of ferrofluid and heat. Our device also facilitates the high-throughput (25,000 cells/min) separation of wild-type Magnetospirillum gryphiswaldense (MSR-1) from nonmagnetic ΔmamAB MSR-1 mutants with a sensitivity of up to 80% and isolation purity of up to 95%, as confirmed with a gold-standard fluorescent-activated cell sorter (FACS) technique. This offers a 25-fold higher throughput than other previously described magnetic microfluidic platforms (1,000 cells/min). The device can also be used to isolate Magnetospirillum magneticum (AMB-1) mutants with different ranges of magnetosome numbers with efficiencies close to theoretical estimates. We believe this technology will facilitate the magnetic characterization of genetically engineered MTB for a variety of applications, including using MTB for large-scale, controlled MNP production.IMPORTANCE Our magnetic microfluidic technology can greatly facilitate biological applications with magnetotactic bacteria, from selection and screening to analysis. This technology will be of interest to microbiologists, chemists, and bioengineers who are interested in the biomineralization and selection of magnetotactic bacteria (MTB) for applications such as directed evolution and magnetogenetics.

Biohybrid and Bioinspired Magnetic Microswimmers

Many motile microorganisms swim and navigate in chemically and mechanically complex environments. These organisms can be functionalized and directly used for applications (biohybrid approach), but also inspire designs for fully synthetic microbots. The most promising designs of biohybrids and bioinspired microswimmers include one or several magnetic components, which lead to sustainable propulsion mechanisms and external controllability. This Review addresses such magnetic microswimmers, which are often studied in view of certain applications, mostly in the biomedical area, but also in the environmental field. First, propulsion systems at the microscale are reviewed and the magnetism of microswimmers is introduced. The review of the magnetic biohybrids and bioinspired microswimmers is structured gradually from mostly biological systems toward purely synthetic approaches. Finally, currently less explored parts of this field ranging from in situ imaging to swarm control are discussed.

Magnetic-field induced rotation of magnetosome chains in silicified magnetotactic bacteria

Understanding the biological processes enabling magnetotactic bacteria to maintain oriented chains of magnetic iron-bearing nanoparticles called magnetosomes is a major challenge. The study aimed to constrain the role of an external applied magnetic field on the alignment of magnetosome chains in Magnetospirillum magneticum AMB-1 magnetotactic bacteria immobilized within a hydrated silica matrix. A deviation of the chain orientation was evidenced, without significant impact on cell viability, which was preserved after the field was turned-off. Transmission electron microscopy showed that the crystallographic orientation of the nanoparticles within the chains were preserved. Off-axis electron holography evidenced that the change in magnetosome orientation was accompanied by a shift from parallel to anti-parallel interactions between individual nanocrystals. The field-induced destructuration of the chain occurs according to two possible mechanisms: (i) each magnetosome responds individually and reorients in the magnetic field direction and/or (ii) short magnetosome chains deviate in the magnetic field direction. This work enlightens the strong dynamic character of the magnetosome assembly and widens the potentialities of magnetotactic bacteria in bionanotechnology.

Magnetotaxis Enables Magnetotactic Bacteria to Navigate in Flow

Magnetotactic bacteria (MTB) play an important role in Earth's biogeochemical cycles by transporting minerals in aquatic ecosystems, and have shown promise for controlled transport of microscale objects in flow conditions. However, how MTB traverse complex flow environments is not clear. Here, using microfluidics and high-speed imaging, it is revealed that magnetotaxis enables directed motion of Magnetospirillum magneticum over long distances in flow velocities ranging from 2 to 1260 µm s^-1 , corresponding to shear rates ranging from 0.2 to 142 s^-1 -a range relevant to both aquatic environments and biomedical applications. The ability of MTB to overcome a current is influenced by the flow, the magnetic field, and their relative orientation. MTB can overcome 2.3-fold higher flow velocities when directed to swim perpendicular to the flow as compared to upstream, as the latter orientation induces higher drag. The results indicate a threshold drag of 9.5 pN, corresponding to a flow velocity of 550 µm s^-1 , where magnetotaxis enables MTB to overcome counterdirectional flow. These findings bring new insights into the interactions of MTB with complex flow environments relevant to aquatic ecosystems, while suggesting opportunities for in vivo applications of MTB in microbiorobotics and targeted drug delivery.

The symbiotic magnetic-sensing hypothesis: do Magnetotactic Bacteria underlie the magnetic sensing capability of animals?

The ability to sense Earth's magnetic field has evolved in various taxa. However, despite great efforts to find the 'magnetic-sensor' in vertebrates, the results of these scientific efforts remain inconclusive. A few decades ago, it was found that bacteria, known as magnetotactic bacteria (MTB), can move along a magnetic field using nanometric chain-like structures. Still, it is not fully clear why these bacteria evolved to have this capacity. Thus, while for MTB the 'magnetic-sensor' is known but the adaptive value is still under debate, for metazoa it is the other way around. In the absence of convincing evidence for any 'magnetic-sensor' in metazoan species sensitive to Earth's magnetic field, we hypothesize that a mutualism between these species and MTB provides one. In this relationship the host benefits from a magnetotactic capacity, while the bacteria benefit a hosting environment and dispersal. We provide support for this hypothesis using existing literature, demonstrating that by placing the MTB as the 'magnetic-sensor', previously contradictory results are now in agreement. We also propose plausible mechanisms and ways to test the hypothesis. If proven correct, this hypothesis would shed light on the forces driving both animal and bacteria magnetotactic abilities.

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.

Measurement of the magnetic moment of single Magnetospirillum gryphiswaldense cells by magnetic tweezers

Magnetospirillum gryphiswaldense is a helix-shaped magnetotactic bacterium that synthesizes iron-oxide nanocrystals, which allow navigation along the geomagnetic field. The bacterium has already been thoroughly investigated at the molecular and cellular levels. However, the fundamental physical property enabling it to perform magnetotaxis, its magnetic moment, remains to be elucidated at the single cell level. We present a method based on magnetic tweezers; in combination with Stokesian dynamics and Boundary Integral Method calculations, this method allows the simultaneous measurement of the magnetic moments of multiple single bacteria. The method is demonstrated by quantifying the distribution of the individual magnetic moments of several hundred cells of M. gryphiswaldense. In contrast to other techniques for measuring the average magnetic moment of bacterial populations, our method accounts for the size and the helical shape of each individual cell. In addition, we determined the distribution of the saturation magnetic moments of the bacteria from electron microscopy data. Our results are in agreement with the known relative magnetization behavior of the bacteria. Our method can be combined with single cell imaging techniques and thus can address novel questions about the functions of components of the molecular magnetosome biosynthesis machinery and their correlation with the resulting magnetic moment.

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.

Supramolecular Adaptive Nanomotors with Magnetotaxis Behavior

With a convenient bottom-up approach, magnetic metallic nickel is grown in situ of a supramolecular nanomotor using the catalytic activities of preloaded platinum nanoparticles. After introducing magnetic segments, simultaneous guidance and steering of catalytically powered motors with additional magnetic fields are achieved. Guided motion in a tissue model is demonstrated.

Magnetic guidance of the magnetotactic bacterium Magnetospirillum gryphiswaldense

Magnetospirillum gryphiswaldense is a magnetotactic bacterium with a permanent magnetic moment capable of swimming using two bipolarly located flagella. In their natural environment these bacteria swim along the field lines of the homogeneous geomagnetic field in a typical run and reversal pattern and thereby create non-differentiable trajectories with sharp edges. In the current work we nevertheless achieve stable guidance along curved lines of mechanical instability by using a heterogeneous magnetic field of a garnet film. The successful guidance of the bacteria depends on the right balance between motility and the magnetic moment of the magnetosome chain.

From invagination to navigation: The story of magnetosome-associated proteins in magnetotactic bacteria

Magnetotactic bacteria (MTB) are a group of Gram-negative microorganisms that are able to sense and change their orientation in accordance with the geomagnetic field. This unique capability is due to the presence of a special suborganelle called the magnetosome, composed of either a magnetite or gregite crystal surrounded by a lipid membrane. MTB were first detected in 1975 and since then numerous efforts have been made to clarify the special mechanism of magnetosome formation at the molecular level. Magnetosome formation can be divided into several steps, beginning with vesicle invagination from the cell membrane, through protein sorting, followed by the combined steps of iron transportation, biomineralization, and the alignment of magnetosomes into a chain. The magnetosome-chain enables the sensing of the magnetic field, and thus, allows the MTB to navigate. It is known that magnetosome formation is tightly controlled by a distinctive set of magnetosome-associated proteins that are encoded mainly in a genomically conserved region within MTB called the magnetosome island (MAI). Most of these proteins were shown to have an impact on the magnetism of MTB. Here, we describe the process in which the magnetosome is formed with an emphasis on the different proteins that participate in each stage of the magnetosome formation scheme.

Opposite and Coordinated Rotation of Amphitrichous Flagella Governs Oriented Swimming and Reversals in a Magnetotactic Spirillum

Current knowledge regarding the mechanism that governs flagellar motor rotation in response to environmental stimuli stems mainly from the study of monotrichous and peritrichous bacteria. Little is known about how two polar flagella, one at each cell pole of the so-called amphitrichous bacterium, are coordinated to steer the swimming. Here we fluorescently labeled the flagella of Magnetospirillum magneticum AMB-1 cells and took advantage of the magnetically controllable swimming of this bacterium to investigate flagellar rotation in moving cells. We identified three motility behaviors (runs, tumbles, and reversals) and two characteristic fluorescence patterns likely corresponding to flagella rotating in opposite directions. Each AMB-1 locomotion mode was systematically associated with particular flagellar patterns at the poles which led us to conclude that, while cell runs are allowed by the asymmetrical rotation of flagellar motors, their symmetrical rotation triggers cell tumbling. Our observations point toward a precise coordination of the two flagellar motors which can be temporarily unsynchronized during tumbling.

IMPORTANCE

Motility is essential for bacteria to search for optimal niches and survive. Many bacteria use one or several flagella to explore their environment. The mechanism by which bipolarly flagellated cells coordinate flagellar rotation is poorly understood. We took advantage of the genetic amenability and magnetically controlled swimming of the spirillum-shaped magnetotactic bacterium Magnetospirillum magneticum AMB-1 to correlate cell motion with flagellar rotation. We found that asymmetric rotation of the flagella (counterclockwise at the lagging pole and clockwise at the leading pole) enables cell runs whereas symmetric rotation triggers cell tumbling. Taking into consideration similar observations in spirochetes, bacteria possessing bipolar ribbons of periplasmic flagella, we propose a conserved motility paradigm for spirillum-shaped bipolarly flagellated bacteria.

Magnetosensitive neurons mediate geomagnetic orientation in Caenorhabditis elegans

Many organisms spanning from bacteria to mammals orient to the earth's magnetic field. For a few animals, central neurons responsive to earth-strength magnetic fields have been identified; however, magnetosensory neurons have yet to be identified in any animal. We show that the nematode Caenorhabditis elegans orients to the earth's magnetic field during vertical burrowing migrations. Well-fed worms migrated up, while starved worms migrated down. Populations isolated from around the world, migrated at angles to the magnetic vector that would optimize vertical translation in their native soil, with northern- and southern-hemisphere worms displaying opposite migratory preferences. Magnetic orientation and vertical migrations required the TAX-4 cyclic nucleotide-gated ion channel in the AFD sensory neuron pair. Calcium imaging showed that these neurons respond to magnetic fields even without synaptic input. C. elegans may have adapted magnetic orientation to simplify their vertical burrowing migration by reducing the orientation task from three dimensions to one.