Isolation of Magnetospirillum magneticum AMB-1 mutants defective in bacterial magnetic particle synthesis by transposon mutagenesis

Will tomorrow's mineral materials be grown?

Biomineralization, the capacity to form minerals, has evolved in a great diversity of bacterial lineages as an adaptation to different environmental conditions and biological functions. Microbial biominerals often display original properties (morphology, composition, structure, association with organics) that significantly differ from those of abiotically formed counterparts, altogether defining the 'mineral phenotype'. In principle, it should be possible to take advantage of microbial biomineralization processes to design and biomanufacture advanced mineral materials for a range of technological applications. In practice, this has rarely been done so far and only for a very limited number of biomineral types. This is mainly due to our poor understanding of the underlying molecular mechanisms controlling microbial biomineralization pathways, preventing us from developing bioengineering strategies aiming at improving biomineral properties for different applications. Another important challenge is the difficulty to upscale microbial biomineralization from the lab to industrial production. Addressing these challenges will require combining expertise from environmental microbiologists and geomicrobiologists, who have historically been working at the forefront of research on microbe-mineral interactions, alongside bioengineers and material scientists. Such interdisciplinary efforts may in the future allow the emergence of a mineral biomanufacturing industry, a critical tool towards the development more sustainable and circular bioeconomies.

Global Analysis of Biomineralization Genes in Magnetospirillum magneticum AMB-1

Magnetotactic bacteria (MTB) are a phylogenetically diverse group of bacteria remarkable for their ability to biomineralize magnetite (Fe₃O₄) or greigite (Fe₃S₄) in organelles called magnetosomes. The majority of genes required for magnetosome formation are encoded by a magnetosome gene island (MAI). Most previous genetic studies of MTB have focused on the MAI, using screens to identify key MAI genes or targeted genetics to isolate specific genes and their function in one specific growth condition. This is the first study that has taken an unbiased approach to look at many different growth conditions to reveal key genes both inside and outside the MAI. Here, we conducted random barcoded transposon mutagenesis (RB-TnSeq) in Magnetospirillum magneticum AMB-1. We generated a library of 184,710 unique strains in a wild-type background, generating ∼34 mutant strains for each gene. RB-TnSeq also allowed us to determine the essential gene set of AMB-1 under standard laboratory growth conditions. To pinpoint novel genes that are important for magnetosome formation, we subjected the library to magnetic selection screens under varied growth conditions. We compared biomineralization under standard growth conditions to biomineralization under high-iron and anaerobic conditions, respectively. Strains with transposon insertions in the MAI gene mamT had an exacerbated biomineralization defect under both high-iron and anaerobic conditions compared to standard conditions, adding to our knowledge of the role of MamT in magnetosome formation. Mutants in an ex-MAI gene, amb4151, are more magnetic than wild-type cells under anaerobic conditions. All three of these phenotypes were validated by creating a markerless deletion strain of the gene and evaluating with TEM imaging. Overall, our results indicate that growth conditions affect which genes are required for biomineralization and that some MAI genes may have more nuanced functions than was previously understood. IMPORTANCE Magnetotactic bacteria (MTB) are a group of bacteria that can form nano-sized crystals of magnetic minerals. MTB are likely an important part of their ecosystems, because they can account for up to a third of the microbial biomass in an aquatic habitat and consume large amounts of iron, potentially impacting the iron cycle. The ecology of MTB is relatively understudied; however, the cell biology and genetics of MTB have been studied for decades. Here, we leverage genetic studies of MTB to inform environmental studies. We expand the genetic toolset for studying MTB in the lab and identify novel genes, or functions of genes, that have an impact on biomineralization.

A review of the ecology, genetics, evolution, and magnetosome –induced behaviours of the magnetotactic bacteria

The discovery of magnetosome and magnetotaxis in its most simple form in the magnetotactic bacteria (MTB) had created the tremendous impetus. MTB, spanning multiple phyla, are distributed worldwide, and they form the organelles called magnetosomes for biomineralization. Eight phylotypes of MTB belong to Alphaproteobacteria and Nitrospirae. MTB show preference for specific redox and oxygen concentration. Magnetosome chains function as the internal compass needle and align the bacterial cells passively along the local geomagnetic field (GMF). The nature of magnetosomes produced by MTB and their phylogeny suggest that bullet-shaped magnetites appeared about 3.2 billion years ago with the first magnetosomes. All MTB contains ten genes in conserved mamAB operon for magnetosome chain synthesis of which nine genes are conserved in greigite-producing MTB. Many candidate genes identify the aero-, redox-, and perhaps phototaxis. Among the prokaryotes, the MTB possess the highest number of O2-binding proteins. Magnetofossils serve as an indicator of oxygen and redox levels of the ancient environments. Most descendants of ancestral MTB lost the magnetosome genes in the course of evolution. Environmental conditions initially favored the evolution of MTB and expansion of magnetosome-formation genes. Subsequent changes in atmospheric oxygen concentration have led to changes in the ecology of MTB, loss of magnetosome genes, and evolution of nonMTB.

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.

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.

A Comparative Study of Receptor-Targeted Magnetosome and HSA-Coated Iron Oxide Nanoparticles as MRI Contrast-Enhancing Agent in Animal Cancer Model

Magnetosomes are specialized organelles arranged in intracellular chains in magnetotactic bacteria. The superparamagnetic property of these magnetite crystals provides potential applications as contrast-enhancing agents for magnetic resonance imaging. In this study, we compared two different nanoparticles that are bacterial magnetosome and HSA-coated iron oxide nanoparticles for targeting breast cancer. Both magnetosomes and HSA-coated iron oxide nanoparticles were chemically conjugated to fluorescent-labeled anti-EGFR antibodies. Antibody-conjugated nanoparticles were able to bind the MDA-MB-231 cell line, as assessed by flow cytometry. To compare the cytotoxic effect of nanoparticles, MTT assay was used, and according to the results, HSA-coated iron oxide nanoparticles were less cytotoxic to breast cancer cells than magnetosomes. Magnetosomes were bound with higher rate to breast cancer cells than HSA-coated iron oxide nanoparticles. While 250 μg/ml of magnetosomes was bound 92 ± 0.2%, 250 μg/ml of HSA-coated iron oxide nanoparticles was bound with a rate of 65 ± 5%. In vivo efficiencies of these nanoparticles on breast cancer generated in nude mice were assessed by MRI imaging. Anti-EGFR-modified nanoparticles provide higher resolution images than unmodified nanoparticles. Also, magnetosome with anti-EGFR produced darker image of the tumor tissue in T2-weighted MRI than HSA-coated iron oxide nanoparticles with anti-EGFR. In vivo MR imaging in a mouse breast cancer model shows effective intratumoral distribution of both nanoparticles in the tumor tissue. However, magnetosome demonstrated higher distribution than HSA-coated iron oxide nanoparticles according to fluorescence microscopy evaluation. According to the results of in vitro and in vivo study results, magnetosomes are promising for targeting and therapy applications of the breast cancer cells.

The magnetosome membrane protein, MmsF, is a major regulator of magnetite biomineralization in Magnetospirillum magneticum AMB-1

Magnetotactic bacteria (MTB) use magnetosomes, membrane-bound crystals of magnetite or greigite, for navigation along geomagnetic fields. In Magnetospirillum magneticum sp. AMB-1, and other MTB, a magnetosome gene island (MAI) is essential for every step of magnetosome formation. An 8-gene region of the MAI encodes several factors implicated in control of crystal size and morphology in previous genetic and proteomic studies. We show that these factors play a minor role in magnetite biomineralization in vivo. In contrast, MmsF, a previously uncharacterized magnetosome membrane protein encoded within the same region plays a dominant role in defining crystal size and morphology and is sufficient for restoring magnetite synthesis in the absence of the other major biomineralization candidates. In addition, we show that the 18 genes of the mamAB gene cluster of the MAI are sufficient for the formation of an immature magnetosome organelle. Addition of MmsF to these 18 genes leads to a significant enhancement of magnetite biomineralization and an increase in the cellular magnetic response. These results define a new biomineralization protein and lay down the foundation for the design of autonomous gene cassettes for the transfer of the magnetic phenotype in other bacteria.

Molecular mechanisms of compartmentalization and biomineralization in magnetotactic bacteria

Magnetotactic bacteria (MB) are remarkable organisms with the ability to exploit the earth's magnetic field for navigational purposes. To do this, they build specialized compartments called magnetosomes that consist of a lipid membrane and a crystalline magnetic mineral. These organisms have the potential to serve as models for the study of compartmentalization as well as biomineralization in bacteria. Additionally, they offer the opportunity to design applications that take advantage of the particular properties of magnetosomes. In recent years, a sustained effort to identify the molecular basis of this process has resulted in a clearer understanding of the magnetosome formation and biomineralization. Here, I present an overview of MB and explore the possible molecular mechanisms of membrane remodeling, protein sorting, cytoskeletal organization, iron transport, and biomineralization that lead to the formation of a functional magnetosome organelle.

mamO and mamE genes are essential for magnetosome crystal biomineralization in Magnetospirillum gryphiswaldense MSR-1

Four non-magnetic mutants of Magnetospirillum gryphiswaldense strain MSR-1 were isolated by transposon mutagenesis and found to contain interruption of either the mamO or mamE gene within the mamAB operon. Studies indicated that mamO and mamE genes are essential for magnetosome crystal biomineralization in MSR-1. This is the first report of a single gene (mamO or mamE) whose mutation affects crystal biomineralization in MSR-1.

Genomics, genetics, and cell biology of magnetosome formation

Magnetosomes are specialized organelles for magnetic navigation that comprise membrane-enveloped, nano-sized crystals of a magnetic iron mineral; they are formed by a diverse group of magnetotactic bacteria (MTB). The synthesis of magnetosomes involves strict genetic control over intracellular differentiation, biomineralization, and their assembly into highly ordered chains. Physicochemical control over biomineralization is achieved by compartmentalization within vesicles of the magnetosome membrane, which is a phospholipid bilayer associated with a specific set of proteins that have known or suspected functions in vesicle formation, iron transport, control of crystallization, and arrangement of magnetite particles. Magnetosome formation is genetically complex, and relevant genes are predominantly located in several operons within a conserved genomic magnetosome island that has been likely transferred horizontally and subsequently adapted between diverse MTB during evolution. This review summarizes the recent progress in our understanding of magnetobacterial cell biology, genomics, and the genetic control of magnetosome formation and magnetotaxis.

Formation of magnetite by bacteria and its application

Magnetic particles offer high technological potential since they can be conveniently collected with an external magnetic field. Magnetotactic bacteria synthesize bacterial magnetic particles (BacMPs) with well-controlled size and morphology. BacMPs are individually covered with thin organic membrane, which confers high and even dispersion in aqueous solutions compared with artificial magnetites, making them ideal biotechnological materials. Recent molecular studies including genome sequence, mutagenesis, gene expression and proteome analyses indicated a number of genes and proteins which play important roles for BacMP biomineralization. Some of the genes and proteins identified from these studies have allowed us to express functional proteins efficiently onto BacMPs, through genetic engineering, permitting the preservation of the protein activity, leading to a simple preparation of functional protein-magnetic particle complexes. They were applicable to high-sensitivity immunoassay, drug screening and cell separation. Furthermore, fully automated single nucleotide polymorphism discrimination and DNA recovery systems have been developed to use these functionalized BacMPs. The nano-sized fine magnetic particles offer vast potential in new nano-techniques.

Genetics and cell biology of magnetosome formation in magnetotactic bacteria

The ability of magnetotactic bacteria (MTB) to orient in magnetic fields is based on the synthesis of magnetosomes, which are unique prokaryotic organelles comprising membrane-enveloped, nano-sized crystals of a magnetic iron mineral that are aligned in well-ordered intracellular chains. Magnetosome crystals have species-specific morphologies, sizes, and arrangements. The magnetosome membrane, which originates from the cytoplasmic membrane by invagination, represents a distinct subcellular compartment and has a unique biochemical composition. The roughly 20 magnetosome-specific proteins have functions in vesicle formation, magnetosomal iron transport, and the control of crystallization and intracellular arrangement of magnetite particles. The assembly of magnetosome chains is under genetic control and involves the action of an acidic protein that links magnetosomes to a novel cytoskeletal structure, presumably formed by a specific actin-like protein. A total of 28 conserved genes present in various magnetic bacteria were identified to be specifically associated with the magnetotactic phenotype, most of which are located in the genomic magnetosome island. The unique properties of magnetosomes attracted broad interdisciplinary interest, and MTB have recently emerged as a model to study prokaryotic organelle formation and evolution.

Inactivation of the flagellin gene flaA in Magnetospirillum gryphiswaldense results in nonmagnetotactic mutants lacking flagellar filaments

Magnetotactic bacteria synthesize magnetosomes, which cause them to orient and migrate along magnetic field lines. The analysis of magnetotaxis and magnetosome biomineralization at the molecular level has been hindered by the unavailability of genetic methods, namely the lack of a means to introduce directed gene-specific mutations. Here we report a method for knockout mutagenesis by homologous recombination in Magnetospirillum gryphiswaldense. Multiple flagellin genes, which are unlinked in the genome, were identified in M. gryphiswaldense. The targeted disruption of the flagellin gene flaA was shown to eliminate flagella formation, motility, and magnetotaxis. The techniques described in this paper will make it possible to take full advantage of the forthcoming genome sequences of M. gryphiswaldense and other magnetotactic bacteria.

Magnetosome vesicles are present before magnetite formation, and MamA is required for their activation

Bacterial magnetosomes are intracellular compartments that house highly ordered magnetite crystals. By using Magnetospirillum sp. AMB-1 as a model system, we show that magnetosome vesicles exist in the absence of magnetite, biomineralization of magnetite proceeds simultaneously in multiple vesicles, and biomineralization proceeds from the same location in each vesicle. The magnetosome-associated protein, MamA, is required for the formation of functional magnetosome vesicles and displays a dynamic subcellular localization throughout the growth cycle of magnetotactic bacteria. Together, these results suggest that the magnetosome precisely coordinates magnetite biomineralization and can serve as a model system for the study of organelle biogenesis in noneukaryotic cells.

Design and application of a new cryptic-plasmid-based shuttle vector for Magnetospirillum magneticum

A 3.7-kb cryptic plasmid designated pMGT was found in Magnetospirillum magneticum MGT-1. It was characterized and used for the development of an improved expression system in strain AMB-1 through the construction of a shuttle vector, pUMG. An electroporation method for magnetic bacteria that uses the cryptic plasmid was also developed.

Characterization of aldehyde ferredoxin oxidoreductase gene defective mutant in Magnetospirillum magneticum AMB-1

A non-magnetic mutant of Magnetospirillum magneticum AMB-1, designated as NMA21, was generated by mini-Tn5 transposon mutagenesis to identify genes involved in bacterial magnetic particle (BMP) synthesis. Alignment of the DNA sequences flanking the transposon allowed the isolation of an open reading frame (ORF2) within an operon consisting of five genes. The amino acid sequence of ORF2 showed homology with tungsten-containing aldehyde ferredoxin oxidoreductase (AOR) from Pyrococcus furiosus (48% identity and 64% similarity), which functions for aldehyde oxidation. AOR was found to be expressed under microaerobic conditions and localized in the cytoplasm of AMB-1. Iron uptake and growth of NMA21 were lower than wild type. Transmission electron microscopy (TEM) of NMA21 revealed that no BMPs were completely synthesized, but polyhydroxybutyrate (PHB)-like granules were persistently produced. These results indicate that AOR may contribute to ferric iron reduction during BMP synthesis in M. magneticum AMB-1 under microaerobic respiration.