Appl Microbiol Biotechnol (1999) 52: 464±473                                                           Ó Springer-Verlag 1...
465the cultivated bacteria exhibit a strong preference for      grown under laboratory conditions include severallow oxyge...
466                                                          magnetite, Fe3O4, and are characterized by narrow sizeMagneto...
467The particles are oriented with a [1 1 1] crystal axis along    the bacteria have elegantly solved the problem of how t...
468antibody cross-reactivity (Okuda et al. 1996; SchulerÈet al. 1997).    Compartmentalization through the formation of th...
469®nal two steps occur within the magnetosome vesicle,         acid sequence, the protein exhibits signi®cant homologywhi...
470Fig. 5 Puri®ed magnetosomesfrom M. gryphiswaldense. Ow-ing to the presence of theenveloping membrane, isolatedmagnetoso...
471of IgG. The procedure involved immobilizing ¯uores-           fundamental understanding of the biochemical and ge-cein-...
472Bahaj AS, Croudace IW, James PAB, Moeschler FD, Warwick PE             Guerinot ML (1994) Microbial iron transport. Ann...
473    magnetic bacterium Aquaspirillum sp. AMB-1. Appl Biochem         Schuler D, Kohler M (1992) The isolation of a new ...
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Bacterial magnetosomes. microbiology, biomineralization and biotechnological applications

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Bacterial magnetosomes. microbiology, biomineralization and biotechnological applications

  1. 1. Appl Microbiol Biotechnol (1999) 52: 464±473 Ó Springer-Verlag 1999 MINI-REVIEW ÈD. Schuler á R. B. FrankelBacterial magnetosomes: microbiology, biomineralizationand biotechnological applicationsReceived: 2 December 1998 / Received revision: 2 March 1999 / Accepted: 5 March 1999Abstract Magnetotactic bacteria orient and migrate One of the most intriguing examples of biologicallyalong geomagnetic ®eld lines. This ability is based on controlled mineralization is the formation of magneticintracellular magnetic structures, the magnetosomes, nano-crystals in magnetosomes within magnetotacticwhich comprise nanometer-sized, membrane-bound bacteria. The formation of magnetosomes in these or-crystals of the magnetic iron minerals magnetite (Fe3O4) ganisms is a well-documented example of the appar-or greigite (Fe3S4). Magnetosome formation is achieved ently widespread occurrence of magnetic minerals inby a mineralization process with biological control over the living world. Biomineralization of ferromagneticthe accumulation of iron and the deposition of the materials, mainly magnetite, has been reported for anmineral particle with speci®c size and orientation within extremely diverse range of organisms including algaea membrane vesicle at speci®c locations in the cell. This (Torres de Araujo et al. 1986), insects (Maher 1998),review focuses on the current knowledge about magne- molluscs (Lowenstam 1981), ®sh (Mann et al. 1988),totactic bacteria and will outline aspects of the physi- birds (Wiltschko and Wiltschko 1995) and even hu-ology and molecular biology of the biomineralization mans (Kirschvink et al. 1992). The discovery of mag-process. Potential biotechnological applications of netotactic bacteria by R. Blakemore (1975) stimulatedmagnetotactic bacteria and their magnetosomes as well interdisciplinary research interest among scientists, in-as perspectives for further research are discussed. cluding microbiologists, physicists, geologists, chemists, and engineers. Commercial uses of bacterial magneto- some particles have been suggested, including the manufacture of magnetic tapes and printing inks,Introduction magnetic targeting of pharmaceuticals, cell separation and their application as contrast-enhancement agents inThe formation of biological hard materials, i.e., bone, magnetic resonance imaging (Blakemore 1983; Mannshell and spicule, involves the deposition of speci®c in- et al. 1990; Matsunaga 1991; Schwartz and Blakemoreorganic minerals on or in an organic matrix in a highly 1984).controlled manner (Lowenstam 1981). Much currentresearch is directed towards understanding these bio-mineralization processes, which are collectively known Ecology and phylogeny of magnetotactic bacteriaas biologically controlled mineralization, because oftheir relevance to the synthesis of advanced materials Magnetotactic bacteria represent a heterogeneous groupwith tailored properties (Mann 1993; Moskowitz 1995). of procaryotes with a variety of morphological types including cocci, rods, vibrios, spirilla and apparently multicellular forms (Bazylinski et al. 1994; Spring and Schleifer 1995). They are constituents of natural micro-D. Schuler (8) È bial communities in sediments and chemically strati®edMax-Planck-Institut fur Marine Mikrobiologie, È water columns and a broad diversity of morphological28 359 Bremen, Germany forms has been found in many marine and freshwatere-mail: dschuele@mpi-bremen.deTel.: +49-421-20 28 746 habitats. The highest numbers of magnetotactic bacteriaFax: +49-421-20 28 580 are generally found at the oxic/anoxic transition zone orR. B. Frankel redoxocline, which is located in many freshwater habi-Department of Physics, California Polytechnic State University, tats at the sediment/water interface or just belowSan Luis Obispo, CA 93407, USA (Bazylinski 1995; Stolz 1993). Accordingly, nearly all of
  2. 2. 465the cultivated bacteria exhibit a strong preference for grown under laboratory conditions include severallow oxygen concentrations and behave as typical mic- strains of Magnetospirillum (Fig. 1; Blakemore et al.roaerophiles. In some environments, magnetotactic 1979; Burgess et al. 1993; Schleifer et al. 1991; Schuler Èbacteria have been shown to be the dominant species of and Kohler 1992). Two strains of a marine magnetic Èthe bacterial population (Spring et al. 1993), implying a vibrio were isolated that can be grown either anaero-signi®cant ecological role for these organisms. bically or microaerobically (Bazylinski et al. 1988; Examinations of natural communities by molecular Meldrum et al. 1993a). The only cultivable magneticphylogenetic techniques have revealed a considerable coccus was grown microaerobically in gradient culturesphylogenetic diversity of natural populations of mag- (Meldrum et al. 1993b). Sakaguchi et al. (1993) were thenetotactic bacteria. The vast majority of magnetotactic ®rst to isolate an obligate anaerobic, sulfate-reducingbacteria, including cocci and rods, as well as all cultiv- magnetotactic bacterium.able vibrios and spirilla, are members of the a-Proteo-bacteria. A morphologically distinct, large magnetic rodwas assigned to the Nitrospira phylum, whereas a Magnetotaxis``multicellular magnetic procaryote and a magneticsulfate-reducing bacterium were found to belong to the d A widely accepted hypothesis about the function ofsubclass of Proteobacteria (DeLong et al. 1993; Spring magnetotaxis is that, because all known magnetotacticand Schleifer 1995). bacteria are either microaerophilic or anaerobic, they seek to avoid high oxygen levels and their navigation along the geomagnetic ®eld lines facilitates migration toIsolation and axenic cultivation their favored position in the oxygen gradient (Frankel and Bazylinski 1994). The preferred motility directionTheir directed migration in magnetic ®elds can easily be found in natural populations of magnetotactic bacteriaused to enrich magnetotactic bacteria and collect them is northward in the geomagnetic ®eld in the northernfrom natural samples in signi®cant numbers. However, hemisphere, whereas it is southward in the southerndespite their ubiquitous occurrence and high abundance, hemisphere. Because of the inclination of the geomag-cultivation of magnetotactic bacteria in the laboratory netic ®eld, migration in these preferred directions wouldhas proven dicult to achieve and only a small minority cause cells in both hemispheres to swim downward.of the broad spectrum of naturally occurring magneto- Recent ®ndings indicate that this process is complex andtactic bacteria have been isolated in pure culture. Di- involves interactions with an aerotactic sensory mecha-culties in isolating and cultivating of magnetotactic nism (Frankel et al. 1997, 1998). Besides magnetotaxis,bacteria arise from their lifestyle, which is adapted to other possible functions of intracellular iron depositionsediments and chemically strati®ed aquatic habitats. As have been discussed, including iron homeostasis, energytypical gradient organisms, magnetotactic bacteria ap- conservation, or redox cycling (Mann et al. 1990;pear to depend on a complex pattern of vertical chemical Guerin and Blakemore 1992; Spring et al. 1993). Thisand redox gradients, which are dicult to mimic under might explain the large numbers of magnetic crystals inlaboratory conditions. Examples of isolates that can be some bacteria.Fig. 1 Electron micrograph ofa Magnetospirillum gryphiswal-dense cell exhibiting the char-acteristic morphology ofmagnetic spirilla. The helicalcells are bipolarly ¯agellatedand contain up to 60 intracel-lular magnetite particles inmagnetosomes, which are ar-ranged in a chain. (Bar equiv-alent to 0.5 lm)
  3. 3. 466 magnetite, Fe3O4, and are characterized by narrow sizeMagnetosomes distributions and uniform, species-speci®c crystal habits (Fig. 2). The crystal habits all consist of various com-The inorganic phase: magnetic minerals binations of the isometric forms {111}, {110} and {100}.Magnetosomes are found in all magnetotactic bacteriaand consist of magnetic iron mineral particles enclosed Fig. 2a±f Electron micrographs of crystal morphologies and intra-within membrane vesicles. In most cases the magneto- cellular organization of magnetosomes found in various magnetotac- tic bacteria. Shapes of magnetic crystals include cubo-octahedral (a),somes are organized in a chain or chains, which are elongated hexagonal prismatic (b, d, e, f) and bullet-shaped morpho-apparently ®xed within the cell. In many magnetotactic logies (c). The particles are arranged in one (a, b, c), two (e) orbacterial strains the iron mineral particles consist of multiple chains (d) or irregularly (f). (Bar equivalent to 100 nm)
  4. 4. 467The particles are oriented with a [1 1 1] crystal axis along the bacteria have elegantly solved the problem of how tothe chain direction. The particle sizes are typically 35± construct a magnetic dipole that will be oriented in the120 nm, which is within the permanent, single-magnetic- geomagnetic ®eld yet ®t inside a micrometer-sized cell.domain-size range for magnetite (Moskowitz 1995). The solution is based on the ability of the bacteria to In several magnetotactic bacteria from marine, sul®- control the mineral type of the iron, the size and orien-dic environments, the magnetosome particles consist of tation of the particles, and their placement in the cell.the iron-sul®de mineral greigite, Fe3S4, which is iso-structural with magnetite and is also ferrimagneticallyordered. Greigite magnetosomes are also characterized The organic phase: the magnetosome membraneby narrow size distributions and species-speci®c crystalhabits, but are oriented with a [1 0 0] crystal axis along In all magnetite-producing magnetotactic bacteria ex-the chain direction. Non-magnetic mackinawite, FeS, amined to date, the magnetosome mineral phase appearshas been found in cells with greigite magnetosomes and to be enveloped by a membrane (Fig. 3; Gorby et al.is thought to be a precursor to greigite mineralization 1988; Matsunga 1991; Schuler and Baeuerlein 1997b). È(Posfai et al. 1998). Although the magnetosome membrane does not appear It should be noted that metabolic activities of dissi- to be contiguous with the cell membrane in electron-milatory iron-reducing bacteria (Lovley et al. 1987) and microscopic images, it can be speculated that some sortsulfate-reducing bacteria can result in the formation of of connection or association with the cytoplasmicextracellular magnetite and greigite respectively, by membrane exists. This would help explain the biosyn-processes known as biologically induced mineralization. thetic origin of the magnetosome compartment. AHowever, unlike the mineral particles in the magneto- membranous ``superstructure may also account for thetactic bacteria, biologically induced mineralization is not integrity of complex intracellular arrangements ofcontrolled by the organism and is characterized by log/ magnetosome particles found in some magnetotacticnormal size distributions and non-unique crystal habits bacteria. Empty and partially ®lled vesicles have been(Sparks et al. 1990). observed in iron-starved cells of M. magnetotacticum When magnetosomes are arranged in a single chain, and M. gryphiswaldense (Gorby et al. 1988; Schuler and Èas in Magnetospirillum species, magnetostatic interac- Baeuerlein 1997b), so it seems likely that the magneto-tions between the single-magnetic domain particles cause some membrane pre-exists as an ``empty vesicle priorthe particle magnetic moments to orient spontaneously to the biomineralization of the mineral phase.parallel to each other along the chain direction (Frankel In strains of Magnetospirillum, the magnetosomeand Blakemore 1980). This results in a permanent membrane was found to consist of a bilayer containingmagnetic dipole associated with the chain with a natural phospholipids and proteins, at least several of whichremanent magnetization approaching the saturation appear unique to this membrane (Gorby et al. 1988;magnetization (Dunin-Borkowski et al. 1998) and su- Schuler and Baeuerlein 1997b). Although the protein Èciently large to be oriented along the geomagnetic ®eld patterns of the magnetosome membrane are distin-at ambient temperature. Since the chain of particles is guishable between di€erent strains of Magnetospirillum,®xed within the cell, the entire cell will be oriented in the at least one major protein with a molecular weight of®eld. This passive orientation results in the migration of about 22±24 kDa appears to be common to all strainsthe cell along the magnetic ®eld lines as it swims. Thus tested so far, as revealed by sequence analysis andFig. 3 Magnetosome particlesisolated from M. gryphiswal-dense. The magnetite crystalsare typically 42 nm in diameterand are surrounded by themagnetosome membrane(arrow). (Bar equivalent to25 nm)
  5. 5. 468antibody cross-reactivity (Okuda et al. 1996; SchulerÈet al. 1997). Compartmentalization through the formation of themagnetosome vesicle enables the processes of mineralformation to be regulated by biochemical pathways. Themembrane may act as a potential gate for compositional,pH and redox di€erentiation between the vesicle and thecellular environment. Although the exact role of themagnetosome-speci®c proteins has not been elucidated,it has been speculated that these have speci®c functionsin the accumulation of iron, nucleation of minerals andredox and pH control (Gorby et al. 1988; Mann et al.1990).Physiology and biochemistry of bacterial magnetitebiomineralization Fig. 4 Growth (A), magnetism (M), and dissolved oxygen concen-The ®rst step in magnetite synthesis in magnetotactic tration (O2) in a culture of M. gryphiswaldense at constant aeration. Changes in magnetism of the cell suspension were monitored bybacteria is the uptake of iron. Because of the large measurement of di€erential light scattering (Schuler et al. 1995); Èamounts of iron required for magnetite synthesis, mag- growth was measured by absorbance. The cells were non-magneticnetotactic bacteria can be expected to use very ecient during aerobic growth. After 20 h the production of Fe3O4 wasuptake systems. The assimilation of iron must be strictly induced by the establishment of microaerobic conditions, correspond- ing to an oxygen concentration of about 1%±3% saturationcontrolled because of the potentially harmful e€ects ofexcess intracellular iron (Guerinot 1994). Several studies,all involving magnetic spirilla, have focused on iron content in non-magnetic cells is relatively low, magneticuptake. Paoletti and Blakemore (1986) reported the cells can contain more then 2% iron on a dry-weightproduction of a hydroxamate-type siderophore if cells of base (Schuler and Baeuerlein 1998). ÈM. magnetotacticum were grown under conditions of In an e€ort to elucidate the relationship betweenhigh iron. However, these results have not been repli- respiratory electron transport in nitrate and oxygencated in other studies and no conclusive evidence for the utilization and magnetite synthesis, cytochromes ininvolvement of siderophores in the formation of mag- M. magnetotacticum have been examined. Tamegai et al.netite has been found so far. Nakamura et al. (1993b) (1993) reported a novel ``cytochrome-a1-like hemo-hypothesized that ferric iron was taken up in Magne- protein that was in greater amounts in magnetic cellstospirillum AMB-1 by a periplasmic binding-protein- than in nonmagnetic cells. A new ccb-type cytochrome cdependent iron-transport system. In M. gryphiswaldense, oxidase (Tamegai and Fukumori 1994) and a cyto-it was found that the major portion of the iron is taken chrome cd1-type nitrite reductase (Yamazaki et al. 1995)up as Fe(III) in an energy-dependent process (Schuler È were isolated and puri®ed from M. magnetotacticum.and Baeuerlein 1996). The observed high rates of ferric The latter protein is of particular interest since it showediron uptake may re¯ect the extraordinary requirement Fe(II):nitrite oxidoreductase activity. It has also beenfor iron in these organisms. Both the amount of mag- observed that the formation of magnetite was enhancednetite formed and the rate of ferric iron uptake were if cells of M. magnetotacticum were growing by dissi-close to saturation at extracellular iron concentrations of milatory nitrate reduction (Blakemore et al. 1985). On15±20 lM Fe, indicating that this bacterium is able to the basis of these observations, it was proposed byaccumulate copious amounts of iron from relatively low Fukumori et al. (1997) that the dissimilatory nitriteenvironmental concentrations. reductase of M. magnetotacticum could function as an The biomineralization of magnetic crystals does not Fe(II) oxidizing enzyme for magnetite synthesis underoccur constitutively, but largely depends on the growth anaerobic conditions.conditions. Besides the availability of micromolar Frankel et al. (1983) examined the nature and dis-amounts of iron, microaerobic conditions are required tribution of major iron compounds in M. magneto-for magnetite formation in Magnetospirillum species tacticum by using Fe-57 Moûbauer spectroscopy. They È(Blakemore et al. 1985; Schuler and Baeuerlein 1998). È proposed a model in which Fe(III) is taken up by the cellCells of M. gryphiswaldense are non-magnetic during and reduced to Fe(II) as it enters the cell. It is thenaerobic growth, but start to produce Fe3O4 immediately thought to be reoxidized to form a low-density hydrousafter the establishment of microaerobic conditions cor- Fe(III) oxide, which is then dehydrated to form a high-responding to an oxygen concentration of about 1%± density Fe(III) oxide (ferrihydrite), which can be directly3% saturation (Fig. 4). The accumulation of iron during observed in cells. In the last step, one-third of the Fe(III)growth is apparently tightly coupled to the induction of ions in ferrihydrite are reduced and, with further dehy-magnetite biomineralization. While the intracellular iron dration, magnetite is produced. It is thought that the
  6. 6. 469®nal two steps occur within the magnetosome vesicle, acid sequence, the protein exhibits signi®cant homologywhich acts as a further constraint on crystal growth. with a number of functionally diverse proteins belonging to the tetratricopeptide repeat family. However, the role of the protein in magnetosome synthesis has not yetMolecular biology of magnetotactic bacteria been elucidated.Attempts to address the molecular biology of magne-tosome formation have been hampered by several Mass cultivation and isolation of magnetosomesproblems including the lack of a signi®cant number ofmagnetotactic bacterial strains, the fastidiousness of the Only a limited number of magnetotactic bacteria haveorganisms in culture, the elaborate techniques required been isolated in pure culture so far. Most of the isolatesfor the growth of these organisms and the inability of are poorly characterized in terms of growth conditionsalmost all these strains to grow on the surface of agar and physiology. Therefore, several Magnetospirillumplates used to screen for mutants. Therefore, our strains have been used for the isolation of magnetosomesknowledge of the genetic determination of magnetosome that can be grown microaerobically on simple mediaformation is still fragmentary. containing short organic acids as a carbon source and Initial studies addressing molecular genetics in mag- ferric chelates as a source of iron (Blakemore et al. 1979;netotactic bacteria were made with M. magnetotacticum Schuler and Baeuerlein 1996; Matsunaga et al. 1990). Èstrain MS-1. It was shown that at least some of the genes The cells have a strictly oxygen-dependent respiratoryof this organism can be functionally expressed in Esc- type of metabolism, but do not tolerate higher oxygenherichia coli and that the transcriptional and transla- concentrations. As growth and magnetosome formationtional elements of the two microorganisms are depend on microaerobic conditions, the control of a lowcompatible. Berson et al. (1989) were able to clone and oxygen concentration in the growth medium (1%±3%)functionally express the recA gene from M. magneto- O2 is of critical importance (Blakemore et al. 1985;tacticum in E. coli. They then cloned a 2-kb DNA Schuler and Baeuerlein 1998). Èfragment from M. magnetotacticum that complemented M. magnetotacticum was the ®rst magnetotacticiron-uptake de®ciencies in E. coli and Salmonella typh- bacterium available in pure culture and was used in theimurium mutants lacking a functional aroD gene (bio- initial studies. One of the reasons for oxygen sensitivitysynthetic dehydroquinase). This suggests that the 2-kb in this organism may be the lack of the oxygen-protec-DNA fragment may have a function in iron uptake in tive enzyme catalase. Addition of catalase to the growthM. magnetotacticum (Berson et al. 1991). medium resulted in increased oxygen tolerance (Blake- Matsunaga et al. (1992) attempted to establish a ge- more et al. 1979). M. gryphiswaldense and Magnetos-netic system in Magnetospirillum strain AMB-1. Cells of pirillum AMB-1 are more oxygen-tolerant and easier tothis species were reported to form magnetic colonies on grow on a large scale. If grown from large inocula,the surface of agar plates when grown under an incu- cultures are tolerant to atmospheric air, which obviatesbation atmosphere containing 2% oxygen. This feature the need for elaborate microaerobic techniques. Thefacilitated the selection of non-magnetic mutants of maximum cell yields reported so far are 0.33 g/l (dryMagnetospirillum strain AMB-1 by Tn5 mutagenesis by weight) for M. gryphiswaldense grown in a 100-l fer-conjugal transfer. It was concluded that at least three menter (Schuler and Baeuerlein 1997a) and 0.34 g/l for Èregions of the chromosome are required for the synthesis Magnetospirillum AMB-1 grown at a 4-l scale (Matsu-of magnetosomes. One of these regions was found to naga et al. 1997).contain a gene, designated magA, that encodes a protein Techniques for the isolation and puri®cation ofthat is homologous to cation e‚ux proteins, in partic- magnetosome particles from Magnetospirillum speciesular the E. coli potassium-ion-translocating protein are based on magnetic separation (Gorby et al. 1988;KefC (Nakamura et al. 1995a). Membrane vesicles Okuda et al. 1996) or a combination of a sucrose-gra-prepared from E. coli cells expressing magA took up iron dient centrifugation and a magnetic separation tech-when ATP was supplied, indicating that energy was re- nique (Schuler and Baeuerlein 1997b). These procedures Èquired for the uptake of iron. The expression of magA leave the surrounding membrane intact and magneto-was enhanced when wild-type Magnetospirillum AMB-1 some preparations are apparently free of contaminatingcells were grown under iron-limited conditions rather material. Owing to the presence of the envelopingthan iron-sucient conditions in which they would membrane, isolated magnetosome particles form stable,produce more magnetosomes (Nakamura et al. 1995b). well-dispersed suspensions (Fig. 5). After solubilizationThus, the role of the magA gene in magnetosome syn- of the membrane by a detergent, the remaining inorganicthesis is unclear. crystals tend to agglomerate as a result of magnetic at- Okuda et al. (1996) took a ``reverse genetics ap- tractive forces. Typically, 2.6 mg bacterial magnetite canproach to the magnetosome problem. They used the be derived from a 1000-ml culture of MagnetospirillumN-terminal amino acid sequence from a 22-kDa protein AMB-1 (Matsunaga et al. 1997), while a similar yieldspeci®cally associated with the magnetosome membrane was reported for M. gryphiswaldense (Schuler and Èto clone and sequence its gene. On the basis of the amino Baeuerlein 1997a).
  7. 7. 470Fig. 5 Puri®ed magnetosomesfrom M. gryphiswaldense. Ow-ing to the presence of theenveloping membrane, isolatedmagnetosome particles formstable, well-dispersed suspen-sions (Bar equivalent to100 nm) netic poles on meteoritic magnetic grains. Bahaj et al.Biotechnological applications (1994, 1998) considered the use of magnetotactic bacte- ria for the removal of heavy metals and radionuclidesThe formation of magnetic particles within biological from wastewater. High-gradient magnetic separation asmembranes is of great interest in terms of forming small well as a low-magnetic ®eld system based on their di-synthetic particles under closely controlled synthesis rected motility have been used to remove metal-loadedconditions. Small magnetic particles can be formed syn- bacteria from water. A ``microbial magnetometer wasthetically by following various routes. However, particles devised for the prediction of oxygen depletion in bottomformed this way are non-uniform, often not fully crys- sediments and overlying water, which is based on thetalline, compositionally nonhomogeneous, and in an magnetotactic and microaerophilic behavior of naturalagglomerated state, which imposes problems in pro- populations of magnetotactic bacteria, tracking theircessing (Sarikaya 1994). Therefore, their synthesis by a position and magnetic strength by a magnetic sensorbiological process promises advantages in terms of con- (Rhoads 1995). All the examples described above rely ontrolling growth and morphological properties. More- living, actively swimming and partially non-cultivableover, biomineralization provides a way to produce highly magnetotactic bacteria. Because of the fastidiousnessuniform magnetite crystals without the drastic regimes of and limited availability of these bacteria, practicaltemperature, pH and pressure that are often needed for applications remain in question.their industrial production (Mann et al. 1990). Soon af-ter the discovery of magnetotactic bacteria, their bio-technological potential was realized and numerous Applications of isolated magnetosome particlescommercial uses of the small magnetic crystals have beencontemplated. Examples for the use of magnetic micro- The small size of isolated magnetosome particles providesorganisms generally fall into two categories, which in- a large surface-to-volume ratio, which makes them usefulvolve either whole, living cells and their magnetotactic as carriers for the immobilization of relatively largebehavior or utilize isolated magnetosome particles. quantities of bioactive substances, which can then sepa- rated by magnetic ®elds. In initial studies, bacterial magnetosomes from non-cultivable magnetotactic bac-Applications of magnetic cells teria were used for immobilizing the enzymes glucose oxidase and uricase as components of medically impor-Applications based on the ®rst category include the use tant biosensors. The glucose oxidase bound to biogenicof magnetotactic bacteria for the nondestructive domain magnetite particles had 40-fold higher activity than theanalysis of soft magnetic materials. When motile mag- same enzyme bound to arti®cial magnetite and Zinc fer-netotactic cocci were applied to the sample surface, the rite particles in this study (Matsunaga and Kamyia 1987).bacteria oriented, migrated and accumulated in the stray Magnetosomes have been also used for the genera-®elds originating from the magnetic domains, allowing tion of magnetic antibodies. Following treatment withvisualization of the domain structure (Harasko et al. glutaraldehyde, bacterial magnetite particles were cou-1993, 1995). Using a similar technique, Funaki et al. pled to antibodies to form conjugates. In one example,(1992) employed magnetotactic bacteria to locate mag- magnetic antibodies could be used for the quanti®cation
  8. 8. 471of IgG. The procedure involved immobilizing ¯uores- fundamental understanding of the biochemical and ge-cein-isothiocyanate-conjugated anti-(mouse IgG) anti- netic principles behind the process of bacterial magnetitebodies on bacterial magnetosome particles (Nakamura biomineralization. More research in this ®eld is clearlyet al. 1991). The presence of the magnetosome mem- required. The most critical issue is the mechanism bybrane resulted in dispersion and handling properties which the organic matrix controls the nucleation andsuperior to those of synthetic magnetic particle conju- growth of the particles. Further investigation of thegates (Matsunga 1991). Other reported examples for the structure and composition of the magnetosome mem-application of antibodies immobilized on bacterial brane and its protein organization is therefore essentialmagnetosomes include the detection and removal of for understanding ion transport through the membraneE. coli cells from bacterial suspensions (Nakamura et al. and the early stages of crystal formation.1993a) as well as the highly sensitive detection of aller- Increased e€orts should be directed to the genetic andgens (Nakamura and Matsunaga 1993). molecular biological analysis of magnetosome forma- Matsunga and coworkers incorporated bacterial tion. The application of genetic engineering techniquesmagnetite particles into eucaryotic cells, which could be might possibly result in magnetosome particles withmanipulated by a magnetic ®eld. Magnetosomes were improved properties and a more dependable productionintroduced into red blood cells by fusion with bacterial process. For example, it has been suggested that mag-spheroplasts mediated by polyethylene glycol. Whole netosome-speci®c proteins might be used for the con-magnetotactic bacteria have also been introduced into struction of gene fusions to couple bioactive substancesleukocytes by phagocytosis (Matsunaga et al. 1989). directly in order to display them on the surface of the Bacterial magnetite particles were also used as carri- magnetosome particles (Nakamura et al. 1995b). Ge-ers for the introduction of DNA into cells. In a method netic engineering might also be used for control of thedescribed by Takeyama et al. (1995), bacterial magnetite chemical composition. The modi®cation of the transportparticles coated with plasmid DNA were used for the system responsible for the accumulation of iron into theballistic transformation of the marine cyanobacterium magnetosome vesicle might result in an altered speci®citySynechocystis using a particle gun. In another experi- for metal uptake, potentially yielding crystals with novelment, DNA oligonucleotides were immobilized on bac- magnetic and crystalline properties. Once the genes forterial magnetite particles and used for the detection of the biomineralization pathway are identi®ed, they mightmRNA (Sode et al. 1993). be expressed in a readily cultivable host organism, Another application of bacterial magnetosomes may thereby potentially eliminating the diculties associatedlie in their potential use as a contrast agent for magnetic with the cultivation of magnetotactic bacteria. Func-resonance imaging and tumor-speci®c drug carriers tional expression of single genes from Magnetospirillumbased on intratumoral enrichment (Bulte and Brooks in E. coli has already been demonstrated (Berson et al.1997). Synthetic liposomes containing superparamag- 1989, 1991), but the heterologous expression of genesnetic iron oxide particles have already been used in related to bacterial biomineralization could be limited bybiomedical applications of this type (Pauser et al. 1997). È the lack of an intracellular compartment. Alternatively,Again, because of their unique magnetic and crystalline bacterial hosts with an existing intracytoplasmaticproperties and the natural presence of a surrounding membrane system, such as phototrophic bacteria, mightmembrane, magnetosomes might be superior in some be more appropriate for expression and intracellularapplications. It has also been demonstrated that apply- targeting of the biomineralization machinery.ing the principle of con®ned biomineralization within Finally, in the light of the dramatic diversity of nat-the cavity of apoferritin resulted in the innovative ma- urally occurring magnetotactic bacteria with variableterial magnetoferritin, which was more e€ective as a crystal morphologies, increased e€ort should be given tomagnetic-resonance contrast agent than chemosynthetic the isolation and study of other magnetotactic speciesparticles (Bulte et al. 1994). with potentially unique features of the biomineralization process. Understanding how di€erent bacteria achieve species-speci®c control over the biomineralization pro-Perspectives cess could be used in tailoring di€erent crystal mor- phologies with the desired properties.Although the biotechnological potential of bacterial Acknowledgements D.S. was supported by grants from the De-magnetite has been demonstrated, to date no application utsche Forschungsgemeinschaft and the Max-Planck-Society.has been exploited on a commercial scale. This is par- R.B.F. was supported by NSF grant CHE 9714101. We are gratefultially because of the problems related to mass cultivation to E. Baeuerlein, D.A. Bazylinski, S. Spring and B. Tebo for dis-of magnetotactic bacteria, which is still too expensive for cussions and collaboration.most practical purposes. However, the use of bacterialmagnetosomes could be competitive in some specialized Referencesapplications, where perfect crystalline properties andrelatively small amounts are required, as in some bio- Bahaj AS, Croudace IW, James PAB (1994) Treatment of heavymedical applications, for example. Another reason for metal contaminants using magnetotactic bacteria. IEEE Transthe limited practicability of many ideas is the lack of a Magnet 30: 4707±4709
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