MINI-REVIEW

D. SchuÈ ler á R. B. Frankel

Bacterial magnetosomes: microbiology, biomineralizationand biotechnological applications

Received: 2 December 1998 / Received revision: 2 March 1999 /Accepted: 5 March 1999

Abstract Magnetotactic bacteria orient and migratealong geomagnetic ®eld lines. This ability is based onintracellular magnetic structures, the magnetosomes,which comprise nanometer-sized, membrane-boundcrystals of the magnetic iron minerals magnetite (Fe3O4)or greigite (Fe3S4). Magnetosome formation is achievedby a mineralization process with biological control overthe accumulation of iron and the deposition of themineral particle with speci®c size and orientation withina membrane vesicle at speci®c locations in the cell. Thisreview focuses on the current knowledge about magne-totactic bacteria and will outline aspects of the physi-ology and molecular biology of the biomineralizationprocess. Potential biotechnological applications ofmagnetotactic bacteria and their magnetosomes as wellas perspectives for further research are discussed.

Introduction

The formation of biological hard materials, i.e., bone,shell and spicule, involves the deposition of speci®c in-organic minerals on or in an organic matrix in a highlycontrolled manner (Lowenstam 1981). Much currentresearch is directed towards understanding these bio-mineralization processes, which are collectively knownas biologically controlled mineralization, because oftheir relevance to the synthesis of advanced materialswith tailored properties (Mann 1993; Moskowitz 1995).

One of the most intriguing examples of biologicallycontrolled mineralization is the formation of magneticnano-crystals in magnetosomes within magnetotacticbacteria. The formation of magnetosomes in these or-ganisms is a well-documented example of the appar-ently widespread occurrence of magnetic minerals inthe living world. Biomineralization of ferromagneticmaterials, mainly magnetite, has been reported for anextremely diverse range of organisms including algae(Torres de Araujo et al. 1986), insects (Maher 1998),molluscs (Lowenstam 1981), ®sh (Mann et al. 1988),birds (Wiltschko and Wiltschko 1995) and even hu-mans (Kirschvink et al. 1992). The discovery of mag-netotactic bacteria by R. Blakemore (1975) stimulatedinterdisciplinary research interest among scientists, in-cluding microbiologists, physicists, geologists, chemists,and engineers. Commercial uses of bacterial magneto-some particles have been suggested, including themanufacture of magnetic tapes and printing inks,magnetic targeting of pharmaceuticals, cell separationand their application as contrast-enhancement agents inmagnetic resonance imaging (Blakemore 1983; Mannet al. 1990; Matsunaga 1991; Schwartz and Blakemore1984).

Ecology and phylogeny of magnetotactic bacteria

Magnetotactic bacteria represent a heterogeneous groupof procaryotes with a variety of morphological typesincluding cocci, rods, vibrios, spirilla and apparentlymulticellular forms (Bazylinski et al. 1994; Spring andSchleifer 1995). They are constituents of natural micro-bial communities in sediments and chemically strati®edwater columns and a broad diversity of morphologicalforms has been found in many marine and freshwaterhabitats. The highest numbers of magnetotactic bacteriaare generally found at the oxic/anoxic transition zone orredoxocline, which is located in many freshwater habi-tats at the sediment/water interface or just below(Bazylinski 1995; Stolz 1993). Accordingly, nearly all of

Appl Microbiol Biotechnol (1999) 52: 464±473 Ó Springer-Verlag 1999

D. SchuÈ ler (&)Max-Planck-Institut fuÈ r Marine Mikrobiologie,28 359 Bremen, Germanye-mail: dschuele@mpi-bremen.deTel.: +49-421-20 28 746Fax: +49-421-20 28 580

R. B. FrankelDepartment of Physics, California Polytechnic State University,San Luis Obispo, CA 93407, USA

the cultivated bacteria exhibit a strong preference forlow oxygen concentrations and behave as typical mic-roaerophiles. In some environments, magnetotacticbacteria have been shown to be the dominant species ofthe bacterial population (Spring et al. 1993), implying asigni®cant ecological role for these organisms.

Examinations of natural communities by molecularphylogenetic techniques have revealed a considerablephylogenetic diversity of natural populations of mag-netotactic bacteria. The vast majority of magnetotacticbacteria, including cocci and rods, as well as all cultiv-able vibrios and spirilla, are members of the a-Proteo-bacteria. A morphologically distinct, large magnetic rodwas assigned to the Nitrospira phylum, whereas a``multicellular magnetic procaryote'' and a magneticsulfate-reducing bacterium were found to belong to the dsubclass of Proteobacteria (DeLong et al. 1993; Springand Schleifer 1995).

Isolation and axenic cultivation

Their directed migration in magnetic ®elds can easily beused to enrich magnetotactic bacteria and collect themfrom natural samples in signi®cant numbers. However,despite their ubiquitous occurrence and high abundance,cultivation of magnetotactic bacteria in the laboratoryhas proven di�cult to achieve and only a small minorityof the broad spectrum of naturally occurring magneto-tactic bacteria have been isolated in pure culture. Di�-culties in isolating and cultivating of magnetotacticbacteria arise from their lifestyle, which is adapted tosediments and chemically strati®ed aquatic habitats. Astypical gradient organisms, magnetotactic bacteria ap-pear to depend on a complex pattern of vertical chemicaland redox gradients, which are di�cult to mimic underlaboratory conditions. Examples of isolates that can be

grown under laboratory conditions include severalstrains of Magnetospirillum (Fig. 1; Blakemore et al.1979; Burgess et al. 1993; Schleifer et al. 1991; SchuÈ lerand KoÈ hler 1992). Two strains of a marine magneticvibrio were isolated that can be grown either anaero-bically or microaerobically (Bazylinski et al. 1988;Meldrum et al. 1993a). The only cultivable magneticcoccus was grown microaerobically in gradient cultures(Meldrum et al. 1993b). Sakaguchi et al. (1993) were the®rst to isolate an obligate anaerobic, sulfate-reducingmagnetotactic bacterium.

Magnetotaxis

A widely accepted hypothesis about the function ofmagnetotaxis is that, because all known magnetotacticbacteria are either microaerophilic or anaerobic, theyseek to avoid high oxygen levels and their navigationalong the geomagnetic ®eld lines facilitates migration totheir favored position in the oxygen gradient (Frankeland Bazylinski 1994). The preferred motility directionfound in natural populations of magnetotactic bacteriais northward in the geomagnetic ®eld in the northernhemisphere, whereas it is southward in the southernhemisphere. Because of the inclination of the geomag-netic ®eld, migration in these preferred directions wouldcause cells in both hemispheres to swim downward.Recent ®ndings indicate that this process is complex andinvolves interactions with an aerotactic sensory mecha-nism (Frankel et al. 1997, 1998). Besides magnetotaxis,other possible functions of intracellular iron depositionhave been discussed, including iron homeostasis, energyconservation, or redox cycling (Mann et al. 1990;Guerin and Blakemore 1992; Spring et al. 1993). Thismight explain the large numbers of magnetic crystals insome 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)

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Magnetosomes

The inorganic phase: magnetic minerals

Magnetosomes are found in all magnetotactic bacteriaand consist of magnetic iron mineral particles enclosedwithin membrane vesicles. In most cases the magneto-somes are organized in a chain or chains, which areapparently ®xed within the cell. In many magnetotacticbacterial strains the iron mineral particles consist of

magnetite, Fe3O4, and are characterized by narrow sizedistributions and uniform, species-speci®c crystal habits(Fig. 2). The crystal habits all consist of various com-binations of the isometric forms {111}, {110} and {100}.

Fig. 2a±f Electron micrographs of crystal morphologies and intra-cellular organization of magnetosomes found in various magnetotac-tic bacteria. Shapes of magnetic crystals include cubo-octahedral (a),elongated hexagonal prismatic (b, d, e, f) and bullet-shaped morpho-logies (c). The particles are arranged in one (a, b, c), two (e) ormultiple chains (d) or irregularly (f). (Bar equivalent to 100 nm)

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The particles are oriented with a [1 1 1] crystal axis alongthe chain direction. The particle sizes are typically 35±120 nm, which is within the permanent, single-magnetic-domain-size range for magnetite (Moskowitz 1995).

In several magnetotactic bacteria from marine, sul®-dic environments, the magnetosome particles consist ofthe iron-sul®de mineral greigite, Fe3S4, which is iso-structural with magnetite and is also ferrimagneticallyordered. Greigite magnetosomes are also characterizedby narrow size distributions and species-speci®c crystalhabits, but are oriented with a [1 0 0] crystal axis alongthe chain direction. Non-magnetic mackinawite, FeS,has been found in cells with greigite magnetosomes andis thought to be a precursor to greigite mineralization(Posfai et al. 1998).

It should be noted that metabolic activities of dissi-milatory iron-reducing bacteria (Lovley et al. 1987) andsulfate-reducing bacteria can result in the formation ofextracellular magnetite and greigite respectively, byprocesses known as biologically induced mineralization.However, unlike the mineral particles in the magneto-tactic bacteria, biologically induced mineralization is notcontrolled by the organism and is characterized by log/normal size distributions and non-unique crystal habits(Sparks et al. 1990).

When magnetosomes are arranged in a single chain,as in Magnetospirillum species, magnetostatic interac-tions between the single-magnetic domain particles causethe particle magnetic moments to orient spontaneouslyparallel to each other along the chain direction (Frankeland Blakemore 1980). This results in a permanentmagnetic dipole associated with the chain with a naturalremanent magnetization approaching the saturationmagnetization (Dunin-Borkowski et al. 1998) and su�-ciently large to be oriented along the geomagnetic ®eldat ambient temperature. Since the chain of particles is®xed within the cell, the entire cell will be oriented in the®eld. This passive orientation results in the migration ofthe cell along the magnetic ®eld lines as it swims. Thus

the bacteria have elegantly solved the problem of how toconstruct a magnetic dipole that will be oriented in thegeomagnetic ®eld yet ®t inside a micrometer-sized cell.The solution is based on the ability of the bacteria tocontrol the mineral type of the iron, the size and orien-tation of the particles, and their placement in the cell.

The organic phase: the magnetosome membrane

In all magnetite-producing magnetotactic bacteria ex-amined to date, the magnetosome mineral phase appearsto be enveloped by a membrane (Fig. 3; Gorby et al.1988; Matsunga 1991; SchuÈ ler and Baeuerlein 1997b).Although the magnetosome membrane does not appearto be contiguous with the cell membrane in electron-microscopic images, it can be speculated that some sortof connection or association with the cytoplasmicmembrane exists. This would help explain the biosyn-thetic origin of the magnetosome compartment. Amembranous ``superstructure'' may also account for theintegrity of complex intracellular arrangements ofmagnetosome particles found in some magnetotacticbacteria. Empty and partially ®lled vesicles have beenobserved in iron-starved cells of M. magnetotacticumand M. gryphiswaldense (Gorby et al. 1988; SchuÈ ler andBaeuerlein 1997b), so it seems likely that the magneto-some membrane pre-exists as an ``empty'' vesicle priorto the biomineralization of the mineral phase.

In strains of Magnetospirillum, the magnetosomemembrane was found to consist of a bilayer containingphospholipids and proteins, at least several of whichappear unique to this membrane (Gorby et al. 1988;SchuÈ ler and Baeuerlein 1997b). Although the proteinpatterns of the magnetosome membrane are distin-guishable between di�erent strains of Magnetospirillum,at least one major protein with a molecular weight ofabout 22±24 kDa appears to be common to all strainstested so far, as revealed by sequence analysis and

Fig. 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)

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antibody cross-reactivity (Okuda et al. 1996; SchuÈ leret 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

The ®rst step in magnetite synthesis in magnetotacticbacteria is the uptake of iron. Because of the largeamounts of iron required for magnetite synthesis, mag-netotactic bacteria can be expected to use very e�cientuptake systems. The assimilation of iron must be strictlycontrolled because of the potentially harmful e�ects ofexcess intracellular iron (Guerinot 1994). Several studies,all involving magnetic spirilla, have focused on ironuptake. Paoletti and Blakemore (1986) reported theproduction of a hydroxamate-type siderophore if cells ofM. magnetotacticum were grown under conditions ofhigh iron. However, these results have not been repli-cated in other studies and no conclusive evidence for theinvolvement of siderophores in the formation of mag-netite has been found so far. Nakamura et al. (1993b)hypothesized that ferric iron was taken up in Magne-tospirillum AMB-1 by a periplasmic binding-protein-dependent iron-transport system. InM. gryphiswaldense,it was found that the major portion of the iron is takenup as Fe(III) in an energy-dependent process (SchuÈ lerand Baeuerlein 1996). The observed high rates of ferriciron uptake may re¯ect the extraordinary requirementfor iron in these organisms. Both the amount of mag-netite formed and the rate of ferric iron uptake wereclose to saturation at extracellular iron concentrations of15±20 lM Fe, indicating that this bacterium is able toaccumulate copious amounts of iron from relatively lowenvironmental concentrations.

The biomineralization of magnetic crystals does notoccur constitutively, but largely depends on the growthconditions. Besides the availability of micromolaramounts of iron, microaerobic conditions are requiredfor magnetite formation in Magnetospirillum species(Blakemore et al. 1985; SchuÈ ler and Baeuerlein 1998).Cells of M. gryphiswaldense are non-magnetic duringaerobic growth, but start to produce Fe3O4 immediatelyafter the establishment of microaerobic conditions cor-responding to an oxygen concentration of about 1%±3% saturation (Fig. 4). The accumulation of iron duringgrowth is apparently tightly coupled to the induction ofmagnetite biomineralization. While the intracellular iron

content in non-magnetic cells is relatively low, magneticcells can contain more then 2% iron on a dry-weightbase (SchuÈ ler and Baeuerlein 1998).

In an e�ort to elucidate the relationship betweenrespiratory electron transport in nitrate and oxygenutilization and magnetite synthesis, cytochromes inM. magnetotacticum have been examined. Tamegai et al.(1993) reported a novel ``cytochrome-a1-like'' hemo-protein that was in greater amounts in magnetic cellsthan in nonmagnetic cells. A new ccb-type cytochrome coxidase (Tamegai and Fukumori 1994) and a cyto-chrome cd1-type nitrite reductase (Yamazaki et al. 1995)were isolated and puri®ed from M. magnetotacticum.The latter protein is of particular interest since it showedFe(II):nitrite oxidoreductase activity. It has also beenobserved that the formation of magnetite was enhancedif cells of M. magnetotacticum were growing by dissi-milatory nitrate reduction (Blakemore et al. 1985). Onthe basis of these observations, it was proposed byFukumori et al. (1997) that the dissimilatory nitritereductase of M. magnetotacticum could function as anFe(II) oxidizing enzyme for magnetite synthesis underanaerobic conditions.

Frankel et al. (1983) examined the nature and dis-tribution of major iron compounds in M. magneto-tacticum by using Fe-57 MoÈ ûbauer spectroscopy. Theyproposed a model in which Fe(III) is taken up by the celland reduced to Fe(II) as it enters the cell. It is thenthought to be reoxidized to form a low-density hydrousFe(III) oxide, which is then dehydrated to form a high-density Fe(III) oxide (ferrihydrite), which can be directlyobserved in cells. In the last step, one-third of the Fe(III)ions in ferrihydrite are reduced and, with further dehy-dration, magnetite is produced. It is thought that the

Fig. 4 Growth (A), magnetism (M), and dissolved oxygen concen-tration (O2) in a culture of M. gryphiswaldense at constant aeration.Changes in magnetism of the cell suspension were monitored bymeasurement of di�erential light scattering (SchuÈ ler et al. 1995);growth was measured by absorbance. The cells were non-magneticduring aerobic growth. After 20 h the production of Fe3O4 wasinduced by the establishment of microaerobic conditions, correspond-ing to an oxygen concentration of about 1%±3% saturation

468

®nal two steps occur within the magnetosome vesicle,which acts as a further constraint on crystal growth.

Molecular biology of magnetotactic bacteria

Attempts to address the molecular biology of magne-tosome formation have been hampered by severalproblems including the lack of a signi®cant number ofmagnetotactic bacterial strains, the fastidiousness of theorganisms in culture, the elaborate techniques requiredfor the growth of these organisms and the inability ofalmost all these strains to grow on the surface of agarplates used to screen for mutants. Therefore, ourknowledge of the genetic determination of magnetosomeformation is still fragmentary.

Initial studies addressing molecular genetics in mag-netotactic bacteria were made with M. magnetotacticumstrain MS-1. It was shown that at least some of the genesof this organism can be functionally expressed in Esc-herichia coli and that the transcriptional and transla-tional elements of the two microorganisms arecompatible. Berson et al. (1989) were able to clone andfunctionally express the recA gene from M. magneto-tacticum in E. coli. They then cloned a 2-kb DNAfragment from M. magnetotacticum that complementediron-uptake de®ciencies in E. coli and Salmonella typh-imurium mutants lacking a functional aroD gene (bio-synthetic dehydroquinase). This suggests that the 2-kbDNA fragment may have a function in iron uptake inM. magnetotacticum (Berson et al. 1991).

Matsunaga et al. (1992) attempted to establish a ge-netic system in Magnetospirillum strain AMB-1. Cells ofthis species were reported to form magnetic colonies onthe surface of agar plates when grown under an incu-bation atmosphere containing 2% oxygen. This featurefacilitated the selection of non-magnetic mutants ofMagnetospirillum strain AMB-1 by Tn5 mutagenesis byconjugal transfer. It was concluded that at least threeregions of the chromosome are required for the synthesisof magnetosomes. One of these regions was found tocontain a gene, designated magA, that encodes a proteinthat is homologous to cation e�ux proteins, in partic-ular the E. coli potassium-ion-translocating proteinKefC (Nakamura et al. 1995a). Membrane vesiclesprepared from E. coli cells expressing magA took up ironwhen ATP was supplied, indicating that energy was re-quired for the uptake of iron. The expression of magAwas enhanced when wild-type Magnetospirillum AMB-1cells were grown under iron-limited conditions ratherthan iron-su�cient conditions in which they wouldproduce more magnetosomes (Nakamura et al. 1995b).Thus, the role of the magA gene in magnetosome syn-thesis is unclear.

Okuda et al. (1996) took a ``reverse genetics'' ap-proach to the magnetosome problem. They used theN-terminal amino acid sequence from a 22-kDa proteinspeci®cally associated with the magnetosome membraneto clone and sequence its gene. On the basis of the amino

acid sequence, the protein exhibits signi®cant homologywith a number of functionally diverse proteins belongingto the tetratricopeptide repeat family. However, the roleof the protein in magnetosome synthesis has not yetbeen elucidated.

Mass cultivation and isolation of magnetosomes

Only a limited number of magnetotactic bacteria havebeen isolated in pure culture so far. Most of the isolatesare poorly characterized in terms of growth conditionsand physiology. Therefore, several Magnetospirillumstrains have been used for the isolation of magnetosomesthat can be grown microaerobically on simple mediacontaining short organic acids as a carbon source andferric chelates as a source of iron (Blakemore et al. 1979;SchuÈ ler and Baeuerlein 1996; Matsunaga et al. 1990).The cells have a strictly oxygen-dependent respiratorytype of metabolism, but do not tolerate higher oxygenconcentrations. As growth and magnetosome formationdepend on microaerobic conditions, the control of a lowoxygen concentration in the growth medium (1%±3%)O2 is of critical importance (Blakemore et al. 1985;SchuÈ ler and Baeuerlein 1998).

M. magnetotacticum was the ®rst magnetotacticbacterium available in pure culture and was used in theinitial studies. One of the reasons for oxygen sensitivityin this organism may be the lack of the oxygen-protec-tive enzyme catalase. Addition of catalase to the growthmedium resulted in increased oxygen tolerance (Blake-more et al. 1979). M. gryphiswaldense and Magnetos-pirillum AMB-1 are more oxygen-tolerant and easier togrow on a large scale. If grown from large inocula,cultures are tolerant to atmospheric air, which obviatesthe need for elaborate microaerobic techniques. Themaximum cell yields reported so far are 0.33 g/l (dryweight) for M. gryphiswaldense grown in a 100-l fer-menter (SchuÈ ler and Baeuerlein 1997a) and 0.34 g/l forMagnetospirillum AMB-1 grown at a 4-l scale (Matsu-naga et al. 1997).

Techniques for the isolation and puri®cation ofmagnetosome particles from Magnetospirillum speciesare based on magnetic separation (Gorby et al. 1988;Okuda et al. 1996) or a combination of a sucrose-gra-dient centrifugation and a magnetic separation tech-nique (SchuÈ ler and Baeuerlein 1997b). These proceduresleave the surrounding membrane intact and magneto-some preparations are apparently free of contaminatingmaterial. Owing to the presence of the envelopingmembrane, isolated magnetosome particles form stable,well-dispersed suspensions (Fig. 5). After solubilizationof the membrane by a detergent, the remaining inorganiccrystals tend to agglomerate as a result of magnetic at-tractive forces. Typically, 2.6 mg bacterial magnetite canbe derived from a 1000-ml culture of MagnetospirillumAMB-1 (Matsunaga et al. 1997), while a similar yieldwas reported for M. gryphiswaldense (SchuÈ ler andBaeuerlein 1997a).

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Biotechnological applications

The formation of magnetic particles within biologicalmembranes is of great interest in terms of forming smallsynthetic particles under closely controlled synthesisconditions. Small magnetic particles can be formed syn-thetically by following various routes. However, particlesformed this way are non-uniform, often not fully crys-talline, compositionally nonhomogeneous, and in anagglomerated state, which imposes problems in pro-cessing (Sarikaya 1994). Therefore, their synthesis by abiological process promises advantages in terms of con-trolling growth and morphological properties. More-over, biomineralization provides a way to produce highlyuniform magnetite crystals without the drastic regimes oftemperature, pH and pressure that are often needed fortheir industrial production (Mann et al. 1990). Soon af-ter the discovery of magnetotactic bacteria, their bio-technological potential was realized and numerouscommercial uses of the small magnetic crystals have beencontemplated. Examples for the use of magnetic micro-organisms generally fall into two categories, which in-volve either whole, living cells and their magnetotacticbehavior or utilize isolated magnetosome particles.

Applications of magnetic cells

Applications based on the ®rst category include the useof magnetotactic bacteria for the nondestructive domainanalysis of soft magnetic materials. When motile mag-netotactic cocci were applied to the sample surface, thebacteria oriented, migrated and accumulated in the stray®elds originating from the magnetic domains, allowingvisualization of the domain structure (Harasko et al.1993, 1995). Using a similar technique, Funaki et al.(1992) employed magnetotactic bacteria to locate mag-

netic poles on meteoritic magnetic grains. Bahaj et al.(1994, 1998) considered the use of magnetotactic bacte-ria for the removal of heavy metals and radionuclidesfrom wastewater. High-gradient magnetic separation aswell as a low-magnetic ®eld system based on their di-rected motility have been used to remove metal-loadedbacteria from water. A ``microbial magnetometer'' wasdevised for the prediction of oxygen depletion in bottomsediments and overlying water, which is based on themagnetotactic and microaerophilic behavior of naturalpopulations of magnetotactic bacteria, tracking theirposition and magnetic strength by a magnetic sensor(Rhoads 1995). All the examples described above rely onliving, actively swimming and partially non-cultivablemagnetotactic bacteria. Because of the fastidiousnessand limited availability of these bacteria, practicalapplications remain in question.

Applications of isolated magnetosome particles

The small size of isolatedmagnetosome particles providesa large surface-to-volume ratio, which makes them usefulas carriers for the immobilization of relatively largequantities of bioactive substances, which can then sepa-rated by magnetic ®elds. In initial studies, bacterialmagnetosomes from non-cultivable magnetotactic bac-teria were used for immobilizing the enzymes glucoseoxidase and uricase as components of medically impor-tant biosensors. The glucose oxidase bound to biogenicmagnetite particles had 40-fold higher activity than thesame enzyme bound to arti®cial magnetite and Zinc fer-rite particles in this study (Matsunaga andKamyia 1987).

Magnetosomes have been also used for the genera-tion of magnetic antibodies. Following treatment withglutaraldehyde, bacterial magnetite particles were cou-pled to antibodies to form conjugates. In one example,magnetic antibodies could be used for the quanti®cation

Fig. 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)

470

of IgG. The procedure involved immobilizing ¯uores-cein-isothiocyanate-conjugated anti-(mouse IgG) anti-bodies on bacterial magnetosome particles (Nakamuraet al. 1991). The presence of the magnetosome mem-brane resulted in dispersion and handling propertiessuperior to those of synthetic magnetic particle conju-gates (Matsunga 1991). Other reported examples for theapplication of antibodies immobilized on bacterialmagnetosomes include the detection and removal ofE. coli cells from bacterial suspensions (Nakamura et al.1993a) as well as the highly sensitive detection of aller-gens (Nakamura and Matsunaga 1993).

Matsunga and coworkers incorporated bacterialmagnetite particles into eucaryotic cells, which could bemanipulated by a magnetic ®eld. Magnetosomes wereintroduced into red blood cells by fusion with bacterialspheroplasts mediated by polyethylene glycol. Wholemagnetotactic bacteria have also been introduced intoleukocytes by phagocytosis (Matsunaga et al. 1989).

Bacterial magnetite particles were also used as carri-ers for the introduction of DNA into cells. In a methoddescribed by Takeyama et al. (1995), bacterial magnetiteparticles coated with plasmid DNA were used for theballistic transformation of the marine cyanobacteriumSynechocystis using a particle gun. In another experi-ment, DNA oligonucleotides were immobilized on bac-terial magnetite particles and used for the detection ofmRNA (Sode et al. 1993).

Another application of bacterial magnetosomes maylie in their potential use as a contrast agent for magneticresonance imaging and tumor-speci®c drug carriersbased on intratumoral enrichment (Bulte and Brooks1997). Synthetic liposomes containing superparamag-netic iron oxide particles have already been used inbiomedical applications of this type (PaÈ user et al. 1997).Again, because of their unique magnetic and crystallineproperties and the natural presence of a surroundingmembrane, magnetosomes might be superior in someapplications. It has also been demonstrated that apply-ing the principle of con®ned biomineralization withinthe cavity of apoferritin resulted in the innovative ma-terial magnetoferritin, which was more e�ective as amagnetic-resonance contrast agent than chemosyntheticparticles (Bulte et al. 1994).

Perspectives

Although the biotechnological potential of bacterialmagnetite has been demonstrated, to date no applicationhas been exploited on a commercial scale. This is par-tially because of the problems related to mass cultivationof magnetotactic bacteria, which is still too expensive formost practical purposes. However, the use of bacterialmagnetosomes could be competitive in some specializedapplications, where perfect crystalline properties andrelatively small amounts are required, as in some bio-medical applications, for example. Another reason forthe limited practicability of many ideas is the lack of a

fundamental understanding of the biochemical and ge-netic principles behind the process of bacterial magnetitebiomineralization. More research in this ®eld is clearlyrequired. The most critical issue is the mechanism bywhich the organic matrix controls the nucleation andgrowth of the particles. Further investigation of thestructure and composition of the magnetosome mem-brane and its protein organization is therefore essentialfor understanding ion transport through the membraneand the early stages of crystal formation.

Increased e�orts should be directed to the genetic andmolecular biological analysis of magnetosome forma-tion. The application of genetic engineering techniquesmight possibly result in magnetosome particles withimproved properties and a more dependable productionprocess. For example, it has been suggested that mag-netosome-speci®c proteins might be used for the con-struction of gene fusions to couple bioactive substancesdirectly in order to display them on the surface of themagnetosome particles (Nakamura et al. 1995b). Ge-netic engineering might also be used for control of thechemical composition. The modi®cation of the transportsystem responsible for the accumulation of iron into themagnetosome vesicle might result in an altered speci®cityfor metal uptake, potentially yielding crystals with novelmagnetic and crystalline properties. Once the genes forthe biomineralization pathway are identi®ed, they mightbe expressed in a readily cultivable host organism,thereby potentially eliminating the di�culties associatedwith the cultivation of magnetotactic bacteria. Func-tional expression of single genes from Magnetospirillumin E. coli has already been demonstrated (Berson et al.1989, 1991), but the heterologous expression of genesrelated to bacterial biomineralization could be limited bythe lack of an intracellular compartment. Alternatively,bacterial hosts with an existing intracytoplasmaticmembrane system, such as phototrophic bacteria, mightbe more appropriate for expression and intracellulartargeting of the biomineralization machinery.

Finally, in the light of the dramatic diversity of nat-urally occurring magnetotactic bacteria with variablecrystal morphologies, increased e�ort should be given tothe isolation and study of other magnetotactic specieswith potentially unique features of the biomineralizationprocess. Understanding how di�erent bacteria achievespecies-speci®c control over the biomineralization pro-cess could be used in tailoring di�erent crystal mor-phologies with the desired properties.

Acknowledgements D.S. was supported by grants from the De-utsche Forschungsgemeinschaft and the Max-Planck-Society.R.B.F. was supported by NSF grant CHE 9714101. We are gratefulto E. Baeuerlein, D.A. Bazylinski, S. Spring and B. Tebo for dis-cussions and collaboration.

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