A test for magnetic sensitivity: When Damien Faivre holds a test tube with a culture of magnetic bacteria first parallel and then perpendicular to a magnetic field, the medium’s turbidity changes. The bacteria’s magnetosome chains are indeed aligning with the magnetic field.

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Magnets – Made by Microbes

Their medical importance would be considerable: magnetic nanoparticles such as those produced by magnetotactic bacteria could, among other applications, help detect tumors. Damien Faivre and his colleagues at the Max Planck Institute of Colloids and Interfaces in Potsdam are studying how these microbes work in order to harness their sophisticated mechanisms.

TEXT CHRISTIAN MEIER

compass has always been them accordingly: magnetotactic bac- an indispensable tool, not teria. However, these microbes do not only for mariners wishing seek out the North Pole, but the deep- to reach their destination. er regions of their aquatic environ- Certain aquatic bacteria ment. The magnetic field lines that lie also A navigate using Earth’s magnetic away from the equator do not run par- field. Their inner compass consists of a allel to the Earth’s surface but point chain of tiny nanoparticles of the mag- downward. This guides the magneto- netic mineral magnetite. tactic bacteria toward the deeper wa- These particles are produced by the ters, where sediment and water mix. bacteria themselves and have such These oxygen-starved areas provide the unique magnetic properties that they ideal conditions for the bacteria to live are of great interest for medical and oth- and flourish. They are not able to use er technology applications. To date, gravity for vertical orientation, since however, only nature knows how they they are nearly as dense as water and are produced. Damien Faivre, a chemist thus do not perceive their weight. working at the Max Planck Institute of Colloids and Interfaces in Potsdam, GUIDED TO THE BOTTOM hopes to unravel the secret with the BY A COMPASS help of his seven-man team. Once the researchers understand how the bacteria The bacteria owe their compass to the produce the nanoparticles, they hope it magnetosomes, organelles consisting of

will be possible to develop a procedure a single particle of magnetite (Fe3O4) to manufacture the particles, first in the that measures less than 100 nanome- test tube and later on an industrial scale. ters and is surrounded by a membrane Photo: Norbert Michalke In 1975, American microbiologist to prevent the particles from clumping Richard Blakemore discovered that together. Some 20 magnetosomes form some aquatic organisms navigate along chains along protein fibers in the bac- the Earth’s magnetic field, and named terium. They work like the needle of a

3 | 10 MaxPlanckResearch 71 The magnetic properties of the parti- cles are of enormous interest for tech- nical applications. “They display a re- manence and coercivity that cannot be matched by artificially produced crystals,” says Faivre. These two phys- ical parameters mean that the materi- als are magnetically hard, so their per- manent magnetism remains very stable. This is a desirable property in many technical applications, for in- stance for magnetic data storage with unprecedented bit density.

MAGNETITE PARTICLES FOR TUMOR DETECTION

Other applications require uniform magnetic properties, and this is exact- ly what the bacteria’s magnetic nano- particles offer, thanks to their uniform shape and size. Artificially created elongated magnetic particles could be used as a contrast agent in magnetic resonance imaging. Tissues holding the particles would show up as darker areas on the images. If the particles could be guided to a tumor, its loca- tion could be pinpointed at an early stage. The particles could also be used compass, turning in the direction of that should make chemical engineers to ensure that drugs target the focused Earth’s magnetic field, approximately sit up and take notice, as uniform par- area of a disease. By positioning mag- in a north-south orientation. Since the ticle size is an important mark of qual- nets outside the body, the particles magnetosomes bind to the protein fib- ity in nanoparticle production. “Not would remain in those areas. The ac- ers of the bacterium, the whole mi- only that, but the bacteria can even tive substances bound to the particles crobe turns with them. Then, when the control the shape of the particles,” adds would thus remain in the tissue where microbes rotate their flagella, they Faivre. Some types of magnetotactic they are needed, instead of being move along the magnetic field lines to- bacteria produce bullet-shaped nano- flushed along in the bloodstream. ward the bottom of the body of water, particles, while others make needle- Although it is possible to create as if on rails. shaped ones. In fact, each type of bac- magnetite particles in the laboratory, “The bacteria generate perfect mag- teria creates its particles in a uniform these synthetic particles, unlike their netic nanoparticles,” affirms Faivre. shape. In short, these bacteria boast biological counterparts, contain a First, magnetotactic bacteria produce perfect internal quality control in the small amount of oxygen. Damien

the particles in a uniform size – a feat synthesis of magnetite particles. Faivre’s team discovered this while Photos: Norbert Michalke (2)

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left A close-up look at a compass: Damien Faivre inserts a sample of magnetotactic bacteria into an electron microscope and then checks that it is correctly positioned. right Compass needles as a gauge: Different kinds of magnetotactic bacteria produce very specific magnetite particles of characteristic size and shape.

studying the crystal structure and chemical composition of the magnet- ic nanoparticles using X-ray radiation from the Berlin-based synchrotron ra- diation facility, BESSY. There are other difficulties with the synthetic production of magnetic na- noparticles: “So far, the available chemical processes cannot produce magnetic nanoparticles of uniform size and shape in environmentally friendly conditions,” explains Faivre. In this case, environmentally friendly would mean that the particles could be produced at room temperature, nor- mal atmospheric pressure and without harmful solvents, instead of the ener- gy-intensive conditions of high pres- sure and high temperature. With this in mind, Faivre wants to understand terial genome contain the genetic in- and shape as in the wild type. This how nature manages to produce the formation that encodes the magneto- method does deliver worthwhile re- uniform magnetic particles. “Nature some proteins. sults, but “Since magnetotactic bacteria shapes material down to the smallest grow very slowly, the in vivo process is detail, literally down to the smallest LAB TESTS REVEAL THE FUNC- a very protracted one,” laments Faivre. unit, the molecule,” he says. “We can TIONS OF INDIVIDUAL PROTEINS It can take up to two years to study a learn from nature by trying to under- single gene or protein. stand how natural models influence Damien Faivre and his team now hope For this reason, his team uses a sec- complex physicochemical and biolog- to identify the roles played by individ- ond, more efficient method to shed ical phenomena. As soon as the biolog- ual proteins and their components in light on the functions of the magneto- ical processes are fully understood, it biomineralization. There are essential- some proteins. They insert the gene of should be possible to copy them in or- ly two methods they can use in this the relevant protein into the genome of der to develop new materials.” quest. The first involves the generation the fast-growing bacterium Escherichia The researchers have already made of “deletion mutants”: bacteria in coli. The cell machinery of this microbe, some initial discoveries about how na- which a given gene has been deactivat- induced by its genetic information to ture produces magnetite nanoparti- ed. Except for that single inactive gene, produce proteins, is stimulated to man- cles. Magnetotactic bacteria control the genome of the mutant is identical ufacture particularly large amounts of the growth of the magnetic particles to that of the wild type. As researchers the implanted magnetosome protein. through a biological process called bi- study the differences between bacteria This is necessary so that researchers can omineralization – another way of say- with the inactive gene and their unal- achieve the same protein concentration ing biologically controlled crystal tered counterparts, they can learn in the test tube as occurs in the far growth. Some 20 to 30 proteins called about the role of the specific gene. They smaller magnetotactic bacterium. magnetosome proteins are responsible check whether the deletion mutant Finally, researchers isolate the pro- for this process. Biologists have also produces magnetosomes, and if so, teins and study their properties in the

Photo: MPI of Colloids and Interfaces discovered which sections of the bac- whether they occur in the same size test tube. To this end, they mix the pro-

3 3 | 10 MaxPlanckResearch 73 Janet Andert (left) places a culture of magnetotactic bacteria in a fermenter (detail image at right). Meanwhile, Antje Reinecke adjusts the conditions to ensure optimum propagation of the microbes.

tein with iron compounds that, like some proteins. At the very least, the di- ids and therefore constituting only a magnetite, contain divalent and triva- rect contact with the magnetic particle small portion of Mms6, influences the lent iron, gradually altering the pH of would imply that the relevant protein size of the magnetite particle. Faivre the solution until its components are must have an important function. The explains that this finding is vitally im- precipitated and magnetite particles are peptides are then translated into DNA portant, “because the artificial mass formed. During this process, the pro- sequences – that is, the language of ge- production of proteins using host or- tein influences the size or the shape of netic information. Using these se- ganisms is limited, whereas synthetic the particle being formed. “This meth- quences, a computer program scans the peptides can be produced in practical- od enables us to study a single protein genome of magnetotactic bacteria to ly unlimited quantities.” in three to four months,” says Faivre. identify the associated proteins. Meanwhile, the search continues It is clear from the outset that not for proteins involved in the formation all magnetosome proteins are equally A PROTEIN THAT CONTROLS of the magnetic nanoparticles. “Up to important for magnetosome produc- PARTICLE SIZE now, 20 protein candidates have been tion, so before they begin to study in- identified in the magnetosome mem- dividual proteins, the researchers gath- So far, Japanese and American research- brane of the magnetotactic bacterium er information on which ones are more ers have clarified the role of one mag- M. gryphiswaldense, and it is assumed likely to play key roles in biominerali- netosome protein in the synthesis of that they have particular effects on the zation. This saves time by avoiding un- magnetite particles inside magneto- size and shape of magnetite crystals,” necessary test tube experiments. somes. The protein bears the unimpres- explains Faivre. One of the screening tools used for sive name Mms6 and is found only in Some of these candidates are cur- protein preselection is bioinformatics magnetotactic bacteria. Mms6 is locat- rently being studied by Faivre’s team at software. This enables the team to iden- ed in the membrane surrounding the the Max Planck Institute in Potsdam. tify similarities between the genes of magnetite particle, and scientists have The team is also researching how indi- different magnetotactic bacteria. Any discovered that it codetermines the size vidual magnetosomes link together to such similarities indicate important of the magnetic nanoparticles. To date, create a chain so that, bit by bit, the tiny genes and thus important proteins. An- it is the only protein known to play a compass needle is formed to guide the other method used is bio-combinatori- decisive role in vitro in controlling par- microbe to its food. They have shown al engineering. Here, the researchers ticle size. Scientists have yet to discov- that the formation of the chain involves study which peptides, or protein por- er any proteins that determine the a complex interaction of genetically tions, bind to the surface of magnetite shape of the magnetite crystals. controlled processes and magnetic forc- crystals. These peptides enable direct In studying Mms6, researchers hap- es. One of the methods used was “ferro- contact between proteins and magnet- pened across another significant phe- magnetic resonance spectroscopy,” ic particles, and could therefore be nomenon: the peptide at one end of which is similar to nuclear magnetic res-

components of important magneto- the protein, consisting of 25 amino ac- onance. FMR, as it is called, allows the Photos: Norbert Michalke (2)

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magnetic properties of solid bodies to be teins and other biological components ter to order, even specifying that they examined. It can show, for example, the (especially lipids) that direct the biomi- should be, say, round or needle-shaped,” preferred orientation, if any, of a mag- neralization of magnetite particles. The says the biochemist. It would simply be netized crystal. It allows scientists to key for them is to understand the differ- a case of selecting the relevant pro- study both the individual magnetite ent roles the proteins play in particle teins, like tools from a toolbox. For particles and the particle chains. synthesis. “Then it would almost be pos- now, it is only a dream – but the Pots- Faivre and his colleagues hope that sible to make magnetic particles measur- dam research team has taken the first their research will identify all the pro- ing 20, 50 or 100 nanometers in diame- steps toward making it a reality.

GLOSSARY

Magnetosome Magnetotaxis Magnetic resonance imaging Biomineralization A magnetite particle The ability of some Also called nuclear magnetic resonance. Organisms use organic and inorganic enclosed by a membrane. life forms to orient A procedure based on the fact that some atoms, substances to produce inorganic minerals Each particle measures along the Earth’s such as hydrogen, have a magnetic moment. and compound materials very precisely, less than 100 nano meters; magnetic field. Their behavior in a magnetic field depends on controlling their production through different types of magne- the tissue in which they are located. This allows biochemical processes. Notable examples, totactic bacteria produce different tissue types to be identified. Certain apart from magnetosome crystals, them in various charac- magnetic substances can be used to enhance include mother of pearl and the silica teristic shapes and sizes. the contrast between them. skeletons of diatoms.

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