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. 70 MaxPlanckForschung 2 | 10 MATERIALS & TECHNOLOGY_Nanoparticles 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 alsoA 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) 72 MaxPlanckResearch 3 | 10 MATERIALS & TECHNOLOGY_Nanoparticles 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.