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Posted: Jan 14, 2011
Bacterial minicompass uses nanomagnets
(Nanowerk News) Magnetotactic bacteria orient themselves in their environment with the aid of tiny compasses, consisting of chains of membrane sacs called magnetosomes, each containing a single nanomagnet. Only a few of these species grow readily in laboratory cultures. A research team led by LMU microbiologists Dr. Christian Jogler and Professor Dirk Schüler, in cooperation with the Max Planck Institutes for Molecular Genetics (Berlin) and Marine Microbiology (Bremen), has now used a magnetic trap to isolate one such species, Magnetobacterium bavaricum, directly from sediments dredged from the Chiemsee (a lake in Southern Bavaria) ("Conservation of proteobacterial magnetosome genes and structures in an uncultivated member of the deep-branching Nitrospira phylum").
M. bavaricum is of special interest because it is unusually large and contains very many minimagnets. The investigators then compared selected DNA sequences from M. bavaricum with those of other known magnetic bacteria. The results show, for the first time, that the genes required for the assembly of bacterial compasses derive from a single source, although they are now found in widely divergent groups. "The genes were most probably transmitted between otherwise unrelated groups by horizontal gene transfer," says Schüler. In many respects the properties of the biomagnets in magnetosomes make them more suitable for many applications in medical diagnostics and therapy than chemically synthesized nanomagnets. The mechanism of magnetosome formation is therefore of considerable biotechnological interest.
Magnetotactic bacteria found in the muddy bottoms of ponds and lakes use the Earth's magnetic field to distinguish up from down, allowing them to seek out optimal conditions for growth and survival. The sensors are called magnetosomes, each comprising an ordered microscopic crystal of an iron oxide (magnetite) or sulfide (greigite) enclosed in a specialized pocket formed by a fold in the cell membrane. Magnetosomes are arranged in linear chains, so that the nanomagnets they contain act as compass needles that enable the cells to follow geomagnetic field lines.
Until a few years ago little was known about how magnetosomes form, because the vast majority of magnetotactic bacterial species found in sediments cannot be grown under laboratory conditions. Schüler's research team has, however, been able to demonstrate that the synthesis and assembly of the magnetosome is a complex process, which involves contributions from at least 20-30 genes, some of unknown function. "In the few magnetotactic bacteria that have been studied in detail, these genes appear to be located in a single segment of the bacterial chromosome, called the magnetosome island," explains Schüler.
Among the magnetotactic bacteria that have yet to be cultured successfully, Magnetobacterium bavaricum, which is found principally in the Chiemsee, is of particular interest, because it displays a number of unusual features. It is almost 10 times bigger than the largest magnetotactic species so far grown in culture, and each cell contains over 1000 magnetite crystals. Furthermore, these nanomagnets have an unusually extended form. Other magnetic bacteria have only 10-50 magnetite crystals, so M. bavaricum may well be the most magnetic bacterium in the world.
In order to isolate enough cells for genetic and structural analysis of this and other magnetic bacterial species, Schüler and Jogler developed magnetic traps with which they can selectively extract magnetic cells from samples of lake sediments. Because M. bavaricum is only very distantly related to most other types of magnetic bacteria, it seemed possible that the genetic and biochemical basis for its magnetosomes might be quite different from those in other species.
Ultrastructural studies carried out by LMU electron microscopist Professor Gerhard Wanner showed, however, that the architecture of the magnetosomes is similar in all the species of magnetic bacteria examined. The magnetic crystals are enclosed in membrane sacs, which are aligned in chains by interaction with protein filaments that constitute a kind of cytoskeleton. For the genetic analysis individual cells were selected under the microscope, and their DNA was isolated and amplified by cloning.
Using specific DNA probes, the investigators were able, for the first time, to identify segments of the M. bavaricum genome that were related to genes located in the magnetosome islands of other magnetotactic bacteria. This finding immediately suggested that the magnetosome genes derive from a common evolutionary source. Given that they now occur in genetically quite distinct lineages, they must have been propagated between species by the process of horizontal gene transfer. Of course, genetic differences specific to M. bavaricum were also detected, which presumably account for its unique characteristics.
The DNA sequence so far determined makes up only a small part of the M. bavaricum genome. Schüler's next step will be to analyze the entire chromosome of this unusual magnetic bacterium. "If we can understand the process of magnetosome formation in detail and assemble them in the test tube, it might be possible to use this knowledge to engineer magnetic nanomaterials with novel properties," says Schüler. "Such designer nanomagnets could be used for many bio- and nanotechnological applications, including magnetism-based fractionation methods, as well as diagnostic and therapeutic procedures."