Magnetotaxis and magnetotactic Bacteria
Magnetotaxis is one of the important tactic responses. Magnetotactic bacteria exhibits directed response to earth's magnetic field or to the magnets placed near their broth suspension/cultures. Salvatore Bellini was the first to observe magnet directed response in bacterial cultures which he called as magnetosensitive bacteria. In 1975, Richard P.Blakemore coined the word, 'magnetotactic' bacteria and described magnetotaxis in detail.
Characteristics of magnetotactic bacteria: Magnetotactic bacteria are aquatic; principally inhabit niches like freshwater and marine environment. They are strictly anaerobic or microaerophilic. Morphologically, various forms like cocci, rods and Spirilla, helix, vibroids are present. Magnetotactic bacteria from extremophilic environment have still not been reported except sulfate reducing marine bacteria which contain magnetosomes. All Magnetotactic bacteria are Gram negative α-proteobacteria. No Gram positive or Archaebacteria are known to be magnetotactic. Aquaspirillum magnetotactum (also known as Magnetospirillum), Magnetospirillum gryphiswaldense and Magnetococcus species are some of the important examples of magnetotactic bacteria. Magnetotactic bacteria are sensitive to magnetic field as low as 0.2 Gauss and as high as 70 Gauss. Individual bacterial cell contains granules of iron ore, especially magnetite (Fe3O4) as cellular inclusions. These inclusions are termed as magnetosomes. Magnetosomes are membrane bound crystalline entities arranged in a chain like a garland. They are present in octahedral, dodecahedral and cubic forms. Chain arrangement results in the formation of permanent magnetic dipole large enough to orient entire bacterium along geomagnetic field. The presence of magnetosomes within a cell makes it behave like an actual bar magnet: like poles repel while as unlike poles attract. Cells can also orient themselves in north-south direction and parallel to magnetic field, similarly like magnetic needle. When these bacteria are grown in a medium containing very less or almost no iron, they will not contain magnetosomes and cannot be magnetotactic. Dead cells do not express magnetotaxis.
Purpose and ecological significance: Magnetotactic response is chiefly essential for locomotion and displacement of bacteria in an aqueous environment. Magnetosomes orient the cells in magnetic field and thereby determine the direction in which they swim. They have an ability to migrate the cells along the geomagnetic field lines. In northern latitudes, earth's magnetic field is inclined downwards and in southern latitudes it is pointed upwards. Hence bacteria from northern hemisphere are north seeking (move upward) and those from southern hemisphere are south seeking (directs cells in downward direction). Magnetotactic bacteria at the equator are mixed populations composed of equal number of north and south seeking cells. In fact, one of the important purposes of magnetotactic behavior is to avoid high and toxic oxygen concentration. When magnetotactic bacteria in water suspension swim along magnetic field lines and return to bottom, indicates prevalence of microaerophilic conditions which can be favorable for their survival. It has been also found that aerotactic and magnetotactic responses are coordinated. Magnetotaxis is thus very significant phenomenon when other bacteria even from aquatic environment possess flagellar appendages or somewhat simple tactic responses meant for movement are available in nature.
Magnetotactic bacteria are also important agents of biogeochemical cycling of iron and other elements. Formation of magnetosomes is achieved by biological mechanism that controls accumulation of iron and biomineralization of magnetic crystals. They are functionally required in iron homeostasis, energy conservation and redox cycling. After death cells are no more magnetotactic but magnetosomes remain preserved and deposited as fossils contributing magnetization of sediments. Magnetofossils of cretaceous chalk beds of Southern England is well known example of magnetized sediments. Precisely organized magnetosomes are also found in igneous and metamorphic rocks. They have been detected in algae, fishes like salmon, bees, human neurological tissues and components of immune system, species of phylum mollusca, chitons and birds like pigeons.
Industrial applications: Number of industrial and medical applications of magnetotaxis and magnetotactic bacteria has been proposed by the researchers worldwide and very few of them have been practically initiated towards an application. The main hindrance is lack of knowledge regarding mass cultivation of these bacteria, magnetite synthesis and membrane enclosure, their unexploited diversity and genetic and metabolic potential. Some of the pharmaceutical and environmental applications of magnetotactic bacteria include their use in magnetic separation based wastewater treatment, in the synthesis of advanced biomaterials with designed properties and their application in printing ink and magnetic tapes. Their uses such as magnetic resonance imaging contrast agent, model to study magnetites contained in meteorite from extracellular life, to study biomineralization of magnetic materials and in the synthesis of uniform magnetite nanoparticles, microchips and nanoacctuators. These nanoparticles can be used as carriers for immobilization of antibodies, enzymes and tumor specific drugs. Some important environmental applications could be in eradication of heavy metals, their use as pollutant adsorbents and biosensors.
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