page_title: Magnetotactic bacteria
section_title: Magnetism
hierarchical_section_title: Magnetotactic bacteria / Magnetism
caption_attribution_description: English: Magnetotactic bacteria polarities. Italiano: Modello teorico semplificato delle polarita dei batteri magnetotattici
context_page_description: Magnetotactic bacteria are a polyphyletic group of bacteria that orient themselves along the magnetic field lines of Earth's magnetic field. Discovered in 1963 by Salvatore Bellini and rediscovered in 1975 by Richard Blakemore, this alignment is believed to aid these organisms in reaching regions of optimal oxygen concentration. To perform this task, these bacteria have organelles called magnetosomes that contain magnetic crystals. The biological phenomenon of microorganisms tending to move in response to the environment's magnetic characteristics is known as magnetotaxis. However, this term is misleading in that every other application of the term taxis involves a stimulus-response mechanism. In contrast to the magnetoreception of animals, the bacteria contain fixed magnets that force the bacteria into alignment—even dead cells are dragged into alignment, just like a compass needle.
context_section_description: The physical development of a magnetic crystal is governed by two factors: one is moving to align the magnetic force of the molecules in conjunction with the developing crystal, while the other reduces the magnetic force of the crystal, allowing an attachment of the molecule while experiencing an opposite magnetic force.  In nature, this causes the existence of a magnetic domain, surrounding the perimeter of the domain, with a thickness of approximately 150 nm of magnetite, within which the molecules gradually change orientation. For this reason, the iron is not magnetic in the absence of an applied field.  Likewise, extremely small magnetic particles do not exhibit signs of magnetisation at room temperature; their magnetic force is continuously altered by the thermal motions inherent in their composition. Instead, individual magnetite crystals in MTB are of a size between 35 and 120 nm, that is; large enough to have a magnetic field and at the same time small enough to remain a single magnetic domain.
  The inclination of the Earth's magnetic field in the two respective hemispheres selects one of the two possible polarities of the magnetotactic cells (with respect to the flagellated pole of the cell), orienting the biomineralisation of the magnetosomes.
  Aerotaxis is the response by which bacteria migrate to an optimal oxygen concentration in an oxygen gradient. Various experiments have clearly shown that magnetotaxis and aerotaxis work in conjunction in magnetotactic bacteria. It has been shown that, in water droplets, one-way swimming magnetotactic bacteria can reverse their swimming direction and swim backwards under reducing conditions (less than optimal oxygen concentration), as opposed to oxic conditions (greater than optimal oxygen concentration). The behaviour that has been observed in these bacterial strains has been referred to as magneto-aerotaxis.
  Two different magneto-aerotactic mechanisms—known as polar and axial—are found in different MTB strains. Some strains that swim persistently in one direction along the magnetic field (either north-seeking [NS] or south-seeking [SS])—mainly the magnetotactic cocci—are polar magneto-aerotactic. These magnetotactic bacteria will travel along the lines of the earth's magnetic field according to their orientation, but will swerve as a group and reverse direction if exposed to a local, more powerful and oppositely-oriented magnetic field. In this way, they continue to travel in the same magnetic direction, but relative instead to the local field.  Those MTB that swim in either direction along magnetic field lines with frequent, spontaneous reversals of swimming direction without turning around—for example, freshwater spirilla—are axial magneto-aerotactic and the distinction between NS and SS does not apply to these bacteria. The magnetic field provides both an axis and a direction of motility for polar magneto-aerotactic bacteria, whereas it only provides an axis of motility for axial types of bacteria. In both cases, magnetotaxis increases the efficiency of aerotaxis in vertical concentration gradients by reducing a three-dimensional search to a single dimension.
  Scientists have also proposed an extension of the described model of magneto-aerotaxis to a more complex redoxtaxis. In this case, the unidirectional movement of MTB in a drop of water would be only one aspect of a sophisticated redox-controlled response. One hint for the possible function of polar magnetotaxis could be that most of the representative microorganisms are characterised by possessing either large sulfur inclusions or magnetosomes consisting of iron-sulfides. Therefore, it may be speculated the metabolism of these bacteria, being either chemolithoautotrophic or mixotrophic, is strongly dependent on the uptake of reduced sulfur compounds, which occurs in many habitats only in deeper regions at or below the OATZ due to the rapid chemical oxidation of these reduced chemical species by oxygen or other oxidants in the upper layers.
  Microorganisms belonging to the genus Thioploca, for example, use nitrate, 
caption: The MTB polarity model
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