| Homepage | Imprint |
|
||
 |
Magnetotactic bacteriaMagnetotactic bacteria are a class of bacteriadiscovered in the 1970sthat are characterised by being able to orient themselves in response to the Earth's magnetic field(magnetotaxis).
IntroductionThe first description of this class of bacteria appeared 1963in a publication of the Istituto di Microbiologia of University of Paviawritten by the MD, Salvatore Bellini. While observing occasionally bog sediments under his microscope, he noticed a group of bacteria that evidently oriented themselves in a unique direction and soon realised that these microorganisms were following the direction of the North Pole. They were hence denominated "magnetosensitive bacteria". The subject successively appeared in 1975in issue of Science, an essay written by the microbiologist, Richard Blakemore, who, observing swamp sediments under his microscope, noticed a group of bacteria which clearly oriented themselves in a certain direction. He soon realised that these microorganisms were following the direction of the Earth's magnetic field, from south to north, and thus the adjectival descriptor "magnetotactic" [1]. These bacteria have been the subject of many experiments. They have even been aboard the Space Shuttleto examine their magnetotactic properties in the absence of gravity, but a definitive conclusion was not reached [12]. There have also been claims of their existence on Marsbecause of the discovery of magnetic particles on a meteorite said to be of Martian origin, but even on this occasion the results were ambiguous [1]. BiologyMagnetotactic bacteria (MagnetoTactic Bacteria, MTB) are usually found in oxic-anoxic transition zones(OATZ - the transition zone between water and sediment). Various MTB differ by morphotype, number, layout and pattern of the bacterial magnetic particles(BMP) [3]. The MTB can be subdivided into two categories, according to whether they produce particles of magnetite(<math>\mathrm{Fe}_3\mathrm{O}_4</math>), or of greigite(<math>\mathrm{Fe}_3\mathrm{S}_4</math>); some species produce both. Magnetite possesses three times the magnetic properties of greigite[1]. The majority of magnetite mineralising MTB need a microaerobic environment to generate magnetosomes. Although beyond a certain threshold they stop producing more BMP and therefore begin to lose their magnetotaxis capability. However some types produce magnetite, even in anaerobic conditions, using nitric oxideor nitrateas a receptor for electrons. However these are part of the order of Alpha Proteobacteria. The greigite mineralising MTB are usually strictly anaerobic and are related to sulphur reducing bacteria, therefore they are classified within the order of Delta Proteobacteria. It has been suggested that MTB evolved in the early ProterozoicEra, as the increase in atmospheric oxygen reduced the quantity of dissolved iron in the oceans. Organisms began to store iron in some form, and this intracellular iron was later adapted to form magnetosomesfor magnetotaxis. These early MTB may have participated in the formation of the first eukaryotic cells. Biogenic magnetite not too different from that found in magnetotactic bacteria has been founded in higher organisms, from Euglenoidalgae, to salmon, pigeons, and humans. Quite possibly the evolutionary advantage of possessing a system of magnetosomes is linked to the ability to efficiently maintain an optimal orientation in terms of chemical substances and redoxreactions, reducing a potential three dimensional search to a single dimension (see the Magnetism subsection below for a description of this mechanism). BMP interact with each other through the formation of chains. In this microscopic imageit is clearly visible in a Magnetospirillum magneticum. The magnetic dipole of the cell is therefore the sum of the dipoles of the single BMP which are sufficient to passively orientate the cell to overcome the casual thermal forces of a wet environment [2]. In the presence of more than one chain, the inter-chain repulsive forces push them to the edge of the cell, inducing turgor[1]. The diversity of MTB is reflected by the high number of different morphotypesfound in environmental samples of water or sediment. Commonly observed morphotypes include coccoidto ovoidcells, rods, vibriosand spirillaof various dimensions. One of the more unique morphotypes is an apparently multicellular bacterium referred to as the MMP - many-celled magnetotactic prokaryote. Regardless of their morphology, all MTB studied so far are motile by means of flagellaand have a cell wall structure characteristic of Gram-negative bacteria. The arrangement of flagella differs and can be either polar, bipolar, or in tufts. Another trait which shows considerable diversity is the arrangement of magnetosomesinside the bacterial cell. In the majority of MTB, the magnetosomes are aligned in chains of various lengths and numbers along the cell's long axis of the cell, which is magnetically the most efficient orientation. However, dispersed aggregates or clusters of magnetosomes occur in some MTB usually at one side of the cell, which often corresponds to the site of flagellar insertion. Besides magnetosomes, large inclusion bodies containing elemental sulfur, polyphosphate, or poly-?-hydroxybutyrateare common in MTB. The most abundant type of MTB occurring in environmental samples, especially sediments, are coccoid cells possessing two flagellar bundles on one somewhat flattened side. This bilophotrichous type of flagellation gave rise to the tentative genus "Bilophococcus" for these bacteria. In contrast, two of the morphologically more conspicuous MTB, regularly observed in natural samples but never isolated in pure culture, are the MMP and a large rod containing large numbers of hook-shaped magnetosomes (Magnetobacterium bavaricum). MagnetismPhysically, the development of a magnetic crystalis governed by two factors: one moving to align the magnetic forceof 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 macroscopically the ironis not magnetic in the absence of an applied field. Similarly, extremely small magnetic particles do not exhibit signs of magnetisation at room temperature, their magnetic force is continually altered by the thermal motions inherent in their composition [1]. Instead MTB are of a size between 35 e 120 nm, that is, big enough to have a permanent magnetic field and at the same time small enough to remain a single magnetic domain. [2]. The inclination of the Earth's magnetic field in the two respective hemispheres selects one of the two possible polarities Image:MTB polarities.jpgof the magnetotactic cells (with respect to the flaggelated pole of the cell), orienting the biomineralisation of the magnetosomes. Various experiments have clearly shown that magnetotaxisand aerotaxiswork in conjunction in the magnetotactic bacteria. Aerotaxis is the response by which bacteria migrate to an optimal oxygen concentration in an oxygen gradient. 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 [8]. Some strains that swim persistently in one direction along the magnetic field (NS or SS) ? mainly the magnetotactic cocci? are polar magneto-aerotactic. Those 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 characterized by possessing either large sulfur inclusions or magnetosomes consisting of iron-sulfides. Therefore, it may be speculated that the metabolism of these bacteria, being either chemolithoautotrophicor 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, which is stored intracellularly to oxidize sulfide and have developed vertical sheaths in which bundles of motile filaments are located. It is assumed that Thioploca uses these sheaths to efficiently move in a vertical direction in the sediment, thereby accumulating sulfide in deeper layers and nitrate in upper layers [7]. For some MTB, it might also be necessary to perform excursions to anoxic zones of their habitat in order to accumulate reduced sulfur compounds. MagnetosomesThe biomineralisation of the magnetite is brought about by the regulating mechanisms of the concentration of iron, by the nucleationof crystal, of the potential redox and of the pH. The compartmentalisation in magnetosomes permits the biochemical control of such processes. After the sequencing of the genomeof certain species of MTB, a comparative analysis of the proteinsinvolved in the formation of BMP became possible. In this way similarities in the sequence between members of the ubiquitaria family CDF (Cation Diffusion Facilitator) and the serinecompound Htr-simili. The first are involved exclusively in the transport of heavy metals, the second are types of heat shock proteins(HSPs) which degrade those proteins that are poorly absorbed. These proteins of the magnetosomial membrane (MM) beyond the serine proteasesdomain contain PDZ domains. Other MM proteins contain TPR domains (Tetratric Peptide Repeat) [3]. TPR domainThe TPR domains fold into two alpha helicesand produce a sequence of 8 amino acids(of the 34 possible) which are the most commonly found in nature. Apart from these, the remainder of the structure specialises in those functions best adapted to the environment. The more notable compounds that comprise TPR proteins include:
There exist examples of both the TPR-TPR interactions as well as the TPR-nonTPR [6]. The diagrams below show the 8 residual ones conserved as sphere-rods from two different angles: W-LG-Y-A-F(here Y)-A-P. Image:TPR domain 1.pngImage:TPR domain 2.png PDZ domainThe PDZ domains are modular structures which are made of 6 beta filaments and 2 alpha helices which recognise the C amino acids of the proteins in a specific sequence. Often the third from last is fosforilabile, such that it prevents the interaction with the PDZ domain. The only residues conserved are those subordinate to the recognition of the COOH terminal (RKXXXGLGF). They are widespread in nature to the extent that they constitute the structure upon which the multi proteases compounds are built, in particular, those associated with proteins of membranes such as the ducts K+inward rectifier or the beta receptors 2-adrenergic[13]. Membrane and proteinsThe formation of the MM passes through at least three stages. In the first an invaginationof the citoplasmatic membrane is formed from guanosine triphosphate. This process can also be observed amongst the eukaryote. The second stage requires the entrance of ferric ionsinto the newly formed vesiclefrom the external environment. Even in a culture deficient in Fe3+ the MTB succeed at accumulating increased intracellular concentrations of a secretion at an extremely low molecular masshighly specialised in inducing Fe3+ as required. The compound Ferro-sideroforois thereby moved in the cytoplasm, where it subdivides. The ferric ions must be converted into ferrous particles so that they can be accumulated by the BMP with the help of a trans-membrane transporter with a corresponding opposite force Na+/H+. In reality, it is a functioning H+/Fe2+ at a protonic gradient. This is localised as much in the cytoplasmic membrane as the MM, but in an inverted orientation, such that in the first type it is like an effluent of iron, and in the second an influx. Being decidedly more ubiquitous in the MM, the effluent of iron from the cytoplasmic membrane is superfluous. In any case this step is strictly controlled by a system of redox of cytochromes not yet fully integrate and, it would seem, species specific. In the final stage the nucleation of the crystal is triggered by the magnetite by the trans-membrane proteins with acidic and basic domains. One of these proteins, called Mms6, has been used in the artificial synthesis of magnetite, and its presence permits the production of crystals homogeneous in shape and size. Many other proteins associated with MM probably carry out roles in generating concentrations of supersaturations of iron, in maintaining less favourable conditions in the oxidisation of iron or in the partial reduction and dehydration of the ferridrate [4]. BiomineralisationIn cultivating Magnetospirillum magnetotacticum the iron cannot be substituted with other transitory metals (Ti, Cr, Co, Cu, Ni, Hg, Pb) that may be found in the earth, even if they have been treated, in much the same way that oxygenand sulphur are not interchangeable as non-metallic substances of the magnetosome within the same species. This is an indicator of the existence of various genera in the biomineralisation of magmagnetite and grigite [2]. In terms of thermodynamics, the inorganic synthesis of magnetite is preferred when compared to other iron oxidesof a neutral pH and a low potential redox. It would thus appear that the microaerobic conditions, or the anaerobic, create a potential adaptation in the formation of the BMP. Moreover the influx of iron is rapidly converted into magnetite, indicating that the formation of crystals is not predicated on an accumulation of iron under other forms and that the structure and the enzymescreated in the biomineralization process are already present within the cell. This conclusion is also supported by the fact that MTB which is cultivated in aerobic conditions (and thus non magnetic) contains a comparable quantum of iron and a few other species of bacteria [9]. Biotechnology ApplicationsIn certain types of applications, bacterial magnetite offers several advantages compared to chemically synthesized magnetite [11]. Bacterial magnetosome particles, unlike those produced chemically, have a consistent shape, a narrow size distribution within the single magnetic domain range, and a membrane coating consisting of lipidsand proteins. The magnetosome envelope allows for easy couplings of bioactive substances to its surface, a characteristic important for many applications. The size of magnetosomes renders them superparamagnetic, that is, they quickly follow changes in the external magnetic field without any remnants of the preceding polarity. [10] Magnetotactic bacterial cells have been used to determine south magnetic poles in meteorites and rocks containing fine-grained magnetic minerals and for the separation of cells after the introduction of magnetotactic bacterial cells into granulocytesand monocytesby phagocytosis. Magnetotactic bacterial magnetite crystals have been used in studies of magnetic domain analysisand in many commercial applications including: the immobilization of enzymes; the formation of magnetic antibodies, and the quantification of IgG; the detection and removal of Escherichia colicells with a fluorescein isothiocyanateconjugated monoclonal antibody, immobilized on magnetotactic bacterial magnetite particles; and the introduction of genes into cells, a technology in which magnetosomes are coated with DNA and "shot" using a particle gun into cells that are difficult to transform using more standard methods. Unfortunately, the prerequisite for any large scale commercial application is mass cultivation of magnetotactic bacteria or the introduction and expression of the genes responsible for magnetosome synthesis into a bacterium, e.g., E. coli, that can be grown relatively cheaply to extremely large yields. Although some progress has been made, the former has not been achieved with the available pure cultures. Bibliography
Categories: Bacteria| Bacteria stubs
|
