Magnetotactic bacteria

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magnetosomes

Magnetotactic bacteria (or MTB) are a

polyphyletic group of bacteria that orient themselves along the magnetic field lines of Earth's magnetic field.[1] 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.[2] 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.[3]

Introduction

The first description of magnetotactic bacteria was in 1963 by Salvatore Bellini of the University of Pavia.[4][5] While observing bog sediments under his microscope, Bellini noticed a group of bacteria that evidently oriented themselves in a unique direction. He realized these microorganisms moved according to the direction of the North Pole, and hence called them "magnetosensitive bacteria". The publications were academic (peer-reviewed by the Istituto di Microbiologia's editorial committee under responsibility of the Institute's Director Prof. L. Bianchi, as usual in European universities at the time) and communicated in Italian with English, French and German short summaries in the official journal of a well-known institution, yet unexplainedly seem to have attracted little attention until they were brought to the attention of Richard Frankel in 2007. Frankel translated them into English and the translations were published in the Chinese Journal of Oceanography and Limnology.[6][7][8][9]

Richard Blakemore, then a microbiology graduate student[10] at the University of Massachusetts at Amherst, working in the Woods Hole Oceanographic Institution in whose collections the pertinent publications of the Institute of Microbiology of the University of Pavia were extant, observed microorganisms following the direction of Earth's magnetic field.[when?] Blakemore did not mention Bellini's research in his own report, which he published in Science, but was able to observe magnetosome chains using an electron microscope.[8][11] Bellini's terms for this behavior, namely Italian: batteri magnetosensibili, French: bactéries magnétosensibles or bactéries aimantées, German: magnetisch empfindliche Bakterien and English: magnetosensitive bacteria (Bellini's first publication, last page), went forgotten, and Blakemore's "magnetotaxis" was adopted by the scientific community.

These bacteria have been the subject of many experiments. They have even been aboard the Space Shuttle to examine their magnetotactic properties in the absence of gravity, but a definitive conclusion was not reached.[12]

The sensitivity of magnetotactic bacteria to the

ALH84001, but these claims are highly contested.[15]

Biology

Several different morphologies (shapes) of MTB exist, differing in number, layout and pattern of the bacterial magnetic particles (BMPs) they contain.

which?] are capable of producing both. Magnetite possesses a magnetic moment with three times the magnitude of greigite.[15]

Magnetite-producing magnetotactic bacteria are usually found in an

evolutionary advantage of possessing a system of magnetosomes is linked to the ability to efficiently navigate within this zone of sharp chemical gradients by simplifying a potential three-dimensional search for more favorable conditions to a single dimension. (See § Magnetism for a description of this mechanism.) Some types of magnetotactic bacteria can produce magnetite even in anaerobic conditions, using nitric oxide, nitrate, or sulfate as a final acceptor for electrons. The greigite mineralizing MTBs are usually strictly anaerobic.[17]

It has been suggested MTB evolved in the early

pigeons are far less advanced.[21]

Magnetotactic bacteria organize their magnetosomes in linear chains. The magnetic dipole moment of the cell is therefore the sum of the dipole moment of each BMP, which is then sufficient to passively orient the cell and overcome the casual thermal forces found in a water environment.

turgor.[15]

Nearly all of the genes relevant to magnetotaxis in MTB[

kilobase region in the genome called the magnetosome island.[22] There are three main operons in the magnetosome island: the mamAB operon, the mamGFDC operon, and the mms6 operon. There are 9 genes that are essential for the formation and function of modern magnetosomes: mamA, mamB, mamE, mamI, mamK, mamM, mamO, mamP, and mamQ.[23] In addition to these 9 genes that are well conserved across all MTB, there are more than 30 total genes that contribute to magnetotaxis in MTB.[23]
These non-essential genes account for the variation in magnetite/greigite crystal size and shape, as well as the specific alignment of magnetosomes in the cell.

The diversity of MTB is reflected by the high number of different

morphotypes found in environmental samples of water or sediment. Commonly observed morphotypes include spherical or ovoid cells (cocci), rod-shaped (bacilli), and spiral bacteria of various dimensions. One of the more distinctive morphotypes is an apparently multicellular bacterium[24] referred to as the many-celled magnetotactic prokaryote
(MMP).

Regardless of their morphology, all MTB studied so far are

16S rRNA
gene sequence comparisons was performed by P. Eden et al. in 1991.

Another trait that shows considerable diversity is the arrangement of magnetosomes inside 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, 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

poly-β-hydroxybutyrate
are common in MTB.

The most abundant type of MTB occurring in environmental samples, especially sediments, are coccoid cells possessing two flagellar bundles on a 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 copious amounts of hook-shaped magnetosomes (Magnetobacterium bavaricum).

Magnetism

The physical development of a magnetic

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.[15] 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.[17]

The MTB polarity model

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.

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.

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

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

anoxic zones
of their habitat to accumulate reduced sulfur compounds.

Magnetosomes

The

tetratricopeptide repeat (TPR) domains.[16]

TPR domain

Main article: Tetratricopeptide repeat

The TPR domains are characterized by a folding consisting of two α-helices and include a highly conserved consensus sequence of 8 amino acids (of the 34 possible),[29] which is the most common in nature. Apart from these amino acids, the remainder of the structure is found to be specialised in relation to its functional significance. The more notable compounds that comprise TPR domains include:

  1. membrane-bound transport complexes conveying proteins within mitochondria and/or peroxisomes
  2. complexes that recognise DNA-binding proteins and repress DNA
    transcription
  3. the anaphase-promoting complex (APC).

Examples of both the TPR-TPR interactions, as well as TPR-nonTPR interactions, have been reported.[30]

PDZ domain

Main article: PDZ domain

The

multiproteinic complexes are assembled. This is especially true for those associated with membrane proteins, such as the inward rectifier K+ channels or the β2-adrenergic receptors.[31]

Membrane and proteins

The formation of the magnetosome requires at least three steps:

  1. Invagination of the magnetosome membrane (MM)
  2. Entrance of magnetite precursors into the newly formed vesicle
  3. Nucleation and growth of the magnetite crystal

The first formation of an invagination in the cytoplasmic membrane is triggered by a GTPase. It is supposed this process can take place amongst eukaryotes, as well.

The second step requires the entrance of ferric

proton gradient. These transmembrane transporters are localised both in the cytoplasmic membrane and in the MM, but in an inverted orientation; this configuration allows them to generate an efflux of Fe2+ ions at the cytoplasmic membrane, and an influx of this same ion at the MM. This step is strictly controlled by a cytochrome-dependent redox system, which is not yet fully explained and appears to be species-specific.[as of?
]

During the final stage of the process, the magnetite crystal nucleation is by action of transmembrane proteins with acidic and basic domains. One of these proteins, called Mms6, has also been employed for the artificial synthesis of magnetite, where its presence allows the production of crystals homogeneous in shape and size.

It is likely that many other proteins associated with the MM could be involved in other roles, such as generation of supersaturated concentrations of iron, maintenance of reducing conditions, oxidisation of iron, and partial reduction and dehydration of hydrated iron compounds.[32]

Biomineralisation

Several clues led to the hypothesis that different genetic sets exist for the biomineralisation of magnetite and greigite. In cultures of Magnetospirillum magnetotacticum, iron can not be replaced with other transition metals (Ti, Cr, Co, Cu, Ni, Hg, Pb) commonly found in the soil. In a similar manner, oxygen and sulfur are not interchangeable as nonmetallic substances of the magnetosome within the same species.[17]

From a thermodynamic point of view, in the presence of a neutral pH and a low redox potential, the inorganic synthesis of magnetite is favoured when compared to those of other iron oxides.[33] It would thus appear microaerophilic or anaerobic conditions create a suitable potential for the formation of BMPs. Moreover, all iron absorbed by the bacteria is rapidly converted into magnetite, indicating the formation of crystals is not preceded by the accumulation of intermediate iron compounds; this also suggests the structures and the enzymes necessary for biomineralisation are already present within the bacteria. These conclusions are also supported by the fact that MTB cultured in aerobic conditions (and thus nonmagnetic) contain amounts of iron comparable to any other species of bacteria.[34]

Symbiosis with other species

Symbiosis with magnetotactic bacteria has been proposed as the explanation for magnetoreception in some marine protists.[35] Research is underway on whether a similar relationship may underlie magnetoreception in vertebrates as well.[36]

Biotechnology applications

In certain types of applications, bacterial magnetite offers several advantages compared to chemically synthesized magnetite.

lipids and proteins. The magnetosome envelope allows for easy couplings of bioactive
substances to its surface, a characteristic important for many applications.

Magnetotactic bacterial cells have been used to determine south magnetic poles in

antibodies, and the quantification of immunoglobulin G; the detection and removal of Escherichia coli cells with a fluorescein isothiocyanate
conjugated monoclonal antibody, immobilised 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.

However, 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.

Further reading

Bibliography

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External links
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