Everything about Magnetotactic Bacteria totally explained
Magnetotactic bacteria (or
MTB) are a class of
bacteria discovered in the
1960s, that exhibit the peculiar ability to orient themselves along the
magnetic field lines of
Earth's magnetic field. The term
magnetotaxis has been coined to describe the biological phenomenon upon which these
microorganisms tend to move in response to the magnetic characteristics of the environment.
Introduction
The first description of magnetotactic bacteria appeared in
1963 in a publication of the Istituto di Microbiologia (it: Microbiology Institute) of the
University of Pavia written by
Salvatore Bellini . While observing bog sediments under his microscope, he noticed a group of bacteria that evidently oriented themselves in a unique direction. He realised that these microorganisms moved according to the direction of the
North Pole and hence denominated them "magnetosensitive bacteria".
The first peer-reviewed report of magnetotactic bacteria appeared in a
1975 article in
Science by
Richard Blakemore, a
microbiologist at the
Woods Hole Oceanographic Institution, who had similarly observed bacteria capable of orienting themselves in a certain direction: Blakemore realised that these microorganisms were following the direction of Earth's magnetic field, from south to north, and thus coined the adjectival descriptor "magnetotactic" .
These bacteria have been the subject of many experiments: they've even been aboard the
Space Shuttle to examine their magnetotactic properties in the absence of
gravity, but a definitive conclusion wasn't reached .
The sensitivity of magnetotactic bacteria to the
Earth's magnetic field arises from the fact that the bacteria precipitate within their cells chains of crystals of magnetic minerals; all magnetotactic bacteria reported to date precipitate either
magnetite or
greigite. These crystals, and sometimes the chains of crystals, can be preserved in the geological record as
magnetofossils. The oldest unambiguous magnetofossils come from the Cretaceous chalk beds of southern England, though less certain reports of magnetofossils extend back in time to the 1.9 billion year old
Gunflint Chert . There have also been claims of their existence on
Mars based on the shape of
magnetite particles within the Martian meteorite
ALH84001, but these claims are highly contested .
Biology
Several different morphologies (shapes) of MTB exist which further differ for number, layout and pattern of the "bacterial magnetic particles" (BMP) the bacteria contain . The MTB can be subdivided into two categories, according to whether they produce particles of
magnetite (
), although some species are capable of producing both. Magnetite possesses a
magnetic moment three times that of greigite .
It has been suggested that MTB evolved in the early
Proterozoic Era, 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 magnetosomes for magnetotaxis. These early MTB may have participated in the formation of the first eukaryotic cells. . Some strains that swim persistently in one direction along the magnetic field (NS or 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 doesn't 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
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, 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 . For some MTB, it might also be necessary to perform excursions to anoxic zones of their habitat in order to accumulate reduced sulfur compounds.
Magnetosomes
The
biomineralisation of the magnetite requires regulating mechanisms to control the concentration of iron, the crystal
nucleation, the redox potential and the
pH. This is achieved by means of compartmentalisation in structures known as magnetosomes that allow the biochemical control of the above mentioned processes. After the
genome of several MTB species had been
sequenced, a comparative analysis of the
proteins involved in the formation of the BMP became possible. Sequence homology with proteins belonging to the ubiquitous CDF (Cation Diffusion Facilitator) family and the "Htr-like"
serine proteases has been found: while the first group is exclusively dedicated to the transport of heavy metals, the second group consists of
heat shock proteins (HSPs) involved in the degradation of badly
folded proteins. Other than the serine protease domain, some proteins found in the magnetosomial membrane (MM) also contain PDZ domains, while several other MM proteins contain TPR domains (Tetratrico Peptide Repeat) .
PDZ domain
The PDZ domains are modular structures which consist of 6 β-filaments and 2 α-helices which recognise the C terminal aminoacids of proteins in a sequence specific manner. Usually the third residue from the C terminal is
phosphorylated, thus preventing interaction with the PDZ domain. It's not surprising the only conserved residues in these structures are those involved in the recognition of the carboxy terminal (RKXXXGLGF). PDZ domains are quite widespread in nature since they constitute the basic structure upon which multi-proteinic complexes are assembled: this is especially true for those associated with membrane proteins, such as the inward rectifier K
+ channels or the β
2-
adrenergic receptors .
Membrane and proteins
The formation of the MM requires at least three steps. During the first formation of an
invagination in the cytoplasmic membrane is triggered by a
GTPase. It is supposed this process can take place amongst
eukaryote as well.
The second step requires the entrance of ferric
ions into the newly formed
vesicles from the external environment. Even when cultured in a Fe
3+ deficient media, MTBs succeed at accumulating high intracellular concentrations of this ion by secreting upon need a
siderophore, a low
molecular weight ligand displaying an elevated affinity for Fe
3+ ions. The "Fe
3+-siderophore" complex is subsequently moved in the
cytoplasm, where it's cleaved. The ferric ions must then be converted into the ferrous form (Fe
2+), in order to be accumulated within the BMP; this is achieved by means of a trans-membrane transporter which exhibits sequence homology with a Na
+/H
+ antiporter. Actually, the complex is a H
+/Fe
2+ antiporter, which transports ions thanks to the
proton gradient. These trans-membrane transporters are localised both in the cytoplasmic membrane and in the MM, but with in an inverted orientation: this configuration allows to generate an efflux of Fe
2+ 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 isn't yet fully explained and appears to be species specific.
During the final stage of the process the magnetite crystal nucleation by action of trans-membrane 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.
Likely, 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 .
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't 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 non-metallic substances of the magnetosome within the same species .
Biotechnology Applications
In certain types of applications, bacterial magnetite offers several advantages compared to chemically synthesized magnetite . 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
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 meteorites and rocks containing fine-grained magnetic minerals and for the separation of cells after the introduction of magnetotactic bacterial cells into
granulocytes and
monocytes by
phagocytosis. Magnetotactic bacterial magnetite crystals have been used in studies of
magnetic domain analysis and 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 coli cells with a fluorescein
isothiocyanate conjugated 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, for example,
E. coli, that can be grown relatively cheaply to extremely large yields. Although some progress has been made, the former hasn't been achieved with the available pure cultures.
Bibliography
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