Physics in the animal world: magnetically sensitive bacteria and their compass

In 1975, microbiologist Richard Blackmore (Richard P. Blakemore), receiving a diploma at the University of Massachusetts, began collecting bacteria from swamps that are located along the Atlantic coast. He studied the selected samples in the laboratory, and then went back to the training camp. One day he discovered an interesting behavior of one of the strains of selected bacteria. The fact is that these bacteria were always collected on the northern edge of a drop of liquid in which they were placed on a microscope slide. He checked his assumption, and it turned out that the bacteria are really constantly heading north.

Blackmore decided to understand how miniature living organisms, the size of which is about two thousandths of a millimeter in length, are able to determine the directions of the world. First, the scientist checked whether the bacteria react to the magnetic field. He took a small compass, and placed it next to a glass slide with a drop of liquid and bacteria in it. The bacteria, ignoring the north this time, began to move in the direction of the magnetic field lines of this magnet.

The scientist immediately realized that it was in a magnetic field, and not in anything else. In order to describe the magnetic sensitivity of bacteria, he proposed the term "magnetotaxis." It is worth noting that later scientists discovered other bacteria that respond to a magnetic field. Among them, at times, there is nothing in common except the ability to move in the direction of the magnetic field lines. The term "magnetotactic bacteria" unites sticks, spirill, vibrios and other microorganisms.
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As it turned out, tiny magnetite particles are enclosed inside the bacteria. The size of each such particle is only 50 nm on each side. Different bacteria can have either magnetite granules (Fe3O4) or graigit granules (Fe3S4). These granules are surrounded by a lipoprotein membrane.

The organs where the crystals are synthesized are called magnetosomes. Inside bacteria, they can be combined into chains, and in the cells of magnetotactic bacteria their number can be several tens or even hundreds (in one of the bacteria, Candidatus Magnetobacter bavaricum, more than a thousand magnetosomes are found). So, magnetite and greigit crystals line up in the body of such bacteria along the chain, orienting themselves along parallel magnetic dipole moments. As Wikipedia tells us, the magnetosome is a membrane structure of bacteria, characteristic of bacteria possessing magnetotaxis, containing monodomain ferromagnetic crystals. Typically, the cell contains from 15 to 20 magnetite crystals, which together act as a compass needle, helping the bacteria orient themselves with respect to the geomagnetic fields, and thus making it easier for them to find their favorite microaerophilic habitat. Particles of magnetite are also found in eukaryotic magnetotactic algae, the cells of which contain several thousand crystals.


Typically, the total magnetic dipole moment is large enough to orient the cell in the direction of the magnetic lines. Oriented on the cardinal points, the bacteria move with one or several flagella. An interesting point is that the dead cells are also oriented along the lines of the magnetic field (the magnetosomes in the body of the bacteria remain) but for obvious reasons do not move.

Magnetosensitive bacteria from the northern hemisphere of the Earth are moving parallel to the lines of the geomagnetic field. This causes microscopically small organisms to move towards the server. They are called "north-seekers." But the bacteria from the southern hemisphere are moving in the opposite direction, they are called "south-seekers." Actually, the names of the bacteria does not cause questions. Since the vectors of the magnetic field lines are directed upwards in the southern hemisphere and downward in the northern one, the movement of both “southerners” and “northerners” is always directed downwards.

Scientists explain this peculiarity of bacteria by the fact that they need layers of silt with a minimum concentration of oxygen. And the ability to navigate in space leads to the fact that bacteria without problems moving lower and lower. Then, reaching the desired depth, they are deposited on the silt particles. True, there are a number of questions. One of them is that scientists cannot yet explain why certain types of bacteria have hundreds of magnetosomes in a single cell. After all, only a few of these particles are enough for orientation.

The discoverer of bacteria of this type, Richard Blackmore, stated that magnetosomes may have several functions. One of them is to prevent the accumulation of hydrogen peroxide H2O2 in the cell. This assumption is partially confirmed by new experiments, which showed that magnetosomes actually reduce the content of active oxygen forms in the cell. But there is another question related to the previous one. The fact is that the synthesis of magnetosomes begins only in the case of a low concentration of oxygen. Plus, the free forms of ferrous iron ion are toxic to bacteria. But the accumulation in the cell of a large number of magnetosomes can lead to the accumulation of such ions.


(A) specific expression of proteins and fluorophore labels; (B) use of fusion ("crosslinked") proteins, streptavidin tags; © the formation of complexes with gold particles by means of DNA linkers; (D) modified magnetosome membrane proteins and immunoglobulin-binding protein (MM — magnetosome membrane, MMP — magnetosome proteins, SAV — streptavidin) "> (A) specific expression of proteins and fluorophore labels; (B) use of fusion (" cross-linked ") proteins , streptavidin tags; © formation of complexes with gold particles through DNA linkers; (D) modified magnetosomes membrane proteins and immunoglobulin-binding protein (MM — magnetosomal membrane, MMP — magnetosome proteins, SAV — streptavidin)

There is an interesting point in the synthesis process. The fact is that almost all magnetically sensitive bacteria synthesize magnetite Fe3O4 crystals of practically the same shape and with a narrow size distribution. And all this happens at room temperature. Not so long ago, it was found that mms6 protein binds to the binding of iron ions, plus, possibly, other proteins. Now there are various plans for the synthesis of magnetite at room temperature from iron hydroxide. But scientists from the Ames Laboratory and Iowa State University (USA) went even further: they used the bacterial protein mms6 to synthesize cobalt ferrite nanocrystals (CoFe2O4), which living organisms do not know how to produce.

In order to achieve this result, the authors included the above-mentioned protein in the composition of the gel, where its individual molecules were combined into groups. Groups also arranged in a certain way, forming a matrix for the synthesis of nanocrystals. When cobalt and iron salts were added (CoCl2 and FeCl2) , rather large (50-80 nm) thin hexagonal cobalt ferrite plates were obtained .

As it turned out , this material showed better magnetic properties compared to cobalt ferrite, which was synthesized under similar conditions, but without the use of mms6 protein.


CoFe2O4 particles obtained by different methods: a, b - in the presence of a polymer, but without protein; c - in the presence of polymer and protein mms6, not related to each other; d is the same for the C-terminal mms6 fragment; e - in the presence of a polymer bound to mms6; f - in the presence of a polymer bound to the C-terminal fragment of mms6 (TEM). Label size 50 nm

Practical use of magnetically sensitive bacteria

That is, inside the bacteria there are magnetic nanocrystals, plus there is an organic lipoprotein membrane in the cell itself, which allowed scientists to begin planning to use these bacteria as various tools. For example, for immobilization of such enzymes as glucooxidase and uricase. In the case of working with bacteria, the enzymes were 40 more active than in the case of working with artificial magnetic particles.

It turned out to be possible to use magnetosomes with antibodies on their surface to carry out various enzyme immunoassays. Among the varieties of these tests - the definition of allergens and cells of epidermoid cancer. Also, bacterial magnetic particles can work with a fluorescent substance to detect E. coli cells.

Now on the basis of magnetically sensitive bacteria and their magnetosomes, a method is being developed for the targeted delivery of drugs to various organs of the human body and animals. With the help of a magnet, magnetosomes of bacteria with drugs can be delivered directly to the target.

The unique features of the described bacteria can be used not only in medicine. You can work with them and electronics. For example, Scientists from the University of Leeds have proposed their own technology for growing homogeneous magnetite crystals on a substrate using magnetically sensitive bacteria. Japanese scientists used a similar method, only they decided to form the basis for nanowires in microscopically small microcircuits using microorganisms. When creating nanowires, scientists from Japan use copper and indium sulfide particles and zinc sulfide. Such nanowires are placed in a lipid envelope. Scientists were able to form from the lipid molecules something like tubes in which the wires are then placed.

With this method of growing crystals, the bacteria are staggered on a gold substrate. After that, the substrate is placed in a solution of iron salts. At a temperature of 80 ° C in those areas that were covered with bacteria, homogeneous magnetite nanocrystals are formed. Such nanocrystals can hold a charge, and the system can be used to record information.


Canadian scientists from the NanoRobotics Laboratory of the Ecole Polytechnique in Montreal have been able to force bacteria to build a small pyramid system. By controlling the shape and intensity of the magnetic floor with a computer, the specialists were able to organize a detachment of builders from a colony of magnetically sensitive bacteria. In a series of experiments, specialists have achieved the creation of a pyramid-shaped structure, as well as the promotion of bacteria in the bloodstream of a living rat. In the future, Canadians hope to use the technology of the behavior of bacteria to create larger nanostructures. Perhaps magnetically sensitive bacteria can become part of a more complex system.

So far, almost all of the proposed methods of working with magnetically sensitive bacteria are at the laboratory testing stage. The fact is that these microorganisms grow relatively slowly, which means their productivity is not very high. Therefore, at the moment it is more profitable to work with traditional physicochemical methods of growing the same crystals. But the methods of cultivation of magnetotactic bacteria are constantly being improved, therefore, the productivity of the strains increases.

In order to achieve better results, scientists propose the use of genetic engineering.