Chemotaxis

The term chemotaxis can be defined as the movement that occurs in an entity or an organism in response to certain chemical stimuli. The bacterial cells, somatic cells, as well as other multicellular and unicellular organisms direct the movements in their bodies according to the chemicals that are present in their environment such as chemoattractants and chemorepellents.

It is seen in the case of bacteria that the mechanism of chemotaxis is used in order to find food, for example, glucose. The bacteria move toward the molecules that have the highest concentration of food. Also, they can use the mechanism to move away from toxins such as phenol.

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In the case of multicellular organisms, the process is essential for the stages of development. The motility of sperm is one of the main features of chemotaxis. Apart from that, chemotaxis meaning is also used to describe certain health functions such as the movement of leukocytes during an infection or injury in the body of organisms.

General Characteristics of Chemotaxis in Bacteria

Certain species of bacteria like E. coli tend to have several flagella in their cells. Typically, one cell contains 4-10 different flagella. These flagella can perform the action of rotation in two different ways.

• The counterclockwise rotation results in the alignment of the flagella into a particular rotating bundle. This causes the bacterium cell to move in a straightforward motion.

• The clockwise rotation will result in the breaking of the flagella bundle in such a manner that the ends of the flagellum are pointed in different directions. This causes the tumbling motions in the flagella.

Different Types of Chemotaxis

Different types of cells have to go through different types of chemotaxis. Multicellular organisms undergo the Collective Migration (involving stromal cells where the leader cells create different tracks for other cells to follow and there are cell-cell junctions present) and Cell Streaming (involving stromal cells where the individual cells tend to move as well as follow one another and it required no cell-cell junctions). In the case of single-cell organisms, the types of chemotaxis include Amoeboid Migration (a case of fast cell migration where the cells are squeezing through a matrix that is extracellular and use contractile force and don’t require matrix metalloproteinases) and Mesenchymal Migration (a process of slow cell migration with elongated cells and established cell polarity).

The Behaviour of Bacteria in Chemotaxis

The movement that is seen in bacteria can be a result of the alternative selection of swimming and tumbling phases. Hence, the trajectory that a particular bacterium follows through swimming in a certain uniform environment will create the appearance of a random type of walk consisting of relative straight patterns of swimming which are interrupted by some random inclusions of tumbles. This is due to the process of chemotaxis in bacteria.

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The bacterial species of E. coli aren’t able to choose the direction in which they can swim. Hence, these species aren’t able to swim in a particularly straight line for more than a few seconds. This is due to the effect of rotational diffusion. In general terms, it can be said that the bacteria actually forget the direction in which it is supposed to move. In case there is a chemical gradient present in the host, the bacteria will go through chemotaxis.

This means the bacteria will direct the overall motion on the basis of the gradient. In case the bacterium has sensed that the direction of movement is correct, it maintains the straight line in which it is moving and it continues in that path until the next tumbling phase. However, in case the bacterium’s movement is in the wrong direction, the tumbling phase occurs a bit sooner. E. coli can use temporal sensing in order to make sure that the direction of movement is correct or it needs improvement and hence is able to find the right location of the highest concentrated attractant.

The biased form of random movement is basically a result of just the bacteria choosing between these 2 different movement methods namely straight swimming and tumbling. One of the most important factors that play a very important role in these peculiar movements of the bacteria would be the helical nature provided to the flagellar filament. Flagellin is the protein structure that is used in order to create the filament in the flagella.

There are certain instances in biological studies where other species of bacterial don’t seem to follow this particular rule of movement. Another bacterial species like Vibrio has a single flagellum situated on the cell pole and hence are monoflagellated bacteria. The method of chemotaxis tends to be a bit different in their case. These bacteria will facilitate the movement by spinning the entire cell which resembles the shape of a corkscrew.

Signal Transduction in Bacterial Chemotaxis: Process

The proteins such as CheA and CheW are responsible for binding themselves to the receptor. The receptor’s activation due to the presence of this external stimulus causes the process of autophosphorylation to take place in the histidine kinase which is CheA. This happens at a particularly high and conserved histidine residue.

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The kinase CheA then transfers the phosphoryl groups that are produced into the conserved residues of aspartate as a response to the actions of the proteins CheB and CheY. It is a very important thing to keep in mind that the CheA is a histidine and doesn’t actively cause the transfer of the phosphoryl group. The CheB which is the response regulator will take this group from CheA. This completes the process of signal transduction and is referred to as a 2-component system.

In order to understand chemotaxis meaning, it can be said that is a directed motion of any entity or organism towards certain environmental conditions that is deemed attractive or repulsive by the entity. The movement that is seen in the Escherichia coli bacteria is one of the common examples used to describe the entire mechanism. It is characterized as a particular sequence that uses smooth runs of swimming interrupted by intermittent tumbles.