What is Chemotaxis?

The organism’s movement in a particular direction as a response to certain chemicals that are available in the environment is defined as chemotaxis. In brief, it can be defined as the occurrence of an organism’s movement in response to a certain chemical stimulus.

How do Organisms use Chemotaxis?

Chemotaxis is a phenomenon in which bacterial, somatic, and certain single or multicellular organisms move in a particular direction due to the presence of a certain chemical in the environment. Chemotaxis occurs as a response to those chemicals. Chemotaxis plays a significant role in enabling bacteria to identify their food and to protect themselves from poisonous molecules. Bacteria tend to swim in the direction which has the highest concentration of the molecules of food to satisfy their hunger. In multicellular organisms, the chemotaxis phenomenon is considered to be critical to regular functions and development.

Types of Chemotaxis

Chemotaxis is broadly classified into two significant types namely Positive and negative chemotaxis. They are described as follows:

• Positive chemotaxis: If the migration of the organisms and motile cells takes place in the direction towards the chemicals or the substances that are diffusible which are greater in concentration are called a positive chemotaxis type.
• Negative chemotaxis: If the phenomenon of chemotaxis takes place in the opposite or alternate direction to that of the chemical substances present in higher concentrations is called the negative chemotaxis.

Process of Chemotaxis

Chemotaxis which is prevalent in organisms starting from bacteria to human involves 2 fundamental steps.

• At first, the process of formation which is the mechanism by the diffusion of the chemical’s concentration gradient takes place with the help of the chemotactic ligands.
• The next step is the gradient sensation by a specific organism or the cell and the alternation of the movement direction accordingly. The movement can be either towards or away from the specific chemical.

Chemoattractants and Chemorepellants

The chemical substances that are either organic or inorganic which possess the phenomenon of inducing the effect of chemotaxis in the motile cells are named chemoattractants and chemorepellants. Chemoattracts exhibit their effects with the help of chemoreceptors. Methyl-accepting chemotaxis protein (MCP) is one such receptor.

Galactose and ribose are examples of chemoattractants and phenol is an example of a chemorepellant. In the external cell domain of chemoattractants and chemorepellants, MCP is bounded and the domain within the cell helps to relay the variations in the concentration gradient of those ligands that are chemotactic and further follows the processes to achieve the movement of the cells or organism in a specific direction. As per the investigation, the mechanism behind the chemoattracts is well defined than that of the chemorepellants.

Bacterial Chemotaxis

E. coli is a bacterium that contains four to ten flagella per cell. This can rotate in two distinct ways namely clockwise and counterclockwise rotation. The movement occurring via both rotations is tumbling and swimming respectively. This movement is based on the alignment of flagella. The bacterium’s overall motion includes both swimming and tumbling movement in an alternative manner. When a gradient in chemical exists, chemotaxis will be carried out by a bacterium in which the gradient directs the bacterium for the direction of the movement. Based on the sensing by a bacterium, it tends to swim in a straight line for a long span of time before the movement of tumbling, if it moves towards the chemoattractant. If it senses its directional movement is towards the chemorepellent, it gets tumbled in a short period.

Each filament of flagella is helical in nature which is significant for the movement of the bacterial cells. The flagellin is the structure of the molecule of protein that constructs the filament of flagella. It is in conservation in all the bacteria that are flagellated. However, in biology, it is not always standard among all organisms. Some bacteria, for example, Vibrio which is called monoflagellated and contains only a flagellum at a cell’s pole. In this case, chemotaxis occurs in a varied manner.

Eukaryotic Chemotaxis

The chemotaxis phenomenon is entirely different in eukaryotes when compared to prokaryotes. Though the sensing of the gradient of chemicals remains a significant step in both organisms. Prokaryotes like bacteria are smaller in size and thus they are not capable of identifying the concentration gradient in a direct way. They utilize a temporal method to sense the environment and further swims and redirect on their own whenever they observe the modifications in the gradient. But in the case of eukaryotes, as they are larger in size than prokaryotes, they have several receptors that are uniformly embedded over the membrane of the cell. This involves a spatial method of sensing the gradient of concentration. This is achieved by the comparison between the receptor activation in an asymmetric way at varying cell ends. Thus, these receptor’s activation aids in the directional movement towards the chemoattracts or away from the chemorepellants. In eukaryotic cells, chemotaxis plays an important role in the movement of immune cells and the initial embryogenic phase. In the early stage of the embryogenic phase, the germ layers are developed by the guidance of the signal molecule’s gradient.

The motility in the cells of eukaryotes is not clear yet as like bacterial movement. It appears to be that it carries out a mechanism that involves the sensing of an external gradient of chemotactic ligands and is converted into a PIP3 gradient which is intracellular. This then activates the pathway for signaling and ends in the polymerization of the filament of actin molecules.

Measuring Chemotaxis

The activity of chemotaxis in the cells can be evaluated by several broad techniques. The measurement requires four significant essentials which are listed as follows:

• The ability of the gradient of concentration to develop relatively in a fast manner and can able to stand for a longer period in a particular system
• The distinction between the chemokinetic and chemotactic mechanisms
• The cell’s movement is independent towards or away on the specific concentration gradient axis
• The response detected are the finalized results of the active cell’s movement.

The ideal method of assay is not yet discovered, however, agar-plate assays, two-chamber assays, and certain other techniques such as the T-maze technique, orientation assay offer better results.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

• B.Sc. in Biology
• B.Sc in Microbiology
• M.Sc in Microbiology
• Masters in Microbial Biotechnology

Related Concepts

• Motility Fertilization
• Durotaxis
• Haptotaxis
• Mechanotaxis