CHEMOTAXIS: BACTERIAL, IN NEUTROPHILS, IN INFLAMMATION - BIOLOGY - 2025

The chemotaxis is a mechanism by which cells move in response to a chemical stimulus. The stimulus can be any diffusible substance, which is detected by receptors on the surface of the cell. There are two main types of chemotaxis: positive and negative.

Positive chemotaxis is in which the movement is directed towards the source of the stimulus where the concentration is greater. For its part, negative chemotaxis is in which the movement occurs in the opposite direction to the chemical stimulus. In multicellular organisms, chemotaxis is vital for normal body development and function.

In chemotaxis, bacteria and other unicellular or multicellular organisms direct their movements in response to certain chemicals in their environment (stimuli).

It is an important mechanism in the immune system to attract T lymphocytes to places where there is infection. This process can be altered during metastasis.

Bacterial chemotaxis

Bacteria can move through a variety of mechanisms, the most common being the movement of their flagella. This movement is mediated by chemotaxis that serves to bring them closer to favorable substances (chemo-attractants) and to move them away from toxic ones (chemo-repellants).

Bacteria, like Escherichia coli, have several flagella which can rotate in two ways:

- To the right. In this case each flagellum "paddles" in a different direction, causing the bacteria to flip over.

- Toward the left. In this case, the flagella are aligned in only one direction, causing the bacteria to swim in a straight line.

Generally, the movement of a bacterium is the result of alternating these two phases of rotation. Chemotaxis targets the bacteria by regulating the frequency and duration of each one.

This modulation of the direction of movement is the result of very precise changes in the direction of rotation of the flagella. Mechanically, therefore, the essence of bacterial chemotaxis is to control the direction of flagellar rotation.

Neutrophil chemotaxis

Neutrophils are a type of cells of the immune system that are essential in the defense against infections. Within the body, neutrophils migrate to places where there is an infection or tissue injury.

The migration of these cells is mediated by chemotaxis, which acts as the force of attraction to determine the direction in which neutrophils move. This process is activated by the release of specialized proteins of the immune system, called interleukins, at sites where there is tissue damage.

Of the many neutrophils that circulate outside the bone marrow, half are in the tissues and half are in the blood vessels. Of those found in blood vessels, half are within the main stream of blood rapidly circulating throughout the body.

The rest of the blood neutrophils move slowly, with their characteristic amoeboid motion, along the inner walls of the blood vessels. Upon receiving a chemotactic signal, neutrophils rapidly enter tissues to perform their defense function.

Protein action

Chemotaxis in neutrophils is mediated by proteins inserted in the plasma membrane, which function as receptors for certain molecules of the immune system. Binding of receptors to their target molecules causes neutrophils to migrate to the sites of infection.

During chemotaxis, cells move in response to chemical signals. The action of neutrophils is just one example of how the body uses chemotaxis to respond to an infection.

Chemotaxis and inflammation

During inflammation, white blood cells (leukocytes) adhere to cells inside blood vessels, from where they migrate through the endothelial cell layer and move between tissues to the source of inflammation, where they will act as host defense.

Leukocyte chemotaxis is considered essential for migration from the blood to tissues where there is inflammation. This inflammatory response is caused by an infectious agent or an allergy-causing substance.

Inflammation increases blood flow and blood vessel permeability, causing cells and proteins to escape from the blood into the tissues. Due to this response, neutrophils are the first to respond to inflammation (in addition to cells already in the tissues, such as macrophages and mast cells).

Chemotaxis and phagocytosis

During an infection, chemical signals attract phagocytes to the places where the pathogen has invaded the body. These chemicals can come from bacteria or other phagocytes already present. Phagocytes are stimulated by these chemo-attractant molecules and move by chemotaxis.

Phagocytes are a class of cells that includes macrophages, neutrophils, and eosinophils, which are capable of ingesting (engulfing) and destroying the microorganisms responsible for inducing the inflammatory response.

Neutrophils are the first to accumulate around invading agents and initiate the phagocytosis process. The local macrophages - also called professional phagocytes - and the rest of the phagocytes from the blood then migrate into the tissue and initiate phagocytosis.

Factors that interfere

Importantly, some species of bacteria and their products can interfere with the chemotaxis process, inhibiting the ability of phagocytes to travel to the site of infection.

For example, streptococcal streptolysin suppresses neutrophil chemotaxis, even at very low concentrations. Also, Mycobacterium tuberculosis cells are known to inhibit leukocyte migration.

Chemotaxis in immunology

Chemotaxis is a fundamental process for the immune system because it coordinates the direction of the movements of cells important in the defense of the organism. Thanks to this mechanism, neutrophils can go to places where there is an infection or injury.

Along with the inflammatory response, chemotaxis is essential for the migration of other phagocytes that are necessary to eliminate toxins, pathogens, and cellular debris. This is part of the innate immune defense.

References

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