Chemotaxis in Bacteria

Chemotaxis is the process by which the bacteria respond to chemical gradients. Chemo means chemical and taxis means to move. It is the movement in which the bacteria and somatic cell move according to chemical gradients present in the environment.

Types of chemotaxis:

There are two types of chemotaxis:

Positive chemotaxis:

Positive chemotaxis is towards the chemical. It is the movement towards the high concentration gradient of the chemicals.

Negative chemotaxis:

Negative chemotaxis is away from the chemical. It is the movement towards the low concentration gradient of the chemicals. This movement is opposite to the chemical gradient.

Bacterial characteristics:

Chemicals always diffuse from the high gradient to the low gradient. Bacteria move in the response of chemicals. When bacteria tumble, it moves away from the chemicals. When bacteria run, it moves towards the chemicals. Bacteria move in two directions in response to chemicals.

Counterclockwise direction:

When the shaft moves in an anticlockwise direction, bacteria start to run. It aligns the bacterial flagella in a single rotating bundle. The single rotating bundle helps the bacteria to swim and move the bacteria in a straight line.

Clockwise direction:

When shaft moves in the clockwise direction, bacteria tumble. It aligns the bacterial flagella in different rotations. The different rotations help the bacteria to move in a different direction and move the bacteria in tumbling manner.

Bacteria behaviour:

The bacteria move in the tumbling and swimming manner. Bacteria move randomly by different swimming and tumbling manner. This swimming and tumbling movement reorient the bacterial direction. Bacteria have some rotational diffusion pattern that forgets the bacteria to move in a straight line for a few seconds. After a few seconds, bacteria change its movement. Due to the chemical gradient, the wrong direction is detected by bacteria and bacteria tumble. By using repeatedly swimming and tumbling movement, bacteria find its favourable location and get its benefit.

There are two types of bacterial behaviour that respond to chemicals in their environment.

Attractant:

Attractants are the chemicals that attract the bacteria towards themselves. They may be sugars, amino acids, fatty acids etc. Bacteria move towards attractants to get nutrition and to fulfil their other basic requirements. When bacteria move towards attractants; bacteria run fast and delay its tumbling movement. When bacteria moves away from attractants, bacteria tumble fast and delay its run movement.

Repellant:

Repellents are the chemicals that are toxic to the growth of bacteria. These may be antibiotics, toxins and bacteriocins. These chemicals are produced when bacteria are present in a competitive environment. Bacteria produce different bactericidal and bacteriostatic components for survival in a tough environment. When bacteria move towards repellent; bacteria tumble fast and delay its run movement. When bacteria move away from repellent, bacteria run fast and delay its tumbling movement.

Sensing of chemical gradients:

These chemical gradients are sensed by bacterial trans-membrane receptors that are present in the cell membrane. These receptors are called methyl accepting chemotaxis proteins (MCPs). These receptors have receptors for the binding of the attractants and repellent that is directly initiating the movement of bacteria. These receptors interact with the ligand in the Periplasmic space and then, send signal to the cell membrane. Cell membrane transmitted these signals to the cytosol where the Che proteins are activated. These proteins are present in the cytoplasm of the bacterial cell and these proteins alter the tumbling movement of the cell.

Organ for bacterial movement:

The bacteria have the organ for locomotion that helps the bacteria to move freely without any support of other organisms. The bacteria that can move are called motile bacteria. The organ for locomotion that makes the bacteria motile is called as flagella. Flagella are present in the outer surface of bacteria. Flagella are a Latin word that means whip. It is the extracellular component of bacteria that are present outside the cell. Flagella allow the bacteria to move in all 360ᴼ angles. Bacteria move in the form of run and tumble. Flagella are protein in nature. Flagella are the hollow tubes that protrudes out from the cell. It is composed of flagellin proteins. Its size varies from bacteria to bacteria and range from 20-30 nm. Their flagella are evolved from type 3 secretary system. There are three parts of flagella.

• Basal body
• Hook
• Filament

Basal body:

Basal body is responsible for originating the structure that protrudes out from the cell. It is present inside the cell wall. It is a rigid component that consists of the shafts or motor. Shaft allows bacteria to move. Motor moves in clockwise and anticlockwise direction. When shaft moves in the clockwise direction, bacteria tumble. When the shaft moves in anticlockwise direction, bacteria start run. fliM protein is responsible for converting the shifting of the shaft. This protein shifts the motor or shaft from clockwise to the counter-clockwise direction.

The basal body consists of the rings that are present in the membrane of the cell wall. Two basal rings are present in gram positive bacteria. These basal rings are named as M that is present in the plasma membrane and S ring that are present in the membrane space. Four basal rings are present in the gram negative bacteria. The two rings are same as gram positive bacteria and two rings are different. The one ring is L ring that is present in the lipopolysaccharide layer and the other ring is P ring that is towards the Periplasmic space of the bacteria.

Hook:

The hook is the component that allows the filament to move outside from the cell. It forms the bridge between the filament and the basal body. It bonds the filament and basal body and forms a connection between them.

Filament:

The filament is the part that is composed of flagellin protein. It is helical part of the flagella. The protein amino acids arrange to form a helical structure. This helical structure makes the flagella a hollow structure. The ribosomes that are responsible for synthesis of the flagellin protein are present in the cytosol of cell. These proteins are transported to the outside of the cell with the help of the transporter proteins. The amino acid adds into the growing tip of the hollow core and synthesizes the filament of flagella.

Flagellum regulation:

Proteins are present for the regulation of flagella for the movement. These proteins are called as CheW and CheA. These proteins bind to the receptor and then regulate the flagella movement. When the receptor is not active and the ligand is absent for the activation of the receptor, auto-phosphorylation of CheA protein in histidine kinase take place. This auto-phosphorylation in CheA protein is responsible for transferring of the phosphoryl group to the conserved aspartate residues. The phosphoryl group is accepted by CheB present in the cell. The transfer of the phosphoryl group activates the binding of CheB and CheY in the cell. This is called two-component signal transduction in which the two component of the cell sends signal to the cell. CheY is responsible for inducing the tumbling motility in the bacteria cell.