Bacteriology

Bacterial Flagella: Definition, Structure, Types, Functions, Rotation, Examples.

Most of the motile bacteria locomote by using threadlike appendages which is extending outward from the plasma membrane and cell wall is...

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Bacterial Flagella: Definition, Structure, Types, Functions, Rotation, Examples.

Bacterial Flagella Definition

  • Most of the motile bacteria locomote by using threadlike appendages which is extending outward from the plasma membrane and cell wall is known as flagella. Flagella also known as flagellum (Singular form).
  • The Latin meaning of the term flagellum is “ whip”, just because often flagella uses whipping motion for locomotion.
  • Flagella helps in locomotion which is the primary function of flagella but also they helps in attachment to surfaces, and in some bacteria, they function as a sensory organ that can sense alterations in pH and temperature.
  • Those cells contain flagella are known as flagellates. A flagellate can contain one or several flagella.
  • The flagella can be found in bacteria, archaea, and eukaryotes. The function of flagella in these three domains is similar but they are different in protein composition, structure, and mechanism of propulsion. 
  • The archaeal flagella is termed as archaellum, to indicate its distinction from bacterial flagella.’
  • The flagellum of archaea is nonhomologous.
  • The flagellum of bacterial cells are coiled, thread-like structure, sharp bent, consisting of a rotary motor at its base, and are made of the protein flagellin. Between the hook and a basal body a shaft is located which passes through the protein rings in the cell membrane.
  • The flagella of eukaryotic cells are made up of tubulin protein. These flagella are pummeled backward and forward and are seen in protist cells, gametes of plants, and animals. 

Bacterial Flagella Example

  • The ulcer-causing Helicobacter pylori is a flagellated bacterium. It reaches the stomach epithelium by propelling itself through the mucus lining. To propel itself it uses multiple flagella.
  • The mammalian sperm cell is a eukaryotic flagellate cell. It propels itself within the female reproductive tract by using its flagellum.

Bacterial Flagella Types/Pattern of Distribution

There are five types of pattern in bacterial flagella such as;

Atrichous

When the cells are devoid of flagella then this type of cell is called Atrichous.

Example: Lactobacillus lactis

Monotrichous

When the cell contains a single flagellum at one end of the cell is known as Monotrichous. If the flagellum is located at an end, it is called as a polar flagellum.

This type of flagellum can rotate clockwise and anti-clockwise. To move forward it rotate clockwise and to move backward it rotate the flagellum anti-clockwise.

Example: Vibrio choleriae

Lophotrichous

When the cells contain a cluster of flagella at one or both ends are known as Lophotrichous. This type of flagellum is known as polar flagellum and they also rotate clockwise and anti-clockwise.

Example: Pseudomonas fluorescents

Amphitrichous

When the cell contains a single flagellum at each pole is known as Amphitrichous. These flagella also rotate clockwise and anti-clockwise.

Example: Aquaspirillum serpens

Peritrichous

In this type, the flagella are spread evenly over the whole surface of the cell. The term “peri” means “around”. 

To rotate in one direction they rotate the flagella in anti-clockwise and form a bundle. If any of the flagella occur and begin wheeling clockwise, the organism does not go in any direction and starts tumbling.

Example: Salmonella typhie

arrangement of bacterial flagella
Arrangement of bacterial flagella | Image Author: Microbiologynote.com

Bacterial Flagella Structure

Bacterial flagella are composed of flagellin protein. These are 20-30 nm in diameter and about 15µm long. Flagellum is made of three important part such as;

  1. Basal Body
  2. Hook
  3. Filament
Structure of bacterial flagella
Structure of bacterial flagella | Image is modified from https://en.wikipedia.org/wiki/Flagellum#/media/File:Flagellum_base_diagram-en.svg by Microbiologynote.com

Basal Body

  • M.L. De Pamphilis and J. Alder first Isolated the basal body from E. coli and Bacillus subtilis and studied its fine structure and arrangement.
  • The basal body helps in the attachment of flagellum to the cell wall and plasma membrane.
  • It is made of a small central rod that contains a series of rings. The types and numbers of these rings vary in gram-positive and gram-negative bacteria. These rings are inserted into the central rod.
  • In gram-negative bacteria, the basal body contains four rings such as L, P, MS, and C. These rings are connected to a central rod.
  • The L, P, and MS rings are inserted within the cell envelope, and the C ring is on the cytoplasmic side of the MS ring.
  • In gram-positive bacteria, the basal body contains only two rings such as S (Super membrane) ring and M (Membrane) ring.
  • The inner ring in gram-positive bacteria connected to the plasma membrane and an outer one probably attached to the peptidoglycan.

Hook

  • The hook is located at the outside of the cell wall and connects filaments to the basal body.
  • It is a short and curved segment and acts as a flexible coupling.
  • The hook is composed of different protein subunits.
  • The hook of gram-positive bacteria is slightly larger than the gram-negative bacteria.

Filament

  • The longest and most obvious portion of the flagellum is known as filament. It extends from the cell surface to the tip.
  • It is composed of a globular protein known as flagellin, which varies in molecular mass from 30,000 to 60,000 daltons, depending on the bacterial species.
  • The flagellins are arranged in several chains that inter-twist and form a helix around a hollow core.
  • At the end, the filament contains a capping protein. Some bacteria contain sheaths surrounding their flagella.i.e, Vibrio cholerae flagella contain lipopolysaccharide sheaths.

Mot Protein

  • Besides, there is another protein called Mot-protein which controls the rotation of flegella.
  • This Mot-protein is anchored in the cytoplasm membrane and cell-wall.
  • The Motor conceit of a small central rod that passes through a system of rings.
Bacterial flagella of Gram-positive and Gram-negative bacteria
Bacterial flagella of Gram-positive and Gram-negative bacteria

Motor

  • The bacterial flagellum contains a rotary engine (Mot complex) at the flagellum’s anchor point on the inner cell membrane. It controls the rotation of flagella.
  • It is made up of protein which is known as Mot-protein.
  • The proton motive force powered this rotary engine. In proton motive force, the hydrogen ions or protons move across the bacterial cell membrane due to a concentration gradient which is set up by the cell’s metabolism.
  • The rotor carries protons crossed the membrane, and is used in the process. The rotor solely can spin at 6,000 to 17,000 rpm, but besides the flagellar filament attached normally only gives 200 to 1000 rpm.
  • The direction of rotation can be altered by the flagellar motor switch almost immediately, affected by a slight alteration in the state of a protein, FliG, in the rotor.
  • The flagellum uses very little energy, which means it is highly energy-efficient.
  • The definite mechanism for torque formation is yet badly understood. Because there are no on-off switch for the flagellar motor, the protein epsE is utilized as a mechanical link to release the motor from the rotor, therefore stopping the flagellum and subtracting the bacterium to settle in one place.
  • The rotational velocity of flagella changes in response to the strength of the proton motive force, some bacteria achieve roughly 60 cell lengths per second. At this speed, a bacterium would take about 245 days to cover 1 km.

Bacterial Flagella Functions

  1. It helps in the movement of bacterial cells.
  2. It helps the bacterial cell to get attached to other cells.
  3. They also help in asexual reproduction and in conjugation.
  4. Functions as a virulence factor.
  5. The movement of flagella also helps in the identification of special types of bacteria such as Proteus species show ‘swarming’ type of growth on solid media.

Endoflagella

  • The flagella-like organelles which are found in periplasmic space and encased by the outer membrane are known as Endoflagella.
  • The endoflagella start at each edge of the organism and wind throughout it, spreading to and overlapping at the midpoint. 
  • Inside the endoflagella is the interior membrane (cytoplasmic membrane) that provides osmotic balance and includes the protoplasmic cylinder. 

References

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