Bacterial Flagella

Bacterial flagella can define as a locomotory apparatus that helps the bacteria to swim in the liquid nutrient medium. The bacteria possessing flagella refers as “Motile organisms or Flagellates” while those bacteria possessing flagella refers as “Non-motile organisms or Non-flagellates”. The width of bacterial flagella is much thinner and simpler than the eukaryotic flagella.

The location, number and arrangement of flagella vary considerably in different bacterial species. A basic structure of flagella are composed of three domains, namely hook, basal body and filament. Bacterial flagella are devoid of “9+2” arrangement of microtubules. The presence or absence of flagella or to detect motility, one can perform flagella staining by using special flagellar stains.

Content: Bacterial Flagella

  1. Definition
  2. Features
  3. Structure
  4. Classification
  5. Functions
  6. Bacterial Motility
  7. Polymorphic Transitions
  8. Conclusion

Definition of Bacterial Flagella

Bacterial flagella can define as the twisted and hair-like filament that give swimming motility in certain groups of bacteria. The flagellar apparatus is generally attached to the bacterial cell surface at one end, and the other end remains free for the movement. These structures are too thin, so to check the bacterial motility one can perform flagella staining by using special stains under a light microscope. Flagella are abundantly found in gram-negative rods, and in few gram-positive rods and cocci.


  1. Flagella in prokaryotes are the proteinaceous structures that are composed of protein monomers “Flagellin”.
  2. Inside the flagella, the flagellin subunits are organized in a helical fashion.
  3. Flagella are the protrusions of the bacterial cell, which performs a significant role in locomotion.
  4. Other than locomotion, flagella can perform sensory functions in prokaryotes by sensing the temperature and chemical variation.
  5. The function of flagella is to provide motility in both bacteria and multicellular organisms, but its morphology, chemical composition and propulsion mechanism differ in both the types.
  6. The flagellar distribution helps to distinguish or classify the bacteria, which can be polar or lateral in distribution.
  7. Polar flagella is a type of arrangement, where the flagellar appendages protrude out from one or both the ends of the bacterial surface.
  8. Lateral flagella is a type of arrangement, where the flagellar appendages extend out over the full bacterial surface.
  9. It may also function as “Secretory organelles”.

Structure of Bacterial Flagella

There are three structural elements that constitute the formation of the flagellar apparatus.
Structure of bacterial flagella

Basal Body

It is generally attached to the cell wall and cell membrane, and embedded with the rack of rings arranged one over the other. The rings are composed of protein sub-units and are of four kinds, namely M, S, P and L ring.

M and S rings are associated with the cytoplasmic membrane, while P and L rings are associated with the periplasmic space and cell wall. The protein rings serve to pump protons or H+ ions across the membrane that drives the ATP generation. This ATP (Adenosine triphosphate)  is then used to rotate the rings as well as the filament.

These four rings are encircled by a pair of proteins refer as “Mot” and “Fli”. Mot protein causes motor rotation, while the Fli protein functions as a “Motor switch” that reverses the flagellar rotation in response to the intracellular signals. It also anchors the filament of the flagellum.


It is the broader region present at the base of the flagellum, and performs a key role in connecting filament to the motor region or basal body. Its length is greater in gram-positive bacterial strains. About 120 sub-units of single protein constitute the formation of a short and curved hook.

Hook remains free or not associated with the structures like cell wall and plasma membrane. It promotes the taxis and motility to the bacteria by generating a motor torque to the filament. It comprises a pair of protein, namely FlgK and FlgL that helps in connection with the filament and its synthesis.


It can define as the whip-like structure that is long, coiled, thin, and several times bigger than the whole bacterial cell and appears as a long hollow tube. The composition of filament includes flagellin protein sub-units that are arranged in an intertwined chains.

Filament participates in the propulsion of bacteria. The rotation or movement of flagella depends upon the motor spinning by the basal body. The bacterial flagella can move in either anticlockwise or clockwise direction.


The flagella in prokaryotes can categorize into the following types, depending upon its arrangement on the cell surface.

Arrangement of flagella

Monotrichous is a polar flagellum that usually appears singly and sometimes in pairs.
Example: Vibrio sp, Campylobacter sp etc.

Amphitrichous is another type, where the flagella appear singly at both the ends of the bacterial cell.
Example: Alcaligenes faecalis

Lophotrichous is a type of flagella that appears in the form of tufts at one or both the ends of the bacterial cell:
Example: Spirilla sp.

Peritrichous flagella are found all around the periphery of the bacterial cell.
Example: Members of the Enterobacteriaceae family.


Motility: The fundamental role of flagella is to impart different kinds of motility to the bacterial cell.

Pathogenesis: According to the research, flagella acts as a virulence factor that helps in adhesion of the bacterial cell to the host cell.

Characterization of bacteria: Bacteria showing swimming motility or swarming motility is remarkably used to differentiate the type of motility. The flagella formation is related to the particular environment. Therefore, the flagellation pattern also decides the environmental conditions, where the bacterial cell can live in.

Bacterial Motility

The motility in bacteria is due to the rotation of the motor in the basal body, which causes supercoiling of the filament. The supercoiling results in the appearance of a corkscrew-like shape.

The flagella rotate clockwise when the filament forms a long pitch supercoil that move the bacterial cell in a straight line or unidirection. In contrast, a flagellum rotates anticlockwise, when the filament forms a short pitch supercoil that tumbles the bacterial cell randomly.

Flagellar movement

A long pitch supercoiling allows the formation of the flagellar bundle to assemble, while a short pitch supercoiling cause dissembling of the flagellar bundle. A bacterial cell can set in up or down motion across the stimulus gradient by decreasing and increasing the tumbling frequency.

The tumbling frequency decreases when the filament of the flagella moves in a clockwise direction, while the tumbling frequency increases when the filament of the flagella moves in an opposite or anticlockwise direction.

Polymorphic Transitions of Bacterial Flagella

There are three transitions model performed in the large filament segment.

  1. When motor torque applied corresponding to the running mode
  2. When motor torque applied corresponding to the tumbling mode
  3. Rotation without a motor torque

In the above two transitions, the protofilaments rearrange in a left or right-handed fashion. Oppositely, in the third transition, the arrangement of protofilaments remains stable. Left-handed supercoiling results in a flagellum when the torque is generated anticlockwise direction (reducing the pitch helices).

Right-handed supercoiling results when the torque is produced in a clockwise direction (increasing the pitch helices). Therefore, the rearrangement of protofilaments goes through a polymorphic transition from one helical state to another by the action of motor torque. These transitions trigger the bacterial cell to swim or tumble in vitro by changing pH, temperature, or ionic strength.


Bacterial flagella help in the propulsion of bacterial cell, sensory function and transporting material.

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