Flagellum

Introduction

Flagella are the hair-like extensions in the body of bacteria that are meant for locomotion in them. A bacterium can have one or more flagella. Although the basic function of flagella is to provide motility, they can sometimes act as the sensory organs too. Other than bacteria, some algae, fungi, and moses also possess flagellum. Human sperm cells also have a flagellum which provides it motility to enter into the female reproductive tract. The flagella differ among organisms based on their protein structure. The bacterial flagella structure is a helical-shaped structure with flagellin as the primary protein component. The prokaryotes, like bacteria, derive energy for locomotion from the proton-motive force, or ion gradient, across the cell membrane.

Parts of Bacterial Flagella Structure

The bacterial flagellum has three basic parts, namely filament, hook, and the basal body.

• Filament- The filament of bacteria is a helical structure. It is rigid and extends from the surface of the cell. It is composed of a special protein, i.e. flagellin. The protein flagellin is present in a helical chain that forms a hollow core. During the synthesis of filament, the flagellin molecules come out of the ribosomes and get transported to the hollow core of the filament. In the hollow core, the flagellin attaches to the growing tip of the filament. This causes the filament to increase in length. Filaments are, however, not surrounded by a sheath (except in some like Bdellovibrio and Vibrio cholera).
• Hook- The hook acts as the connector between the filament and the basal body
• Basal Body- The basal body possesses a rod and a series of rings that harbor the flagellum towards the cell wall and also to the cytoplasmic membranes. The bacterial flagella lack internal fibrils to flex. In them, the basal body acts as a molecular rotator motor and enables the flagellum to rotate. It also makes bacteria propel through the fluids. The flagellar motor can rotate at high speed. The motors generate torque for the rotation. For the rotation, the energy comes from the proton motive force. The Basal body consists of rings surrounded by a protein called MotB. The list of these types of rings are as follows-
  o   L-ring: In this type of ring, the outer ring is anchored in the lipopolysaccharide layer and is present in gram +ve bacteria
  o   P-ring: In this type of ring, the outer ring is anchored in the peptidoglycan layer
  o   C-ring: In this type of ring, the outer ring is anchored in the cytoplasm
  o   M-S ring: In this type of ring, the outer ring is anchored in the cytoplasmic membrane

  Bacteria Flagella Types

  According to the arrangement of the flagellum, the bacteria flagella types can be classified into the following four categories:
  • Monotrichous- Monotrichous, as the name suggests, in this type, the bacteria have only a single flagellum at the endings for locomotion. This type of flagella moves in counterclockwise rotation and pushes the cell forward for the movement. It looks like a corkscrew moving inside a cork
  An Example of such type of flagella in bacteria is Vibrio cholerae, the bacteria responsible for causing cholera disease in humans.
  • Amphitrichous- The amphitrichous bacteria have two flagella each protruding from one end. However, only one of them operates at a single time. It allows the bacteria to reverse code rapidly in the direction by switching the flagellum under movement
  An example of such a type of bacteria is Alkaligens faecalis.
  • Lophotrichous- The lophotrichous bacteria have multiple flagella. But all of these flagella are present at the single end. These flagella drive the bacterium in a straight direction only
  An example of a lophotrichous bacterium is Spirillum.
  • Peritrichous- Peritrichous bacteria have numerous flagella throughout their body. These flagella are present on every end of the body. Each of these rotates in a clockwise manner and can randomly change its direction. However, they cannot move transitionally
  An example of a peritrichous bacteria is Salmonella typhi, the bacterium causing typhoid disease in humans.

  Movement of Flagella

  The counterclockwise movement causes the flagellum to possess a left-handed helical structure like in peritrichous bacteria. The bacteria can continuously run upto 10 to 20 times their length. Whenever the bacterium stops, it changes its direction due to Brownian motion and fluid currents. When the flagellum rotates counterclockwise, it pushes the bacterium in a forward direction. Similarly, when the flagella rotates clockwise, it pushes the bacterium in the backward direction.

  Functions of Flagella

  The basic function of flagella is the locomotion or movement from one place to another in bacteria. However, there are some of its other functions. The functions other than the locomotion in the bacterial flagella are as follows-
  • A sensory organ for detection of change in temperature and pH of the surroundings
  • Signal transduction
  • Adhesion onto the surfaces
  • In eukaryotes, it provides motility to the male gamete, i.e. sperm cell
  • It acts as an important virulence factor

  Conclusion

  Flagella are the basic locomotory organs present in the bacterial cell wall. The bacterial flagella structure has three parts, a filament, a hook, and a basal part. Other than locomotion, the flagella also serve as a sensory and adhesive organ. They differ according to the numbers and position of arrangement of the flagella over the bacterial cell wall. On such basis, the flagella are of four types, Monotrichous (having one flagellum), Amphitrichous (single flagellum at both ends), Lophotrichous (numerous flagella as a tuft), and Peritrichous (flagella distributed all over the bacterial cell except at the poles).