Structure and arrangement of flagella in bacterial cell and types of flagellar motility

  • Bacterial flagella are hairlike, helical appendages that protrude through the cell wall.
  • They are responsible for motility in bacteria and are much thinner than the flagella or cilia of eukaryotes.
  • They are much simpler in structure and are 0.01-0.02µm in diameter.
  • A flagellum is composed of three parts:
    • Basal body:
      • It is associated with the cytoplasmic membrane and cell wall of bacteria.
      • Basal body of Gram negative bacteria consists of 4 rings; L, P, S and M rings.
      • L- ring is associated with the outer membrane and P- ring is with the peptidoglycan layer.
      • Both S and M rings are associated with the cell membrane.
      • In case of Gram positive bacteria, the outer L-ring is absent.
      • Basal body acts as a motor for the rotation of flagella.
    • A short hook
    • Filament: a thin helical filament which is usually several times as long as the cell
                                                Structure of Bacterial flagellum


  • Some Gram negative bacteria have a sheath surrounding the flagellum which is continuous with the outer membrane of the Gram negative cell wall.
  • The filament of the flagellum is made up of single fibril, composed of a protein called flagellin.
  • Unlike a hair, a flagellum grows at its tip rather than at the base.

Arrangement of flagella in bacteria:

  • On the basis of arrangement in the bacterial cell, flagella are of following types:
    • Monotrichous: it is a single polar(at one or both ends of the cell) flagellum e.g. Pseudomonas aeruginosa
    • Lophotrichous: a cluster of flagella lie at one end or pole of the cell. e.g. Pseudomonas florescence
    • Amphitrichous: either single or cluster of flagella lies at both the ends of the cell. e.g. Aquaspirillum serpens
    • Peritrichous: flagella are present all over the cell surface. e.g. Escherichia coli

Mechanism of flagellar rotation:

  • Flagella are the organelles of motility in bacteria and are responsible for swimming motility.
  • Mot protein acts as a starter to rotate flagellum and helical filament of flagellum rotates in screw type motion either in clockwise or anti-clockwise direction.
  • This rotation of flagellum either pushes or pulls the bacterial cell.
  • Proton gradient provides the necessary energy for the rotation of flagella.
  • When proton flows through the Mot protein, it attracts the opposite charge on M and S rings of flagellar basal body, thus rotating the flagella in screw type motion.

Types of flagellar motility:

  • Reversible flagellar motility:
    • Bacteria with polar flagella move by two mechanisms.
    • Flagella of some bacteria rotate in both directions, such flagella are called reversible flagella.
    • In such bacteria, at first flagella rotate in one direction and make the bacteria move forward and to move in reverse direction, bacteria change the direction of rotation of flagella.
  • Unidirectional flagellar motility:
    • Flagella of some bacteria rotate only in one direction.
    • In such bacteria, to change the direction of movement, bacteria first should stop and orient to change the direction again.
    • Such motility is unidirectional motility of flagella.
  • Peritrichous flagellar motility:
    • In bacteria with peritrichous flagella, flagellar motility is different.
    • At first, peritrichous flagella assemble at one pole forming a bundle which then rotates in anti-clockwise direction and pushes the bacteria forward.
    • To change the direction of movement, flagella rotate in clockwise direction, by which flagella unwind or release from the bundle and bacteria tumble.
    • Flagellar bundle is formed again on the other pole and bundle rotates in anti-clockwise direction and bacteria move in a new direction.

Types of motility in bacterial cells:

  • There are two types of motility in bacteria;
    • Gliding motility:
      • This type of motility occurs in bacteria without flagella.
      • Bacteria glide on solid surface only and is comparatively slower than swimming motility. e.g. Flavobacterium, Myxobacteria.
    • Swimming motility:
      • This type of motility occurs in water with the help of flagella.
      • It can be reversible, unidirectional or like in peritrichous flagella.
      • It is faster than gliding motility. e.g. Proteus spp, Escherichia coli, Vibrio cholerae etc.

Structure and arrangement of flagella in bacterial cell and types of flagellar motility