The Bacterial Flagella and Pili Movement, Attachment, and Communication

 

2.5 – FLAGELLA and PILI (Bacteria)

Flagella

• Many kinds of bacteria possess flagella → enable locomotion (movement).
• Secondary function: detect and respond to chemical signals (chemotaxis).
• Atrichous bacteria: Bacteria without flagella (a = without, trichous = hair/flagella).

Types of Flagellar Arrangement

1. Monotrichous – single polar flagellum (mono = one, trichous = hair).
2. Lophotrichous – a tuft (cluster) of flagella at one pole.
3. Amphitrichous – flagella at both poles.
4. Peritrichous – flagella surrounding the whole cell.

(Figure 2.7 shows these arrangements)

Structure of Bacterial Flagellum

• Different from eukaryotic flagellum:

  1. Not built on 9+2 microtubule pattern.

  2. Composed of flagellin protein.

• Main parts:

  1. Basal body – present just beneath cell membrane.

• Made of rotating rings.
• Gram-positive bacteria → one pair of rings.
• Gram-negative bacteria → two pairs of rings.
• Function: anchors flagellum in cell membrane and cell wall.

1. Hook – a curved structure connecting basal body with filament.

2. Filament – long, whip-like structure for movement

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Pili (Singular: Pilus)

• Non-helical, filamentous appendages.
• Smaller and thinner than flagella.

• Functions:

1. Attachment → help bacteria stick to various surfaces.

2. Conjugation → act as sex pili to transfer DNA between bacterial cells (mating process).

2.5 Bacterial Flagella (Whip-like structures for motility in bacteria)

• Many kinds of bacteria possess flagella, which primarily enable them to move (locomotion through rotation).
• Secondary function of flagella is to detect and respond to chemical signals (known as chemotaxis, allowing bacteria to move towards or away from chemicals).
• Bacteria which do not possess flagella are called atrichous (meaning "without hair" or non-motile due to absence of flagella).

Arrangements of Bacterial Flagella (Based on number and position; refer to Figure 2.7 for visual)

• Atrichous: No flagella at all (non-motile bacteria).
• Monotrichous: Bacteria with a single polar flagellum (one flagellum at one end of the cell).
• Lophotrichous: Bacteria with a tuft of flagella at one pole (bundle of flagella emerging from one end, aiding in faster movement).
• Amphitrichous: Bacteria with flagella at each of two poles (flagella at both ends of the cell, allowing bidirectional movement).
• Peritrichous: Bacteria with flagella surrounding the whole cell (flagella distributed all over the surface, enabling random tumbling motion).

Figure 2.7: Illustrates the different arrangements of bacterial flagella (Atrichous, Monotrichous, Lophotrichous, Amphitrichous, Peritrichous).

Structure of Bacterial Flagella

• Bacterial flagella are entirely different in structure from eukaryotic flagella (eukaryotic ones are complex with internal structure).
• They are not built on the 9+2 pattern of microtubules (eukaryotic flagella have 9 outer microtubule doublets + 2 central singles for bending motion).
• Composed primarily of flagellin protein (a globular protein that assembles into a helical filament).
• Consists of three main parts:
  • Basal body: Present just beneath the cell membrane; acts as a motor.
    • Made up of rotating rings (one pair in Gram-positive bacteria [thicker peptidoglycan layer] and two pairs in Gram-negative bacteria [thinner peptidoglycan with outer membrane]).
    • The rings anchor the flagellum in the cell membrane and cell wall (provides stability and rotation powered by proton motive force).
  • Hook: A curved structure that connects the basal body with the filament (acts as a universal joint for flexible transmission of rotation).
  • Filament: The long, helical, whip-like outer part (extends outward and propels the bacterium by rotating like a propeller).

Related Structures: Pili (Also known as fimbriae in some contexts)

• Some bacteria have pili (singular: pilus; hair-like appendages).
• These are non-helical, filamentous appendages (straight, not spiral like flagella).
• Smaller and thinner than flagella (typically 3-10 nm diameter vs. 20 nm for flagella).
• Functions:
  • Used for attachment of bacteria to various surfaces (adhesion to host cells or substrates, aiding in colonization).
  • Involved in the mating process (conjugation) between cells (transfer of genetic material via sex pilus in bacterial reproduction).