Bacterial flagella: Structure and Function

Bacterial flagella are thread-like appendages, protruding from the cell wall, they confer motility to the bacteria (organs of locomotion). They measure 5-20 μm in length and 0.01- 0.02 μm in thickness.

Arrangement of bacterial flagella

There are various patterns of arrangement of bacterial flagella with respect to the bacterial surface (Fig; 2.17):

• Monotrichous (single polar flagellum), e.g. Vibirio cholerae, Pseuodomonas and Campylobacter 

• Lophotrichous (multiple polar flagella), e.g. Spirillum. 

• Peritrichous (flagella distributed over the entire cell surface)-e.g. Salmonellsa typhi, Escherichia coli. 

• Amphitrichous (single flagellum at both the ends)- e.g. Alcaligenes faecalis

 

Ultra-structure of bacterial flagella

Electron microscope reveals that the bacterial 

flagellum is composed of three parts (Fig. 2.18).

 

 

• Filament: It is the longest portion of the flagellum that extends from the cell surface to the tip. It is a hollow, rigid cylinder, made up of a single protein flagellin. 

• The basal body: This is the portion of flagellum which is embedded in the cell. It is the most  complex part of a flagellum, made up of 2-4 rings connected to a central rod.

• In most gram-negative bacteria, there are four rings named as- 

• The outer L and P rings associate with the LPS and peptidoglycan layers, respectively. 

• The inner S ring lies in periplasmic space and M ring contacts the plasma membrane. 

• Gram-positive bacteria have only two basal body rings, an inner ring connected to the plasma membrane and an outer one probably attached to the peptidoglycan. 

• Hook: It is a short, curved flexible segment that links the filament to its basal body.

 

Detection of bacterial flagella

Flagella can be demonstrated by:

Direct demonstration of flagella:

• Tannic acid staining (Leifson's method and Ryu's method) 

• Dark ground, phase contrast or electron microscope

Indirect means by demonstrating the 

motility:

• Cragie tube method 

• Hanging drop method 

• Semisolid medium, e.g. mannitol motility medium 

Bacterial Motility

Bacteria can produce characteristic type of motility  which helps in their identification (Table 2. 7).

• Bacterial Growth Curve 
• Bacterial Fimbriae
• Bacterial Capsule