Bacterial Flagella: The Ultra Structure and Mechanism of Movement

 Flagella are the specialized structures of bacteria that are responsible for bacterial motility. (Singular, flagellum, which means whip). Some others can move by gliding. Aquatic bacteria move through their gas- filled vacuoles. 
    Flagellum, is a long helical and thread like loco motor appendage that exends outward from the plasma membrane and cell wall. 

Structure of Flagella 

 1). Bacterial flagella are long, slender, delicate and undulating structures. 
 2). They measure 5 to 20 mm in length and 20 nm in diameter. The flagella of bacterial genus Bdellovlbrio ar very thick because they are enclosed by sheath of extended plasma membrane, whereas that of Vibrio cholerae has a sheath of lipoplysaccharide.
 3). The flagella difer in their shapes. However, each flagellum has definite wavelength and amplitude. They can be coiled, curled normal or wavy. 
 4). The flagella are so thin that they cannot be seen directly under the bright-field microscope. It can be observed only after special staining techniques.

  These techniques make use of mordents like tannic acid that can be deposited on the surface to increase the thickness (Leifson's method and Gray's method are used). However, its ultra structure can be studied under the electron microscope only. 

Arrangement of Flagella
  
  Bacterial species show differences in number of flagella and their pattern of arrangement. They can be Monotrichous, Amphytrichous, Lophotrichous or Peritrichous. It helps in identification of bacteria. 

The distribution of flagella in bacteria  
Monotrichous : Single polar flagellation, for example Pseudomonas sp. (Mono means one and Trichous means hair). Amphitrichous : Single flagellum at each pole of a cell as seen in genus Spirillum. (Amphi means two). 
Lophotrichous : A cluster of two or more flagella at one end or both the ends of a bacterium. (Lopho means tuft) as seen in genus Spirillum.
Peritrichous : Flagella distributed on the entire surface of bacteria as seen in genera of many gram-negative bacteria like Escherichia, Proteus etc. (Peri means all around). 
Mixed flagellation : Members of genus Vibrio possess two types of flagella - polar sheathed flagella and  peritrichous unsheathed flagella. 


Ultra structure of bacterial Flagella 

 Electron microscopic studies of bacterial flagella reveal Its ultra structure. It consists of three parts.
    A. Basal body,
    B. Hook and,
    C. Filament.

A) Basal body
  Basal body is also called motor. It is anchored into the cytoplasmic membrane and the cell wall. It consists of a small central rod that passes through a system of rings. Basal body of gram positive bacteria is simple compared to that of gram negative bacteria. 

 1. In gram-negative bacteria (e.g. E. coli) the basal body consists of two pairs of rings, four rings. 
 a). The outer pair of rings includes the L ring and P rings that are associated with
lipopolysaccharide and peptidoglycan layers respectively.
 b). The inner pair of rings includes S and M rings that remain in contact with plasma membrane. layers respectively.

 2. In gram-positive bacteria, lipopolysaccharide layer is absent. It possesses only one, inner, pair of rings. Outer one attaches to the peptidoglycan  and the inner one attaches to the plasma membrane. It is believed that the S-ring is attached to the cell wall.

 The basal body acts as a motor and allows movement of flagella. 
 a).The inner ring is surrounded by a pair of proteins called Mot proteins. Mot proteins drive the flagellar motor such that it causes rotation of filament.
 b). Another protein called Fli protein also links with the flagellar motor and functions as a motor switch. It can reverse the rotation of flagella in response to the inner signals.
 c). M-ring and the rod rotate during flagellar movement. S-ring in gram-positive bacteria does not rotate.
 d). The P and L rings of gram-negative bacteria function as bearings for the rotating rod.

 B) Hook
   It is a slightly wider part of the flagellum that connects basal body and the filament. It consists of a single type of protein. 

C) Filament 
  It is a helical structure and extends from the cell surface to the tip. It is a hollow, rigid, cylinder, which is constructed of protein subunits called flagellin. Its molecular weight ranges between 30,000 and 60,000 Daltons and varies with the bacterial species. The filament ends with a capping protein.


Mechanism of flagellar movement

  Movement of flagella like a propeller enables the bacteria to move. It spins at a rate of 270 revolutions or higher per second. The basal body functions as a motor. It imparts rotary motion to the flagellum. The required energy comes from proton motive force across the membrane through the mot complex
 The nature of bacterial movement depends upon the direction of flagellar rotation. 

1). In monotrichously flagellated bacteria, the polar flagella move counter clockwise. This will thrust the cell forward and lagella trailing. The monotrichous bacteria stop and tumble randomly by reversing direction of flagellar rotation.
 
2). In peritrichously flagellated bacteria, counter clockwise rotation of flagella pushes the cell forward. During this, they bend their hooks to form a rotating bundle. The bundle disrupts, when the flagella rotates clockwise and the cell tumbles. In Proteus sp.cells move in wavelike motion, which is evidenced by swarming growth.