VMC 211: General Veterinary Microbiology (1+1)

STRUCTURE OF FLAGELLUM AND MOTILITY

• It is made of three parts.
  • Basal body associated with the cytoplasmic membrane and cell wall.
  • A short hook
  • Helical filament which is several times as long as the cell.

• Chemical composition of the basal body is not known. Gram negative bacteria have four basal rings. Gram positive bacteria have only two rings. Hook and filament are composed of protein subunits arranged in helical fashion. The protein of the filament is known as flagellin. Bacteria move by rotating the helical flagella. The process is analogous to rotation of a cork screw. The nature of the rotary motor that spins the flagellum is not known. The rings in the basal body are probably involved. The enery for thia motor is from proton motive force.
• Bacteria with polar flagella swim in a back and forth manner. They reverse the direction by reversing the direction of flagellar rotation. Bacteria with lateral flagella swim in a more complicated manner. The flagella form a bundle that extend behind the cell. When the motors reverse conformational changes occur along the flagella. The bundle flies apart and the cell tumbles wildly. Finally the flagellar motors resume their normal direction, the bundles again forms and the cell begins to swim in a different direction.
• This sequence of events occurs repeatedly so the motility becomes a series of swimming (runs) punctuated by periods of tumbling (twiddles) with a change in direction after each tumble.
• Spirochetes exhibit swimming motility though they lack external flagella. They have flagella like structures within the cell just beneath the outer cell envelope. They are called as periplasmic flagella. Other names are axial filament or endoflagella. Spiroplasmas also exhibit motility but they lack any organelle for motility even periplasmic flagella. Mechanism of this motility is not known.
• Gliding motility is seen in some bacteria.Eg: Cytophaga species. They are motile only when they are in contact with solid surface. They exhibit a sinuous, flexing motion. It is a slow process. The movement is only a few um/second. The mechanism of this motility is not known.