BACTERIAL FLAGELLA

 BACTERIAL SURFACE APPENDAGES

·      Bacterial Surface Appendages are Proteinaceous tubular or fibrous structure found on the surface of the bacterial cells.

·       The main functions of Bacterial surface appendages are Locomotion, Adhesion, Attachment and Assisting in Genetic exchange.

·       The three Surface appendages of bacteria are Flagella, Fimbriae and Pili.  

FLAGELLA

·  Bacterial flagella are thread-like or whip-like appendages intricately embedded in the cell envelope.

·    Flagella are responsible for conferring motility to the bacteria. They do not have any role in surface attachment.

·       Flagella are 20 - 30 μm in diameter and 15 μm in length.

·       Flagella are many times longer than Pili and Fimbriae.

·       Flagella are made up of protein called Flagellin.

·       Flagella are helical and non-straight in nature.

·       Flagella are interior in the originated from the Cytoplasmic membrane.

·       The formation of Flagella is controlled by the gene present in the Nucleoid region.

·    Spirochetes are motile bacteria but without any external flagella. They are motile due to the presence of an Axial filament. Axial filament consists of a bundle of flagellum-like structures that lie between the cell surface and an outer sheath, and connects one end of the cell to the other. They are some-times called the Endoflagellates.

Structure of Flagella

·       The structure of Flagella contains three main components. They are (a) Filament, (b) Hook and (c) Motor.

Structure of Bacterial flagella (Source: onlinebiologynotes.com)

a) Filament

·       Filament is a long thin appendage that extends from the cell surface.

·       The filament is composed of the protein Flagellin and is hollow.

·       Flagellin proteins are transcribed in the cell cytoplasm and then transported across the cell membrane and cell wall.

·       A bacterial flagellar filament grows from its tip (unlike the hair on your head), adding more and more Flagellin units to extend the length until the correct size is reached.

·       The Flagellin units are guided into place by a Protein cap.

b) Hook

·       Hook is a curved coupler that attaches the filament to the flagellar motor.

·       Length of Hook is longer in Gram positive bacteria than Gram negative bacteria.

c) Motor

·       A rotary motor that spans both the cell membrane and the cell wall, with additional components for the Gram negative outer membrane.

·       The motor has two components: (i) Basal body, which provides the rotation and (ii) Stator, which provides the torque necessary for rotation to occur.

·       The Basal body consists of a central shaft surrounded by Protein rings, two in the Gram positive bacteria and four in the Gram negative bacteria.

·       The Stator consists of “Mot” proteins that surround the rings embedded within the Cell membrane.

Arrangement of Flagella

i.       Atrichous – Flagella absent. Example – All non-motile bacteria.  

ii.    Monotrichous – Single flagellum at one pole. Example – Pseudomonas aeruginosa and Vibrio cholerae.

iii.   Lopotrichous – Tuft of flagella at one pole. Example – Pseudomonas fluorescence and Bartonella bacilliformis.  

iv.   Amphitrichous – Flagella at both poles. Example – Aquaspirillum serpens

v.     Amphilopotrichous – Tuft of flagella at both ends.

vi.   Peritrichous – Flagella all over the bacterial cell. Example – Proteus mirabilis, Escherichia coli and Salmonella typhi.

Types of Bacterial motility

i.       Tumbling motility – Listeria sp.

ii.     Gliding motility – Mycoplasma sp.

iii.   Stately motility – Clostridium sp.

iv.   Dartling motility – Vibrio cholerae and Campylobacter jejuni

v.     Swarming motility – Proteus sp., Bacillus subtilis and Clostridium tetani

vi.   Corkscrew motility - Spirochetes

Functions of Flagella

a)     Flagella are primarily responsible for motility of bacteria.

b)     Flagella may play a role in bacterial survival and pathogenesis.

c)     Flagella are highly antigenic, they possess H - antigens, and some of the immune responses to infection are directed against these proteins. The flagella of different bacteria differ antigenically. Flagellar antibodies are not protective but help in Serodiagnosis.

d)     Signal transduction.

e)     Sensation