Most prokaryotes are motile by swimming, and this function is typically due to a structure called the flagellum. The plural form of this word is flagella which means whip. Bacterial flagellum is a threadlike locomotor appendages extending outgrowth from the plasma membrane and cell wall.
Bacterial flagella are thin, rigid structures, about 20 nm across and up to 15-20 μm long.Flagella are so thin that a single flagellum can be seen with the light microscope only after being stained with special stains that increase their diameter. However, flagella are easily seen with the electron microscope.
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Arrangement of flagella
- Atrichous: Bacteria have no flagellum. e.g. Micrococcus corchorii.
- Monotrichous: (mono means one; trichous means hair)Bacteria have one flagellum; if it is located at an end, It is said to be a polar flagellum. e.g. Pseudomonas aeruginosa.
- Amphitrichous: (amphi means on both sides) Bacteria have a single flagellum or clusters at each pole. e.g. Aquaspirillum serpens.
- Lophotrichous: (lopho means tuft) Bacteria have a cluster of flagella at one end. e.g. Rhodospirillum photometricum.
- Peritrichous: (peri means around) Flagella are spread fairly evenly over the whole surface of bacterial cell. e.g. Proteus vulgaris.
Flagella are not straight but are helical. When flattened, flagella show a constant distance between adjacent curves called the wave and this wave length is charecteristics for the flagella of any given species. A flagella consists of several components an moves by rotation, much like a propellar of a boat motor. The base of the flagellum is structurally different from the fillament. There is a wide region at the base of the fillament called the hook. The hook consist of a single type of proteins and connects the fillament to the motor portion of the base.
Transmission electon microscope(TEM) studies have shown that the bacterial flagellum is composed of three parts.
- Filament: The longest and most obvious portion is the flagellar filament, which extends from the cell surface to the tip.
- Basal body: The basal body anchors the flagellum to the cell wall and plasma membrane.
- Hook: A short, curved segment consists of different protein, the flagellar hook links the filament to its basal body and acts as a flexible coupling.The motor is anchored in the cytoplasmic membrane and cell wall. The motor consists of a central rod that passes through a series of rings.
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