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.
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.