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In this article we will discuss about the definition and locomotion of Flagella.
Definition of Flagella:
The motile bacterium may possess a flagellum (plural flagella). The flagellum is hair like helical and surface appendages emerging from the cell wall. It is of 20-30 nm in diameter and 15 µm long. It provides various types of motility to the bacterial cell. The flagella of prokaryotes are several time thinner than that of eukaryotes.
In addition, the number and position of flagella vary. The arrangement may be monotrichous (a single polar flagellum e.g. V. cholerae), lophotrichous (a clusture of polar flagella e.g. Spirillum), amphitrichous (flagella at both the ends either singly or in clusture), cephalotrichous (two or more flagella at one end of bacterial cell e.g. Pseudomonas), peritrichous (cell surface evenly surrounded by several lateral flagella e.g. Proteus vulgaris) or atrichous (cells devoid of flagella e.g. Lactobacillus).
Locomotion of Flagella:
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There are three types of movement in bacteria.
(a) Flagellar movement:
Bacterial flagella are motile and help in locomotion of bacterial cells. Prokaryotic flagellum is semi rigid, helical rotor that moves the cell by rotating from the basal body either clockwise or counter clockwise around its axis. The helical waves are generated from the base to the tip of flagellum. The rotating flagellum forms a bundle that pushes against water and propels the bacterium.
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The basal body acts as motor and causes rotation. Berg (1975) suggested that a turning motion is generated between S-ring and M- ring, where the former acts as a starter and the later acts as rotor. The P-rings and L-ring are just bashings. The basal body gives a universal joint to the cell and allows complete rotation of the hook and shaft both clockwise and counter clock-wise.
Doetsch and Sjoblad (1980) have reviewed the mechanics of prokaryotic flagella. According to them the flagella function as a propeller of a boat. The polar flagellum rotates anticlockwise but the cell rotates clockwise when moving normally. Rotation of flagellum in anticlockwise direction results in movement of bacterial cell in opposite direction.
The peritrichous flagelia as a trailing bundle also rotates in anticlockwise direction (Fig. 4.5). Cells of spirilla possess a non- helical tuft of polar flagella. The flagella rotate either at one end or both and result in cell movement. There is coordination between the flagella of both ends for rotation and movement of cell.
Energy is required for the movement of flagella. In eukaryotes, energy is generated by ATP. The mechanism of energy generation for the movement of prokaryotic flagella is not known. However, the basal body requires energy to cause motion. Movement of ions between M-ring and S- ring possibly energises the flagellar motor.
(b) Spirochaetial movement:
The spirochaetes show several types of movements such as flexing, spining, free swimming and creeping as they are flexible and helical bacteria lack flagella. Just within the cell envelope they have flagella like structure which are known as periplasmic flagella or axial fibrils or endo-flagella.
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The axial fibrils are present in the space between inner and outer membrane of cell envelope. The mechanism of motility is not known. Berg (1975) postulated that the axial fibrils rotate in periplasmic space and cause the rotation of periplasmic cylinder on the body axis in the opposite direction.
(c) Gliding movement:
Some bacteria such as the species of cyanobacteria (e.g. Cytophaga) and mycoplasma show gliding movement when come in contact with a solid surface. However, no organelles are associated with the movement. Except mycoplasma, in others two Gram-negative type cell walls are present.
In the members of cytophagales and cyanobacteria, movement helps to find out the substratum e.g. wood, bark, shell, etc. for anchorage and reproduction. They secrete slime with the help of which they get attached to the substratum. In Oscillatoria princeps fibrils of 5- 8 nm thick are present near the cell surface and located helically around the cell. Oscillation is observed in this alga.
