Many but not all bacteria exhibit motility, i.e. self-propelled motion, under appropriate circumstances. Motion can be achieved by one of three mechanisms.
Most motile bacteria move by the use of flagella, rigid structures 20 nm in diameter and 15-20 µm long which protrude from the cell surface, e.g. the Chromatium cells in the video. In some bacteria, there is only a single flagellum – such cells are called monotrichous. In these circumstances, the flagellum is usually located at one end of the cell (polar). Some bacteria have a single flagellum at both ends – amphitrichous. However, many bacteria have numerous flagella; if these are located as a tuft at one end of the cell, this is described as lophotrichous (e.g. Chromatium), if they are distributed all over the cell, as peritrichous.
Flagella consist of a hollow, rigid cylinder composed of a protein called flagellin, which forms a filament anchored to the cell by a curved structure called the hook, which is attached to the basal body. Flagellae are, in effect, rotary motors comprising a number of proteinaceous rings embedded in the cell wall. These molecular motors are powered by the phosphorylation cascade responsible for generating energy within the cell. In action, the filament rotates at speeds from 200 to more than 1,000 revolutions per second, driving the rotation of the flagellum. The organization of these structures is quite different from that of eukaryotic flagella. The direction of rotation determines the movement of the cell. Periodically the direction of rotation is briefly reversed, causing what is known as a “tumble”, and results in reorientation of the cell. When anticlockwise rotation is resumed, the cell moves off in a new direction. Watch for the tumbles in this video. This allows bacteria to change direction. Bacteria can sense nutrients and move towards them – a process is known as chemotaxis. Additionally, they can also move away from harmful substances such as waste products and in response to temperature, light, gravity, etc. This apparently intelligent behavior is achieved by changes in the frequency of tumbles. When moving towards a favourable stimulus or away from an unfavourable one, the frequency of tumbles is low, thus the cells moves towards or away from the stimulus as appropriate. However, when swimming towards an unfavourable or away from a favourable stimulus, the frequency of tumbles increases, allowing the cell to reorient itself and move to a more suitable growth.
The second type of motility is shown by Spirochaetes, helical bacteria which have a specialized internal structure known as the axial filament which is responsible for rotation of the cell in a spiral fashion and consequent locomotion. The video shows highly motile Rhodospirillum rubrum cells. Watch the corkscrew motion of the cells through the medium.
The third mechanism is gliding motility. Gliding motility is the movement of cells over surfaces without the aid of flagella, a trait common to many bacteria. Gliding bacteria all secrete copious slime, but the exact mechanism which propels the cells is not known. The gliding motility apparatus which propels the cells involves a complex of proteins, yet the full nature of this “motor” and how the components interact is not understood. You can watch an Oscillatoria cell gliding in real time in the video.
However, beware for not everything that moves is motile! Under the microscope, motile bacteria seem to move in a purposeful way, though they may frequently change direction. However, even dead cells, such as those in this video, move. Rapid movement is due to capillary action or convection currents on the microscope slide. However, the motion which causes most problems is Brownian motion, first observed in 1827 by the English botanist Robert Brown. This is due to random molecular bombardment of tiny bacterial cells by the molecules of the solvent. A microbiologist needs to learn to distinguish the effects of Brownian motion from true bacterial motility.