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Bacterial Motility

Many but not all bacteria exhibit motility, i.e. self-propelled motion, under appropriate circumstances. Motion can be achieved by one of three mechanisms:

The following digital video shows motile Rhodospirillum rubrum cells, ~1 µm in diameter and variable in length. Watch the corkscrew motion of the cells through the medium:

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. The following digital video shows motile Chromatium cells: short, Gram-negative rods, ~1 µm in diameter and 3-4 µm long. Watch for the tumbles as the cells change direction:

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 phosporylation 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. Anticlockwise rotation of monotrichious polar flagella thrusts the cell forward with the flagellum trailing behind. Peritrichous cells operate in the same way.

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. This ability is important, since it allows bacteria to change direction. Bacteria can sense nutrient molecules such as sugars or amino acids 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 behaviour 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.

For more examples of motile bacteria, see the page on the genus Bacillus.

Gliding motility is the movement of cells over surfaces without the aid of flagella, a trait common to many bacteria, yet the mechanism of gliding motility is unknown. The gliding motility apparatus which propels the cells involves a complex of proteins, yet the actual nature of the "motor" and how the components interact is not understood. You can watch Oscillatoria cells gliding in real time in this video:

Not everything that moves is motile!

Under the microscope, motile bacteria seem to move in a purposeful way, though they may frequently change direction - look at the videos above to see this. However, even dead cells such as those in the video below (killed with disinfectant) 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, who was investigating a suspension of microscopic pollen particles in solution and observed movement even in pollen samples that had been dead for more than 100 years. 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! The following video shows a whole range of highly motile bacteria cultured from an aquatic environment, including rods, Spiroplasma and others:

 


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