Bacterial movement

Bacterial movement refers to the self-propelled movement of bacteria . This movement is also referred to motility. The jiggling movement seen in some nonmotile bacteria that are incapable of self-propelled movement is due to the bombardment of the bacteria by water molecules. This so-called Brownian motion is not considered to represent bacterial movement.

There are several types of bacteria movement. The most common occurs by the use of appendages called flagella. A bacterium can contain a single flagellum, several flagella located at one or both poles of the cell, or many flagella dispersed all over the bacterial surface. Flagella can rotate in a clockwise or counterclockwise direction. When the motion is counterclockwise, even multiple flagella can unite into a flagellar bundle that functions as a propeller. This occurs when the bacterium is moving towards a chemical attractant or away from a repellent in the behavior known as chemotaxis. If the flagella turn in the opposite direction, the coordinated motion of the flagella stops, and a bacterium will "tumble," or move in an undirected and random way.

Spirochaete bacteria have flagella that are internal. These so-called axial filaments provide the rigidity that enables the spiral bacterium to twist around the axis of the filament. As a result, the bacterium literally screws itself through the fluid. Reversal of the twist will send the bacterium in a reverse direction. Examples of bacteria that move in this manner include Treponema pallidum and Rhodospirillum rubrum.

The bacteria that are known as gliding bacteria exhibit another type of bacterial movement. One example of a gliding bacterium is the cyanobacterium Oscillatoria. Gliding movement is exactly that; a constant gliding of a bacterium over a surface. The basis of this movement is still not clear, although it is known to involve a complex of proteins.

In a human host, disease causing bacteria such as Salmonella typhymurium can move along the surface of the host cells. This movement is due to another bacterial appendage called a pilus. A bacterium can have numerous pili on its surface. These hair-like appendages act to bind to surface receptors and, when retracted, pull the bacteria along the surface. Movement stops when a suitable area of the host cell surface is reached.

See also Bacterial appendages