7.4: Bacterial Motility

Many bacteria are capable of motility (the ability to move under their own power). Most motile bacteria propel themselves by special organelles termed flagella.

A bacterial flagellum has 3 basic parts: a filament, a hook, and a basal body.

1) The filament is the rigid, helical structure that extends from the cell surface. It is composed of the protein flagellin arranged in helical chains so as to form a hollow core. During synthesis of the flagellar filament, flagellin molecules coming off of the ribosomes are transported through the hollow core of the filament where they attach to the growing tip of the filament causing it to lengthen. With the exception of a few bacteria, such as Bdellovibrio and Vibrio cholerae, the flagellar filament is not surrounded by a sheath. (See Fig. \(\PageIndex{14}\).)

2) The hook is a flexible coupling between the filament and the basal body. (See \(\PageIndex{14}\).)

3) The basal body consists of a rod and a series of rings that anchor the flagellum to the cell wall and the cytoplasmic membrane. (See Fig. \(\PageIndex{1}\).) Unlike eukaryotic flagella, the bacterial flagellum has no internal fibrils and does not flex. Instead, the basal body acts as a rotary molecular motor, enabling the flagellum to rotate and propel the bacterium through the surrounding fluid. In fact, the flagellar motor rotates very rapidly.

The MotA and MotB proteins form the stator of the flagellar motor and function to generate torque for rotation of the flagellum. The MS and C rings function as the rotor. (See \(\PageIndex{1}\).) Energy for rotation comes from the proton motive force provided by protons moving through the Mot proteins along a concentration gradient from the peptidoglycan and periplasm towards the cytoplasm.

Figure \(\PageIndex{1}\): Structure of a Bacterial Flagellum. The filament of the bacterial flagellum is connected to a hook which, in turn, is attached to a rod. The basal body of the flagellum consists of a rod and a series of rings that anchor the flagellum to the cell wall and the cytoplasmic membrane. In Gram-negative bacteria, the L ring anchors the flagellum to the lipopolysaccharide layer of the outer membrane while the P ring anchors the flagellum to the peptidoglycan portion of the cell wall. The MS ring is located in the cytoplasmic membrane and the C ring (the rotor) in the cytoplasm. The stator, composed of MotA and MotB proteins surround the MS and C rings of the motor and function to generate torque for rotation of the flagellum. Energy for rotation comes from proton motive force. Energy for rotation comes from the proton motive force provided by protons moving through the Mot proteins. (Copyright; Gary E. Kaiser, Ph.D. The Community College of Baltimore County, Catonsville Campus CC-BY-3.0)

Bacterial motility constitutes unicellular behavior. In other words, motile bacteria are capable of a behavior called taxis. Taxis is a motile response to an environmental stimulus and functions to keep bacteria in an optimum environment.

The arrangement of the flagella about the bacterium is of use in classification and identification. The following flagellar arrangements may be found. (See Fig. \(\PageIndex{2}\).)

Figure \(\PageIndex{2}\): Bacterial Flagellar Arrangements. (Copyright; Gary E. Kaiser, Ph.D. The Community College of Baltimore County, Catonsville Campus CC-BY-3.0)