II. THE PROKARYOTIC CELL: BACTERIA

B. PROKARYOTIC CELL STRUCTURE

4. STRUCTURES LOCATED OUTSIDE THE CELL WALL

b. Flagella

The overall purpose of this Learning Object is:
1) to learn the chemical makeup and functions associated with bacterial flagella;
2) to learn how bacterial use motility and taxis to respond to their environment;
3) to introduce the relationship between components of bacterial flagella the initiation of body defenses; and
4) to introduce the relationship between bacteriality motility and chemotaxis and bacterial pathogenicity.

LEARNING OBJECTIVES FOR THIS SECTION

In this section on Prokaryotic Cell Structure we are looking at the various organelles or structures that make up a bacterium. As mentioned in the introduction to this section, a typical bacterium usually consists of:

Structures located outside the cell wall of bacteria include the glycocalyx (capsule), flagella, and pili. We will now look at bacterial flagella.

 

Flagella (def)

 

A. Structure and Composition

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. 1).

2) The hook is a flexible coupling between the filament and the basal body (see Fig. 1).

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. 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. (Some flagella can rotate up to 300 revolutions per second!)

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 Fig. 1). Energy for rotation comes from the proton motive force (def) provided by protons moving through the Mot proteins along a concentration gradient from the peptidoglycan and periplasm towards the cytoplasm.

 

Bacteria flagella (see Fig. 2 and Fig. 3) are 10-20 µm long and between 0.01 and 0.02 µm in diameter and come in a number of distinct arrangements:

 

B. Flagellar Arrangements (see Fig. 4)

1. monotrichous: a single flagellum, usually at one pole

2. amphitrichous: a single flagellum at both ends of the organism

3. lophotrichous: two or more flagella at one or both poles

4. peritrichous: flagella over the entire surface

5. axial filaments: internal flagella found only in the spirochetes. Axial filaments are composed of from two to over a hundred axial fibrils (or endoflagella) that extend from both ends of the bacterium between the outer membrane and the cell wall, often overlapping in the center of the cell. (see Fig. 5 and Fig. 6). A popular theory as to the mechanism behind spirochete motility presumes that as the endoflagella rotate in the periplasmic space between the outer membrane and the cell wall, this could cause the corkscrew-shaped outer membrane of the spirochete to rotate and propell the bacterium through the surrounding fluid.

 

C. Functions

Flagella are the organelles of locomotion for most of the bacteria that are capable of motility. Two proteins in the flagellar motor, called MotA and MotB, form a proton channel through the cytoplasmic membrane and rotation of the flagellum is driven by a proton gradient. This driving proton motive force (def) occurs as protons accumulating in the space between the cytoplasmic membrane and the cell wall as a result of the electron transport system travel through the channel back into the bacterium's cytoplasm. Most bacterial flagella can rotate both counterclockwise and clockwise and this rotation contributes to the bacterium's ability to change direction as it swims. A protein switch in the molecular motor of the basal body controls the direction of rotation.

If a bacterium has a peritrichous arrangement of flagella, counterclockwise rotation of the flagella causes them to form a single bundle that propels the bacterium in long, straight or curved runs without a change in direction.Counterclockwise rotation causes the flagellum to exhibit a left-handed helix. During a run, that lasts about one second, the bacterium moves 10 - 20 times its length before it stops. This occurs when some of the the flagella rotate clockwise, disengage from the bundle, and trigger a tumbling motion. Clockwise rotation causes the flagellum to assume a right-handed helix. A tumble only lasts about one-tenth of a second and no real forward progress is made. After a “tumble”, the direction of the next bacterial run is random because everytime the bacterium stops swimming, brownian motion and fluid currents cause the bacterium to reorient in a new direction.

When bacteria with a peritrichous arrangement grow on a nutrient-rich solid surface, they can exhibit a swarming motility wherein the bacteria elongate, synthesize additional flagella, secrete wetting agents, and move across the surface in coordinated manner.

 

D. Taxis

Around half of all known bacteria are motile. Motility serves to keep bacteria in an optimum environment via taxis (def). Taxis is a motile response to an environmental stimulus. Bacteria can respond to chemicals (chemotaxis), light (phototaxis), osmotic pressure (osmotaxis), oxygen (aerotaxis), and temperature (thermotaxis).

Chemotaxis is a response to a chemical gradient of attractant or repellent molecules in the bacterium's environment.