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 and body defenses; and
4) to introduce the relationship between bacteriality motility and chemotaxis and 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 propell the bacterium through the surrounding fluid. In fact, the flagellar motor rotates very rapidly. (The motor of E. coli rotates 270 revolutions per second!)

The Mot 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 (def). Protons moving through the Mot proteins drives rotation.

The Fli proteins act as the motor switch to trigger either clockwise or counterclockwise rotation of the flagellum and to possibly disengage the rod in order to stop motility.

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.

The bacterial flagellum can rotate both counterclockwise and clockwise. A protein switch in the molecular motor of the basal body controls rotation. Clockwise rotation results in a tumbling motion and changes the direction of bacterial movement. On the other hand, counterclockwise rotation leads to long, straight or curved runs without a change in direction. During a run, that lasts about one second, the bacterium moves 10 - 20 times its length before it stops. In the case of a tumble, the movement lasts only about one-tenth of a second and no real forward progress is made.

To view videos showing motile bacteria, see The Microbiology Video Library.

 

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. In an environment that lacks such a gradient, the bacterium moves randomly. It travels in a straight line, or runs, for a few seconds, then stops, tumbles, and runs in a different direction. However, when the bacterium is exposed to a chemical gradient of, for example, an attractant, it tumbles less frequently (has longer runs) as it moves up the gradient, but tumbles at the normal rate if it travels down the gradient. In this way, the net movement is towards a more optimum environment.

Chemotaxis is regulated by chemoreceptors located in the cytoplasmic membrane or periplasm of the bacterium bind chemical attractants or repellents. This leads to either the methylation or demethylation of methyl-accepting chemotaxis proteins (MCPs) that in turn, eventually trigger either a counterclockwise or clockwise rotation of the flagellum. For example, if the concentration of an attractant (def) stays the same or decreases, the MCPs become demethylated and this eventually leads to a clockwise rotation of the flagellum. In the case of E. coli, when an attractant molecule is not bound to an MCP, ATP can donate a phosphate to a protein called CheA that, in turn donates the phoshate to another protein called CheY. The phosphorylated CheY then interacts with a switch protein called FliM at the base of the flagellum promoting clockwise rotation and tumbling. When attractant are bound to the MCPs, CheA molecules become dephosphorylated and the CheY molecules are unable to interact with the switch proteins and throw the switch. This leads to counterclockwise flagellar rotation and swimming in a run.

Therefore, an increasing concentration of attractant or decreasing concentration of repellent (def) (both conditions beneficial) causes less tumbling and longer runs; a decreasing concentration of attractant or increasing concentration of repellent (both conditions harmful) causes normal tumbling and a greater chance of reorienting in a "better" direction. As a result, the organism's net movement is toward the optimum environment (see Fig. 7).

For additional information on movement and chemotaxis see John Brown's Bugs in the News web page at the University of Kansas.

 

E. Significance to Initiating Body Defense

In order to protect against infection, one of the things the body must initially do is detect the presence of microorganisms. The body does this by recognizing molecules unique to microorganisms that are not associated with human cells. These unique molecules are called pathogen-associated molecular patterns or PAMPs. (Because all microbes, not just pathogenic microbes, possess PAMPs, pathogen-associated molecular patterns are sometimes referred to as microbe-associated molecular patterns or MAMPs.)

The protein flagellin in bacterial flagella is a PAMP that binds to pattern-recognition receptors or PRRs on a variety of defense cells of the body and triggers innate immune defenses such as inflammation, fever, and phagocytosis.

 

F. Significance of Motility to Bacteria Causing Infections

Motility and chemotaxis probably help some intestinal pathogens to move through the mucous layer so they can attach to the epithelial cells of the mucous membranes. They also enable spirochetes to move through viscous environments and penetrate cell membranes. This will be discussed in more detail under Bacterial Pathogenesis in Unit 2.

Examples of bacteria that use motility to help colonize the body include Treponema pallidum (inf), Leptospira (inf), and Borrelia burgdorferi ) (inf). Flagella may also enable Helicobacter pylori (inf) to penetrate the mucous covering in the stomach and thus colonize the gastric mucosa.

 


Highlighted Bacterium: Treponema pallidum

Click on this link, read the description of Treponema pallidum, and be able to match the bacterium with its description on an exam.

 

E-Medicine article on infections associated with organisms mentioned in this Learning Object. Registration to access this website is free.

 

For further information on bacterial flagella and motility, see the online Microbiology Web Textbook at the University of Wisconsin-Madison.
For more information on bacterial flagellar structure and motility, see
Motile Behavior of Bacteria in Physics Today on the Web

 

 

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