II. THE PROKARYOTIC CELL: BACTERIA
B. PROKARYOTIC CELL STRUCTURE
4. STRUCTURES LOCATED OUTSIDE THE CELL WALL
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.
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.
- Electron micrograph and illustration of the basal body of bacterial flagella; Cover photo of Molecular Biology of the Cell, May 1, 2000.
- Animation of a rotating bacterial flagellum from the ARN Molecular Museum
- YouTube movie of bacterial motility and animation of the basal body of bacterial flagella; from the Garland Science book Essential Cell Biology; 3rd. ed.
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
- Scanning electron micrograph showing monotrichous flagellum of Vibrio; courtesy of CDC.
2. amphitrichous: a single flagellum at both ends of the organism
3. lophotrichous: two or more flagella at one or both poles
- Scanning electron micrograph of Helicobacter pylori showing lophotrichous arrangement of flagella ; from Science Photolab.com
4. peritrichous: flagella over the entire surface
- Transmission electron micrograph of Proteus mirabilis showing peritrichous arrangement of flagella; from Masterfile, firstname.lastname@example.org
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.
- Axial filaments of the spirochete Leptospira; Midlands Technical College, Bio 255 course site
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.
- A bacterium with peritrichous flagella:
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.
- Illustration of runs and tumbles of a bacterium with peritrichous flagella; from Clear Science
- A bacterium with polar flagella:
- Most bacteria with polar flagella, like the peritrichous above, can rotate their flagella both clockwise and counterclockwise. If the flagellum is rotating counterclockwise, it pushes the bacterium forward. When it rotates clockwise, it pulls the bacterium backward. These bacteria change direction by changing the rotation of their flagella. See Fig. 8B.
- Some bacteria with polar flagella can only rotate their flagellum clockwise. In this case, clockwise rotation pushes the bacterium forward. Everytime the bacterium stops, brownian motion and fluid currents cause the bacterium to reorient in a new direction. See Fig. 8C.
- Movie of motile Escherichia coli with fluorescent labelled-flagella #1 Courtesy of Dr. Howard C. Berg from the Roland Institute at Harvard.
- Movie of motile Escherichia coli with fluorescent labelled-flagella #2 Courtesy of Dr. Howard C. Berg from the Roland Institute at Harvard.
- Movie of motile Escherichia coli with fluorescent labelled-flagella #3 Courtesy of Dr. Howard C. Berg from the Roland Institute at Harvard.
- Movie of motile Escherichia coli with fluorescent labelled-flagella #4 Courtesy of Dr. Howard C. Berg from the Roland Institute at Harvard.
- Movie of swimming Escherichia coli as seen with phase contrast microscopy Courtesy of Dr. Howard C. Berg from the Roland Institute at Harvard.
- Movie of tethered Escherichia coli showing that the bacterial flagella rotate Courtesy of Dr. Howard C. Berg from the Roland Institute at Harvard.
- Movie of swarming motility of Escherichia coli Courtesy of Dr. Howard C. Berg from the Roland Institute at Harvard.
- Movie of motile Pseudomonas from YouTube.
- Movie of motile Rhodobacter spheroides with fluorescent labelled-flagella Courtesy of Dr. Howard C. Berg from the Roland Institute at Harvard.
- Movie of motile Borrelia bergdorferi, the spirochete that causes Lyme disease.. From You Tube, courtesy of CytoVivo.
- Movie of motile Borrelia bergdorferi, the spirochete that causes Lyme disease.
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 a gradient of attractant or repellent, the bacterium moves randomly. In this way the bacterium keeps searching for a gradient.
- In an environment that has a gradient of attractant or repellent, the net movement of the bacterium is towards the attractant or away from the repellent.
If a bacterium has a peritrichous arrangement of flagella, such as Escherichia coli, Salmonella, Proteus, and Enterobacter, 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. Clockwise rotation of some of the flagella in the bundle causes those flagella to be pushed apart from the bundle triggering a tumbling motion. Everytime the bacterium tumbles it reorients itself in a new direction. In the presence of a chemical gradient, these movements become biased. When the bacterium is moving away from higher concentrations of repellents or towards higher concentrations of attractants the runs become longer and the tumbles less frequent.
- Illustration of runs and tumbles of a bacterium with peritrichous flagella; from Clear Science
Most bacteria with polar flagella, such as Pseudomonas aeruginosa, can rotate their flagella both clockwise and counterclockwise. If the flagellum is rotating counterclockwise, it pushes the bacterium forward. When it rotates clockwise, it pulls the bacterium backward. These bacteria change direction by changing the rotation of their flagella. Some bacteria with polar flagella, such as Rhodobacter sphaeroides, can only rotate their flagellum clockwise. In this case, clockwise rotation pushes the bacterium forward. Everytime the bacterium stops, it reorients itself in a new direction.
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, in Escherichia coli, which posesses peritrichous flagella, 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. 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.
Once a bacterium has been successful in responding to its environment, it needs to reset its chemosensing system. This is accomplished by CheB molecules. CheB is a methylase; it removes methyl groups from the MCPs. When fully methylated, MCPs can no longer respond to attractants. As CheB molecules demethylate the MCPs, they become "reset" and can again respond to attractants. Fully methylated MCPs, on the other hand, respond best to increasing gradients of repellents.
In summary, 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..
E. Significance of Flagella in the Initiation of 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 Bacterial Pathogenicity
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. In fact, many bacteria that can colonize the mucous membranes of the bladder and the intestines are motile. Motility probably helps these bacteria move through the mucus in places where it is less viscous.
Motility and chemotaxis also enable spirochetes to move through viscous environments and penetrate cell membranes. Examples include Treponema pallidum (inf), Leptospira (inf), and Borrelia burgdorferi ) (inf). Because of their thinness, their internal flagella (axial filaments), and their motility, spirochetes are more readily able to penetrate host mucous membranes, skin abrasions, etc., and enter the body. Motility and invasins may also enable the spirochetes to penetrate deeper in tissue and enter the lymphatics and bloodstream and disseminate to other body sites.
This will be discussed in more detail under Bacterial Pathogenesis in Unit 2.
E-Medicine article on infections associated with organisms mentioned in this Learning Object. Registration to access this website is free.
For more information on bacterial flagellar structure and motility, see
Motile Behavior of Bacteria in Physics Today on the Web