II. THE
PROKARYOTIC CELL: BACTERIA
B.
PROKARYOTIC CELL ANATOMY
4.
STRUCTURES LOCATED OUTSIDE THE CELL WALL
b.
Flagella
Fundamental Statements for this Learning Object:
1. Many bacteria are motile and use flagella to swim through liquid environments.
2. The basal body of a bacterial flagellum functions as a rotary molecular motor, enabling the flagellum to rotate and propel the bacterium through the surrounding fluid.
3. Bacterial flagella appear in several arrangements, each unique to a particular organism.
4. Motility serves to keep bacteria in an optimum environment via taxis.
5. Taxis refers to a motile response to an environmental stimulus enabling the net movement of bacteria towards some beneficial attractant or away from some harmful repellent.
6. Most bacterial flagella can rotate both clockwise and counterclockwise enabling to stop and change direction.
7. The protein flagellin that forms the filament of bacterial flagella functions as a pathogen-associated molecular pattern or PAMP that binds to pattern-recognition receptors or PRRs on a variety of defense cells of the body to trigger innate immune defenses.
8. 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 and colonize the intestines.
9. Innate immunity is an antigen-nonspecific defense mechanisms that a host uses immediately or within several hours after exposure to almost any microbe. This is the immunity one is born with and is the initial response by the body to eliminate microbes and prevent infection.
10. An antigen is a molecular shape that reacts with antigen receptors on lymphocytes to initiate an adaptive immune response.
11. The actual portions or fragments of an antigen that react with antibodies and with receptors on B-lymphocytes and T-lymphocytes are called epitopes.
12. Adaptive (acquired) immunity refers to antigen-specific defense mechanisms that take several days to become protective and are designed to react with and remove a specific antigen. This is the immunity one develops throughout life.
13. Humoral immunity involves the production of antibody molecules in response to an antigen.
14. Cell-mediated immunity involves the production of cytotoxic T-lymphocytes, activated macrophages, activated NK cells, and cytokines in response to an antigens. These defense cells help to remove infected cells and cancer cells displaying foreign epitopes.
15. Antibodies made against bacterial flagella can immobilize bacteria and/or promote phagocytosis.
16. Motility enables some spirochetes to penetrate deeper in tissue and enter the lymphatics and bloodstream and disseminate to other body sites.
LEARNING
OBJECTIVES FOR THIS SECTION
In this section on Prokaryotic Cell
Anatomy we are looking at the various cellular parts that make up
a bacterium. As mentioned in the introduction to this section, a typical bacterium
usually consists of:
- a cytoplasmic membrane surrounded
by a peptidoglycan cell wall and maybe an outer membrane;
- a fluid cytoplasm containing
a nuclear region (nucleoid) and numerous ribosomes; and
- often various external structures
such as a glycocalyx, 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 (def) of the flagellar motor and function to generate torque for rotation of the flagellum. The MS and C rings function as the rotor (def). (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) are 10-20 µm long and between
0.01 and 0.02 µm in diameter.
B. Flagellar Arrangements
(see Fig. 4)
1. Monotrichous:
A single flagellum, usually at one pole. (See Fig. 4A and Fig. 4B)
2. Amphitrichous:
A single flagellum at both ends of the organism. (See Fig. 4C)
3.
Lophotrichous: Two or more flagella at one or both poles. (See Fig. 4D)
4.
Peritrichous: Flagella over the entire surface. ( See Fig. 4E and Fig. 4F)
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 theory as to
the mechanism behind the corkscrewing motility of spirochetes is shown in Fig. 7.
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.
1. 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 every time 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.
2. 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.
Movie of Spirillum volutans, a spiral-shaped bacterium with a bundle of flagella at either end.
Movies made available for download by Dr. Howard C. Berg, PI, Bacterial Motility and Behavior. The Roland Institute at Harvard University. |
Some bacteria with polar flagella can only rotate their flagellum clockwise. In this case, clockwise rotation pushes the bacterium forward. Every time the bacterium stops, Brownian motion and fluid currents cause the bacterium to reorient in a new direction.
Movie of Rhodobacter spheroides with fluorescent-labelled flagella.
The flagellum can only rotate clockwise.
Movies made available for download by Dr. Howard C. Berg, PI, Bacterial Motility and Behavior. The Roland Institute at Harvard University. |
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 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. Every time 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.
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. Every time 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. In most cases, 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. An increasing concentration
of attractant (def) 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
Initiation of Innate Immunity
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.
Initiation of Adaptive Immunity
Proteins associated with bacterial flagella function as antigens and initiate adaptive immunity. An antigen (def) is defined as a molecular shape that reacts with antibody molecules and with antigen receptors on lymphocytes. We recognize those molecular shapes as foreign or different from our body's molecular shapes because they fit specific antigen receptors on our B-lymphocytes and T-lymphocytes, the cells that carry out adaptive immunity.
The actual portions or fragments of an antigen that react with antibodies and with receptors on B-lymphocytes and T-lymphocytes are called epitopes (def). An epitope is typically a group of 5-15 amino acids with a unique shape that makes up a portion of a protein antigen, or 3-4 sugar residues branching off of a polysaccharide antigen. A single microorganism has many hundreds of different shaped epitopes that our lymphocytes can recognize as foreign and mount an adaptive immune response against.
The body recognizes an antigen as foreign when epitopes of that antigen bind to B-lymphocytes (def) and T-lymphocytes (def) by means of epitope-specific receptor molecules having a shape complementary to that of the epitope. The epitope receptor on the surface of a B-lymphocyte is called a B-cell receptor and is actually an antibody molecule. The receptor on a T-lymphocyte is called a T-cell receptor (TCR).
There are two major branches of the adaptive immune responses: humoral immunity and cell-mediated immunity.
1. Humoral immunity (def): Humoral immunity involves the production of antibody molecules in response to an antigen (def) and is mediated by B-lymphocytes. Through a variety of mechanisms, these antibodies are able to remove or neutralize microorganisms and their toxins after binding to their epitopes. For example, antibodies made against flagellar antigens can stick bacteria to phagocytes, a process called opsonization (def). They can also interfere with bacterial motility. Antibodies made against the flagella of motile bacteria can bind to these locomotor organelles and arrest the organism's movement blocking its ability to spread.
2. Cell-mediated immunity (def): Cell-mediated immunity involves the production of cytotoxic T-lymphocytes, activated macrophages, activated NK cells, and cytokines in response to an antigen (def) and is mediated by T-lymphocytes. These defense cells help to remove infected cells and cancer cells displaying foreign epitopes.
Adaptive immunity will be discussed in greater detail in Unit 6.
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.
Electron micrograph of Treponema pallidum invading a host cell.
This will be discussed in more detail under Bacterial Pathogenesis in Unit 3.
Medscape
article on infections associated with organisms mentioned in this
Learning Object. Registration to access this website is free.
|
Gary E. Kaiser, Ph.D.
Professor of Microbiology
The Community College of Baltimore County, Catonsville Campus
This work is licensed under a Creative Commons Attribution 4.0 International License.
Based on a work The Grapes of Staph at https://cwoer.ccbcmd.edu/science/microbiology/index_gos.html.
Last updated: Feb., 2021
Please send comments and inquiries to Dr.
Gary Kaiser