| Bacterial Motility - flagella and nanotechnology |
The diagram above shows a model of a close-up view of the basal components of a bacterial flagella.
These are the wheels at the roots of the flagella as mentioned in the introduction to bacteria, if you have
not read this introduction then click here to learn the basics of bacterial structure. The diagrams below add
some labels to this structure. Note that only a tiny portion of the filament is shown here, since this is the
long helix-like propeller that we saw previously. Remember the flagellum rotates like a propeller to propel
the bacterial cell along, as shown by the arrows in the diagram below.
These are the wheels at the roots of the flagella as mentioned in the introduction to bacteria, if you have
not read this introduction then click here to learn the basics of bacterial structure. The diagrams below add
some labels to this structure. Note that only a tiny portion of the filament is shown here, since this is the
long helix-like propeller that we saw previously. Remember the flagellum rotates like a propeller to propel
the bacterial cell along, as shown by the arrows in the diagram below.
Above: the flagellum emerges from the bacterial surface or wall (or cell envelope)` which consists of
three main layers: the outer membrane (OM), the periplasm (P) which contains a mesh of very strong
fibrous material called peptidoglycan (or murein) and an inner cytoplasmic membrane (CM). (Click here
to learn about cell membranes). Note how different this arrangement is from an animal cell which has a
single cell membrane rather than this double membrane structure. Beneath these layers is the cell
interior (the intracellular compartment) or protoplast (consisting of cytoplasm and nucleoid) and external
to these layers is the extracellular (external) environment, such as the water the bacterium is swimming
in. (Additional layers may exist outside the OM, including a slime capsule, but we shall look at these
possibilities later). This type of wall structure is particular to some types of bacteria called Gram negative
bacteria, but other wall structures occur, as we shall see later. The root of the flagellum consists of a
series of rings (made of proteins) that anchor the structure in the cell envelope.
three main layers: the outer membrane (OM), the periplasm (P) which contains a mesh of very strong
fibrous material called peptidoglycan (or murein) and an inner cytoplasmic membrane (CM). (Click here
to learn about cell membranes). Note how different this arrangement is from an animal cell which has a
single cell membrane rather than this double membrane structure. Beneath these layers is the cell
interior (the intracellular compartment) or protoplast (consisting of cytoplasm and nucleoid) and external
to these layers is the extracellular (external) environment, such as the water the bacterium is swimming
in. (Additional layers may exist outside the OM, including a slime capsule, but we shall look at these
possibilities later). This type of wall structure is particular to some types of bacteria called Gram negative
bacteria, but other wall structures occur, as we shall see later. The root of the flagellum consists of a
series of rings (made of proteins) that anchor the structure in the cell envelope.
Above: the flagellar basal complex with some outer structures (Mot proteins) removed to show the
internal structure of the motor. The labels are shown in the figure below:
internal structure of the motor. The labels are shown in the figure below:
Above: the flagellar motor rotates, causing the flagellum to rotate (and the cell to rotate in the opposite
sense). Notice the scale bar: the line illustrates a real-life length of 20 nanometres (20 millionths of a
millimetre!) so we are dealing with a minute machine - a nanomachine! These minute electric motors
evolved by natural means, on Earth, long before human beings existed.
sense). Notice the scale bar: the line illustrates a real-life length of 20 nanometres (20 millionths of a
millimetre!) so we are dealing with a minute machine - a nanomachine! These minute electric motors
evolved by natural means, on Earth, long before human beings existed.
The basal structure consists of a series of rings connected by a rod, the rings are the L ring (embedded in
the lipid bilayer of the outer membrane), the P ring (embedded in the periplasm), the S and the M rings
(the rotating motor or rotor) and the C ring. The L and P rings act as a bushing (a bushing is a ring-like
structure that constrains moving mechanical parts, in this case the rotating rod and may also be lubricated
to reduce friction). The motor proteins (Mot) conduct electric current carried by positively charged protons
('positive electricity' as opposed to negative electricity in which the current is carried by negatively charged
electrons as in a metal wire) from the periplasm into the cell cytoplasm. The electric charge is thought to
flow into the M (motor) ring where it is converted into rotary mechanical motion, causing the M-ring to
rotate. The M ring is attached to the rod, causing the rod to rotate. The M ring acts against the fixed S
(stator) ring, which rotates slowly in the opposite direction to the M ring - slowly because it is fixed to the
bacterial cell wall and so causes the whole bacterial cell to rotate in a direction opposite to the M ring and
rod.
The rod is attached to the filament via a flexible hook (which acts as a universal joint, transferring rotary
motion to the filament via the hook associated proteins (HAPs)). The filament is actually much too long to
show more than a tiny segment of it in these diagrams, it is about 20 nanometres in diameter, but 10 - 15
micrometres (10 - 15 thousand nanometres) long, which is longer than a typical bacterial cell which is
about 2 micrometres long. The filament is made up of about 30 000 subunits of a protein called flagellin
and is a corkscrew or helix shape. (The flagellin is arranged into typically 11 strands that are twined
together). This shape is important, mutants with straight filaments are immotile - the filament is the
propeller driven by this remarkable microscopic electric motor! (The cell body may contribute to thrust in
some forms in which the body is also helical). The helical filament is hollow, and flagellin is transported
from inside the cell, through the C ring (which has a hole in its centre) and along the filament, in its hollow
core, to its tip to which they are added - the filament constantly grows, as it must do to compensate for
breakages. A cap protein forms the tip and stabilises the filament.
the lipid bilayer of the outer membrane), the P ring (embedded in the periplasm), the S and the M rings
(the rotating motor or rotor) and the C ring. The L and P rings act as a bushing (a bushing is a ring-like
structure that constrains moving mechanical parts, in this case the rotating rod and may also be lubricated
to reduce friction). The motor proteins (Mot) conduct electric current carried by positively charged protons
('positive electricity' as opposed to negative electricity in which the current is carried by negatively charged
electrons as in a metal wire) from the periplasm into the cell cytoplasm. The electric charge is thought to
flow into the M (motor) ring where it is converted into rotary mechanical motion, causing the M-ring to
rotate. The M ring is attached to the rod, causing the rod to rotate. The M ring acts against the fixed S
(stator) ring, which rotates slowly in the opposite direction to the M ring - slowly because it is fixed to the
bacterial cell wall and so causes the whole bacterial cell to rotate in a direction opposite to the M ring and
rod.
The rod is attached to the filament via a flexible hook (which acts as a universal joint, transferring rotary
motion to the filament via the hook associated proteins (HAPs)). The filament is actually much too long to
show more than a tiny segment of it in these diagrams, it is about 20 nanometres in diameter, but 10 - 15
micrometres (10 - 15 thousand nanometres) long, which is longer than a typical bacterial cell which is
about 2 micrometres long. The filament is made up of about 30 000 subunits of a protein called flagellin
and is a corkscrew or helix shape. (The flagellin is arranged into typically 11 strands that are twined
together). This shape is important, mutants with straight filaments are immotile - the filament is the
propeller driven by this remarkable microscopic electric motor! (The cell body may contribute to thrust in
some forms in which the body is also helical). The helical filament is hollow, and flagellin is transported
from inside the cell, through the C ring (which has a hole in its centre) and along the filament, in its hollow
core, to its tip to which they are added - the filament constantly grows, as it must do to compensate for
breakages. A cap protein forms the tip and stabilises the filament.
Above a diagram of the bacterial flagellum showing some additional details. The C ring is made up of the
proteins FliM and FliN and the Mot proteins consist of two subunits: Mot A and MotB. The hook
associated proteins HAP3 and HAP1 are also known as FlgL and FlgK respectively. The rod is made up
of a variety of proteins and the protein FliF spans the region between the M and S rings. It is thought
that as protons (H+ or hydrogen nuclei) move through the Mot ring complex, MotA undergoes a shape
change, exerting a torque (rotary force) on FliG which connects to the M ring. We will look at models for
this process in more detail in a future update.
Performance of the bacterial flagella motor
Not all bacteria are motile and of those that are more than half use one or more helical flagella, like the
one we have described above. Compared to other bacterial propulsion systems (which we shall look at
later) the flagella propellers have the advantage of speed. For example, a cell of the bacterium
Escherichia coli is about 2 micrometres (2 thousandths of a millimetre) long and has six flagella that
originate from various points on the cell surface but the filaments come together to form a bundle which
propels the cell at about 20 micrometres per second, or ten body lengths per second! This is a clear
advantage that more than pays back the high cost of flagella - each flagellum contains about 1% of the
bacterium's total protein and requires some 50 genes for its production (about 2 % of the genome of
about 2 500 genes). The flagella enable swarmer cells to disperse from the biofilm (colony) and locate
new sites for colonisation. They enable the bacteria to swim to a source of nutrients and to avoid
harmful irritants. The drawback of flagella is that they require a fluid medium in which to work most
effectively, although bacteria can use them to move across moist surfaces. Flagella also fail to work well
in highly viscous media, such as the muddy ooze at the bottom of ponds, and bacteria may employ
different propulsion systems in these environments.
The helical filament rotates in a rigid manner as determined by attaching latex beads along the filaments
of mutants in which the filaments are straight. This ruled out the possibility that they undulate like whips
(as do the flagella of many non-bacterial cells) or that they move by winding and unwinding. More recent
experiments using laser dark-field microscopy have enabled individual flagella to be directly observed
rotating. They can flex, however, by virtue of the flexible hook. When the six flagella of Escherichia coli
rotate counterclockwise (CCW) forces exerted on them by the surrounding water (hydrodynamic forces)
force them to come together into a single bundle, trailing behind the tail end of the cell.
The flagella of Vibrio alginolyticus can rotate at about 1000 revolutions per second (rps) and propel the
cells at up to 116 micrometres per second (compare to the flagellum of Escherichia coli with 270 rps and
a top speed of 36 micrometres per second). However, removal of the filament (which loads the motor)
may increase the engine rotation rate to 200 000 rps!
Models of flagella rotation - how does the rotor work? Click here to find out!
Learn about motility in the strange spirochaetes which drill through materials (including people!).
Download an illustrated essay on bacterial motility and navigation in pdf format: Prokaryotes_motility.
How can something as complex as a bacterial flagellum evolve?
proteins FliM and FliN and the Mot proteins consist of two subunits: Mot A and MotB. The hook
associated proteins HAP3 and HAP1 are also known as FlgL and FlgK respectively. The rod is made up
of a variety of proteins and the protein FliF spans the region between the M and S rings. It is thought
that as protons (H+ or hydrogen nuclei) move through the Mot ring complex, MotA undergoes a shape
change, exerting a torque (rotary force) on FliG which connects to the M ring. We will look at models for
this process in more detail in a future update.
Performance of the bacterial flagella motor
Not all bacteria are motile and of those that are more than half use one or more helical flagella, like the
one we have described above. Compared to other bacterial propulsion systems (which we shall look at
later) the flagella propellers have the advantage of speed. For example, a cell of the bacterium
Escherichia coli is about 2 micrometres (2 thousandths of a millimetre) long and has six flagella that
originate from various points on the cell surface but the filaments come together to form a bundle which
propels the cell at about 20 micrometres per second, or ten body lengths per second! This is a clear
advantage that more than pays back the high cost of flagella - each flagellum contains about 1% of the
bacterium's total protein and requires some 50 genes for its production (about 2 % of the genome of
about 2 500 genes). The flagella enable swarmer cells to disperse from the biofilm (colony) and locate
new sites for colonisation. They enable the bacteria to swim to a source of nutrients and to avoid
harmful irritants. The drawback of flagella is that they require a fluid medium in which to work most
effectively, although bacteria can use them to move across moist surfaces. Flagella also fail to work well
in highly viscous media, such as the muddy ooze at the bottom of ponds, and bacteria may employ
different propulsion systems in these environments.
The helical filament rotates in a rigid manner as determined by attaching latex beads along the filaments
of mutants in which the filaments are straight. This ruled out the possibility that they undulate like whips
(as do the flagella of many non-bacterial cells) or that they move by winding and unwinding. More recent
experiments using laser dark-field microscopy have enabled individual flagella to be directly observed
rotating. They can flex, however, by virtue of the flexible hook. When the six flagella of Escherichia coli
rotate counterclockwise (CCW) forces exerted on them by the surrounding water (hydrodynamic forces)
force them to come together into a single bundle, trailing behind the tail end of the cell.
The flagella of Vibrio alginolyticus can rotate at about 1000 revolutions per second (rps) and propel the
cells at up to 116 micrometres per second (compare to the flagellum of Escherichia coli with 270 rps and
a top speed of 36 micrometres per second). However, removal of the filament (which loads the motor)
may increase the engine rotation rate to 200 000 rps!
Models of flagella rotation - how does the rotor work? Click here to find out!
Learn about motility in the strange spirochaetes which drill through materials (including people!).
Download an illustrated essay on bacterial motility and navigation in pdf format: Prokaryotes_motility.
How can something as complex as a bacterial flagellum evolve?
Above: illustrating the flow of protons (carrying positive electric charge) across the Mot ring from the
periplasm into the cell, powering the motor.
periplasm into the cell, powering the motor.
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