Motility - endoflagella
These are found in the corkscrew shaped spirochetes. Although these bacteria are gram negative (with
a double membrane envelope structure), they possess only 2, to 4 rings (the S, M and presumably the
C rings and sometimes an extra pair) in the basal complex - they lack the P and L rings. The result is
that the basal complex does not cross the outer membrane and the filaments lie between the
peptidoglycan and outer membrane (in the periplasm) wrapped around the cell (giving it a spiral
appearance) as shown above. Otherwise, these flagella possess the usual basal body (discs), hook and
filament. These internal flagella form a bundle or axial filament. The filaments contain a core of flagellin
(FlaB) and in some species also contain a sheath of a second type of flagellin (FlaA). The sheathed
filaments are thicker (about 25 nanometres in diameter). These non-emergent flagella, or endoflagella
(endo- meaning 'internal') are illustrated in the diagram below:
These are found in the corkscrew shaped spirochetes. Although these bacteria are gram negative (with
a double membrane envelope structure), they possess only 2, to 4 rings (the S, M and presumably the
C rings and sometimes an extra pair) in the basal complex - they lack the P and L rings. The result is
that the basal complex does not cross the outer membrane and the filaments lie between the
peptidoglycan and outer membrane (in the periplasm) wrapped around the cell (giving it a spiral
appearance) as shown above. Otherwise, these flagella possess the usual basal body (discs), hook and
filament. These internal flagella form a bundle or axial filament. The filaments contain a core of flagellin
(FlaB) and in some species also contain a sheath of a second type of flagellin (FlaA). The sheathed
filaments are thicker (about 25 nanometres in diameter). These non-emergent flagella, or endoflagella
(endo- meaning 'internal') are illustrated in the diagram below:
| Spirochaetes |
Why a corkscrew shape?
The flagella cause the cells to rotate like a corkscrew. This enables spirochetes to travel with ease
through highly viscous media, like mud, mucus and the host connective tissue matrix (such as the
cartilage in your joints) as in lyme-disease spirochetes (Kimsey & Spielman, 1990). The trouble with
emergent flagella is that they fail to operate effectively in such highly viscous media (velocity drops
rapidly at viscosities above about 0.005 Pas). (Some research has suggested that bacteria that
possess a bundle of flagells, such as Escherichia coli, do not gain any additional speed than if they
had but one flagellum, which begs the question why have 6 flagella. One possible answer is that a
flagella bundle operates better in more viscous media, but the endoflagella are still superior in
extremely viscous media). Spirochaetes achieve maximum velocity only in highly viscous fluids with
a viscosity about that of engine oil (0.3 to 0.5 Pas) and become immotile at about 1 Pas. Indeed,
the spirochaete Leptospira has been shown to be positively viscotactic - meaning that it seeks out
regions of high viscosity.
What is viscosity? Viscosity is a measure of the stickiness or 'thickness' of a fluid. This stickiness
creates resistance when a fluid is set in motion - treacle is highly viscous and sticks to itself creating
high internal friction which resists motion. Water is moderately viscous, since water is sticky
(droplets will stick to your skin) and becomes highly viscous on the micrometre scale - to a
bacterium water is rather like treacle. As mentioned in the introduction to bacteria, the bacterium
flagellum is designed to function in such high viscosities. However, mud is even more viscous and
emergent flagella fail to work well above a certain viscosity. Spirochaetes, on the other hand, can
drill through thick mud and even human cartilage in the case of Lyme's disease. Lyme's
disease-causing spirochaetes are carried by deer ticks, which may spread the bacteria to humans,
resulting in a form of arthritis as the joints are attacked. Viscosity is usually measured in units of
Pascal seconds (Pas) or else in centipoise (cP, 1000 cP = 1 Pas).
For example the spirochaetes Leptospira and Spirochaeta have two endoflagella, Treponema has
2 to 16, Borrelia 30 to 40 and some large spirochaetes have more than 200. More or less equal
numbers of endoflagella are inserted at each end of the cell. (The insertion is subpolar, meaning
just short of the ends or poles of the cell).
How do endoflagella propel these cells?
Endoflagella work in several different ways, depending upon the species. In one model,
endoflagella work by generating torque (rotary force). As the flagella rotate in one sense, say
clockwise, they exert a torque on the outer membrane in the anticlockwise direction. (Try sitting in a
revolving chair and taking hold of a rotating bicycle wheel by the axil and see how the wheel exerts
a torque on you, causing you to swivel in the chair - the mechanism is essentially the same). If the
outer sheath is loosely attached and free to rotate (lubricated by fluid in the periplasm) then it will
rotate counterclockwise such that fluid outside the cell rotates clockwise relative to the cell and so
resists the cell's rotation. This is illustrated in the figure below:
The flagella cause the cells to rotate like a corkscrew. This enables spirochetes to travel with ease
through highly viscous media, like mud, mucus and the host connective tissue matrix (such as the
cartilage in your joints) as in lyme-disease spirochetes (Kimsey & Spielman, 1990). The trouble with
emergent flagella is that they fail to operate effectively in such highly viscous media (velocity drops
rapidly at viscosities above about 0.005 Pas). (Some research has suggested that bacteria that
possess a bundle of flagells, such as Escherichia coli, do not gain any additional speed than if they
had but one flagellum, which begs the question why have 6 flagella. One possible answer is that a
flagella bundle operates better in more viscous media, but the endoflagella are still superior in
extremely viscous media). Spirochaetes achieve maximum velocity only in highly viscous fluids with
a viscosity about that of engine oil (0.3 to 0.5 Pas) and become immotile at about 1 Pas. Indeed,
the spirochaete Leptospira has been shown to be positively viscotactic - meaning that it seeks out
regions of high viscosity.
What is viscosity? Viscosity is a measure of the stickiness or 'thickness' of a fluid. This stickiness
creates resistance when a fluid is set in motion - treacle is highly viscous and sticks to itself creating
high internal friction which resists motion. Water is moderately viscous, since water is sticky
(droplets will stick to your skin) and becomes highly viscous on the micrometre scale - to a
bacterium water is rather like treacle. As mentioned in the introduction to bacteria, the bacterium
flagellum is designed to function in such high viscosities. However, mud is even more viscous and
emergent flagella fail to work well above a certain viscosity. Spirochaetes, on the other hand, can
drill through thick mud and even human cartilage in the case of Lyme's disease. Lyme's
disease-causing spirochaetes are carried by deer ticks, which may spread the bacteria to humans,
resulting in a form of arthritis as the joints are attacked. Viscosity is usually measured in units of
Pascal seconds (Pas) or else in centipoise (cP, 1000 cP = 1 Pas).
For example the spirochaetes Leptospira and Spirochaeta have two endoflagella, Treponema has
2 to 16, Borrelia 30 to 40 and some large spirochaetes have more than 200. More or less equal
numbers of endoflagella are inserted at each end of the cell. (The insertion is subpolar, meaning
just short of the ends or poles of the cell).
How do endoflagella propel these cells?
Endoflagella work in several different ways, depending upon the species. In one model,
endoflagella work by generating torque (rotary force). As the flagella rotate in one sense, say
clockwise, they exert a torque on the outer membrane in the anticlockwise direction. (Try sitting in a
revolving chair and taking hold of a rotating bicycle wheel by the axil and see how the wheel exerts
a torque on you, causing you to swivel in the chair - the mechanism is essentially the same). If the
outer sheath is loosely attached and free to rotate (lubricated by fluid in the periplasm) then it will
rotate counterclockwise such that fluid outside the cell rotates clockwise relative to the cell and so
resists the cell's rotation. This is illustrated in the figure below:
In this way the bacterium is seen to rotate in the water, and since the helical filaments of the flagella
cause the cell to twist into a corkscrew shape, the cell drills its way through the medium.
In a second model, a helical wave propogates down the flagella and hence the cell, since the cell is
flexible. These waves may be flat, in which case the cell will undulate from side to side, much like an eel
(which also lives in thick mud!) or they may be circular, in which case the cell again assumes a
corkscrew shape.
The reality is a bit more complicated with different species of spirochaete adopting different methods.
Some examples will now be given.
Borrelia burgdorferi, the causative agent of Lyme disease, is 0.33 micrometres in diameter and 10-20
micrometres long. The endoflagella account for the shape of the cells, since mutants lacking flagellin
arestraight rods. When the two bundles (one coming from each end of the cell and each comprising
7-11 endoflagella) rotate in the same direction there is no translation, only cell flexion, but when they
rotate in opposite directions the cells translate as backward-moving waves pass along the cell. These
waves may be flat or circular depending on species, and are flat in Borrelia burgdorferi, such that the
cells undulate like eels (with a wavelength of 2.8 micrometres and an amplitude of 0.78 micrometres).
The PFs are LH-helices. Waves pass down the cells at 5-10 Hz (1 Hz = one cycle per second) and the
cells gyrate CCW as seen from behind. Thus, in this species, the propagating helical wave model
appears the most appropriate.
What are LH and RH helices? A helix (plural helixes or helices) is a twisted shape like a corkscrew or
'spiral' (mathematically a spiral is flat and a helix is a spiral pulled out in one direction). If you look down
the long axis of a helix as it rotates clockwise, then if it moves away from you it is a right-handed (RH)
helix. If it comes toward you when rotating clockwise and must rotate counterclockwise to move away
from you, then it is a left-handed (LH) helix. Note that this is a fundamental property of the helix - a LH
helix can never be a RH helix, no matter how you look at it. The two are mirror images of one another.
This asymmetry is essential to the spirochaete - as a helix rotates in the correct sense it displaces fluid
behind it and drills forward, otherwise the spirochaete would simply rotate and go nowhere! Most screws
are RH helices. What type of helix is a corkscrew for wine bottle corks?
Treponema is the causative agent of such diseases as venereal syphilis, endemic syphilis, yaws and
pinta, the so-called treponematoses and lives in the oral cavity, intestinal tract, stomach and rumen of
ruminants. Treponema denticola is about 6-16 micrometres long and 0.21-0.25 micrometres in diameter
and has two bundles of two periplasmic fibrils (PF) (a periplasmic fibril is the filament of an
endoflagellum) that emerge from just beneath each cell pole and overlap in the centre of the cell. These
cells form irregular twisted shapes with helical and flat planar regions though some cells form
right-handed helices (there are thus two stable forms with transitions from one form to the other being
rare). If the outer membrane is removed then they assume a right-handed helix, as do flagella-less
mutants, suggesting that the spiral shape is due to the peptidoglycan layer exerting tension on the cell.
Treponema phagedenis is 14-15.5 micrometres long and adopts a right-handed (RH) helix in the middle
of the cell and flagellar bundles of 4-8 PFs extend from the subpolar regions, but do not reach the
central RH helical part of the cell. The ends of the cells are often left handed (LH) helices and bent.
Mutants lacking the PFs also lack the bent ends.
What these Treponema studies tell us is that the shape of the cell is governed both by the
peptidoglycan layer, the endoflagella and the outer membrane. The whole system is under tension,
giving these bacteria their elasticity - they are flexible but will always relax to their default shape. These
examples are perhaps explained by the model in which the PFs rotate inside the OM and impart a
LH-helical shape to the otherwise RH-helical cells.
The Leptospiracae include Leptonema illini, Leptospira biflexa (a saprophyte) and Leptospira
interrogans (a pathogen). These bacteria are thin RH-helical cells, 6-20 micrometres long by 0.1-0.2
micrometres in diameter. They have a short PF at each end, attached subterminally (i.e.
subpolarly)which extends toward the cell centre but do not overlap and coil like springs. Resting and
dead cells have hook-shaped ends. Mutants lacking PFs or with straight uncoiled PFs are helical but
with straight ends and retain this shape if the OM is removed. The PFs exert tension on the helical cells,
causing the ends to bend. In translating cells, the anterior (front) end is spiral whilst the posterior end is
hook-shaped and these ends can readily reverse roles with the posterior hook-shaped end becoming
the anterior spiral end when the cell reverses direction. In the spiral anterior end the PFs rotate CCW
and the hooked end they are presumably rotating CW or else not rotating at all. Thus, in these bacteria
the PFs rotate in opposite directions in translating cells and reversal in the directions of PF rotation
cause the cell to reverse direction.
Other forms of locomotion in spirochaetes
Spirochaetes are long cells for bacteria and easily visible with the light microscope. I have seen them
many a time and made a nice video, however, this recording is not in my position at present, but if I
obtain it again then I shall post clips and stills here in the future. They are remarkable to watch, the very
long ones flex about as they swim, and exhibit many worm-like movements. Some of these vermiform
(worm-like) bacteria have even been described moving like inchworms - placing their (presumably
adhesive) anterior end against a surface and then flexing their body to bring up the rear end and then
stretching forward again. They may exhibit thrashing, lashing and writhing movements in addition to
translation. They can swim in suspension or glide or creep along a solid surface.
Download an illustrated essay on bacterial motility and navigation in pdf format: Prokaryotes motility.
Back to bacteria introduction...
Back to bacterial motility...
cause the cell to twist into a corkscrew shape, the cell drills its way through the medium.
In a second model, a helical wave propogates down the flagella and hence the cell, since the cell is
flexible. These waves may be flat, in which case the cell will undulate from side to side, much like an eel
(which also lives in thick mud!) or they may be circular, in which case the cell again assumes a
corkscrew shape.
The reality is a bit more complicated with different species of spirochaete adopting different methods.
Some examples will now be given.
Borrelia burgdorferi, the causative agent of Lyme disease, is 0.33 micrometres in diameter and 10-20
micrometres long. The endoflagella account for the shape of the cells, since mutants lacking flagellin
arestraight rods. When the two bundles (one coming from each end of the cell and each comprising
7-11 endoflagella) rotate in the same direction there is no translation, only cell flexion, but when they
rotate in opposite directions the cells translate as backward-moving waves pass along the cell. These
waves may be flat or circular depending on species, and are flat in Borrelia burgdorferi, such that the
cells undulate like eels (with a wavelength of 2.8 micrometres and an amplitude of 0.78 micrometres).
The PFs are LH-helices. Waves pass down the cells at 5-10 Hz (1 Hz = one cycle per second) and the
cells gyrate CCW as seen from behind. Thus, in this species, the propagating helical wave model
appears the most appropriate.
What are LH and RH helices? A helix (plural helixes or helices) is a twisted shape like a corkscrew or
'spiral' (mathematically a spiral is flat and a helix is a spiral pulled out in one direction). If you look down
the long axis of a helix as it rotates clockwise, then if it moves away from you it is a right-handed (RH)
helix. If it comes toward you when rotating clockwise and must rotate counterclockwise to move away
from you, then it is a left-handed (LH) helix. Note that this is a fundamental property of the helix - a LH
helix can never be a RH helix, no matter how you look at it. The two are mirror images of one another.
This asymmetry is essential to the spirochaete - as a helix rotates in the correct sense it displaces fluid
behind it and drills forward, otherwise the spirochaete would simply rotate and go nowhere! Most screws
are RH helices. What type of helix is a corkscrew for wine bottle corks?
Treponema is the causative agent of such diseases as venereal syphilis, endemic syphilis, yaws and
pinta, the so-called treponematoses and lives in the oral cavity, intestinal tract, stomach and rumen of
ruminants. Treponema denticola is about 6-16 micrometres long and 0.21-0.25 micrometres in diameter
and has two bundles of two periplasmic fibrils (PF) (a periplasmic fibril is the filament of an
endoflagellum) that emerge from just beneath each cell pole and overlap in the centre of the cell. These
cells form irregular twisted shapes with helical and flat planar regions though some cells form
right-handed helices (there are thus two stable forms with transitions from one form to the other being
rare). If the outer membrane is removed then they assume a right-handed helix, as do flagella-less
mutants, suggesting that the spiral shape is due to the peptidoglycan layer exerting tension on the cell.
Treponema phagedenis is 14-15.5 micrometres long and adopts a right-handed (RH) helix in the middle
of the cell and flagellar bundles of 4-8 PFs extend from the subpolar regions, but do not reach the
central RH helical part of the cell. The ends of the cells are often left handed (LH) helices and bent.
Mutants lacking the PFs also lack the bent ends.
What these Treponema studies tell us is that the shape of the cell is governed both by the
peptidoglycan layer, the endoflagella and the outer membrane. The whole system is under tension,
giving these bacteria their elasticity - they are flexible but will always relax to their default shape. These
examples are perhaps explained by the model in which the PFs rotate inside the OM and impart a
LH-helical shape to the otherwise RH-helical cells.
The Leptospiracae include Leptonema illini, Leptospira biflexa (a saprophyte) and Leptospira
interrogans (a pathogen). These bacteria are thin RH-helical cells, 6-20 micrometres long by 0.1-0.2
micrometres in diameter. They have a short PF at each end, attached subterminally (i.e.
subpolarly)which extends toward the cell centre but do not overlap and coil like springs. Resting and
dead cells have hook-shaped ends. Mutants lacking PFs or with straight uncoiled PFs are helical but
with straight ends and retain this shape if the OM is removed. The PFs exert tension on the helical cells,
causing the ends to bend. In translating cells, the anterior (front) end is spiral whilst the posterior end is
hook-shaped and these ends can readily reverse roles with the posterior hook-shaped end becoming
the anterior spiral end when the cell reverses direction. In the spiral anterior end the PFs rotate CCW
and the hooked end they are presumably rotating CW or else not rotating at all. Thus, in these bacteria
the PFs rotate in opposite directions in translating cells and reversal in the directions of PF rotation
cause the cell to reverse direction.
Other forms of locomotion in spirochaetes
Spirochaetes are long cells for bacteria and easily visible with the light microscope. I have seen them
many a time and made a nice video, however, this recording is not in my position at present, but if I
obtain it again then I shall post clips and stills here in the future. They are remarkable to watch, the very
long ones flex about as they swim, and exhibit many worm-like movements. Some of these vermiform
(worm-like) bacteria have even been described moving like inchworms - placing their (presumably
adhesive) anterior end against a surface and then flexing their body to bring up the rear end and then
stretching forward again. They may exhibit thrashing, lashing and writhing movements in addition to
translation. They can swim in suspension or glide or creep along a solid surface.
Download an illustrated essay on bacterial motility and navigation in pdf format: Prokaryotes motility.
Back to bacteria introduction...
Back to bacterial motility...
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