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:
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:
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.

Nature of the endoflagella

Endoflagella are variants on the typical bacterial flagella rotor (see flagella structure) - they each have a rotor at the base to drive rotation of the helical filament. The helical filaments are made of multiple copies of the protein flagellin. In spirochaetes the filaments do not emerge through the outer membrane, but reside in the periplasm between the inner and outer membranes instead. There are other notable modifications: in Borrelia and Leptospira the filament consists of a core of FlaB (flagellin B) whilst a second flagellin, FlaA forms a sheath around the filament core. The combined structure is helical. In most bacteria studied with emergent filaments the filament consists of a helical core of a single flagellin with no protein sheath. Additionally, the motor has a large rotor ring in spirochaetes (about 50% larger in diameter than that of Salmonella enterica) with more stator proteins around it (12 stators) which enables it to generate greater torque, which is presumably an adaptation to using the endoflagella to twist the cell through high viscous media. The torque generation (a torque or moment can be loosely thought of as a force causing rotation) is twice that of Escherichia coli (4000 pN nm compared to 2000 pN nm; pN = picoNewton, nm = nanometer).

A number of beautiful studies have enabled detailed observation of the rotors using a technique called cryo-electron tomography (rapidly frozen unstained specimens are sectioned and a series of sections viewed under the electron microscope and the 3D structure reconstructed using computer algorithms) - a technique which minimizes artifacts to give a more detailed view of a structure. For example: Xu et al., 2011. Chemoreceptors and Flagellar Motors Are Subterminally Located in Close Proximity at the Two Cell Poles in Spirochetes; J. Bacteriology 193(10): 2652–2656.

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.

Snakelike crawling occurs in Leptospira. This process is accompanied by the movement of adhesins (molecules that adhere to the substrate) such as LPS (lipopolysaccharide - see envelope structure) in the outer membrane. This suggests that adhesins may adhere to the substrate, move back along the cell body (moving the cell forward relative to the substrate) and then detach in a conveyor-belt mechanism. Furthermore, this crawling utilises the proton motive force (PMF, see power systems)

Download an illustrated essay on bacterial motility and navigation in pdf format: Prokaryote motility.

Article updated: 20 Aug 2020

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Back to bacterial motility...

A spirochaete
A spirochaete
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
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
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
- 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). (See also: biorheology).
A spirochaete
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.