- 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.
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.
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.
Above: the flagella 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:
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 Mot (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 S ring is now known to be part of the rotor, along
with the M ring, and forms a socket for the rod, but is not part
of the stator. Confusion may arise when the stator ring is
referred to as the S-ring.
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: illustrating the flow of protons (carrying positive electric charge) across the Mot ring from the periplasm into the cell, powering the motor.
a diagram of the bacterial flagellum showing the detailed
structure of the flagellum basal complex (in a Gram negative
bacterium). The units of protein FliG (about 25-45 units) form a
ring extension to the M-ring (the M ring is shown in section
an extension on top of the C-ring according to the model. Units of the proteins
FliM (about 35 units) and FliN (about 110 units) form the 45 nm
diameter C ring, which together with the M and S rings forms the
rotor. This rotor drives the rod, which is a rotor-shaft
connected through the centre of the L and P rings to the hook.
The proteins MotA and MotB form a ring of 10 studs embedded in the cytoplasmic membrane,
forming the stator, which is tethered by connections to the rigid Peptidoglycan layer (PG). The L
and P rings act as bushing.The C ring is made up of the proteins FliM and FliN (and perhaps also FliG) and the Mot proteins consist of two subunits: Mot A and MotB. Studies suggest that 4 MotA subunits complex with 2 MotB subunits and that mostly MotA, but also part of MotB contributes to the formation of a proton channel.
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.
The basal complex anchors the flagellum into the surface of the cell. There are two structural variations according to the type of bacterial wall it is anchored in. Gram positive bacteria (stain purple with Gram's stain) like Bacillus possess a thick peptidoglycan wall (about 80 nm) overlying the bilipid cytoplasmic membrane (CM). In this case the basal body has three rings, the 26 nm diameter M (membrane) ring embedded in the CM, the S (supramembrane) ring or socket attached to the inner surface of the peptidoglycan wall by techoic acids and the C (cytoplasmic) ring. The S ring is an extension of the M-ring, to which it is attached, and both are composed of the same ring of protein FliF, thus the M and S rings are sometimes considered to be a single double-ring, the MS ring. A rod (7nm diameter) passes into the S ring socket and its other (distal) end attaches to the hook. More details of these structures are illustrated below:
that it is difficult to map proteins from their predicted or
measured shapes to electron density maps, such as those
generated by electron cryomicroscopy. Some newer models have
FliG as the top of the C-ring, the U-shaped structure at top in
yellow in the fig. above, rather than as part of the M ring,
whilst other models have it spanning the C and M rings. Whatever
its exact position its interactions with Mot are thought to
drive the rotor, including both the C and MS rings, which must,
therefore, be connected in some way.
There are some 25 or 26 FliG molecules in the flagellum base and about 11 Mot complexes, each containing 4 MotA and 2 MotB. It should also be realised that different bacterial species have different flagellum base architectures. An updated and more detailed model is shown below:
The table below summarises the various substructures shown in the figures above and their encoding genes.
Mechanism of Torque Generation - A Model
Quite a number of ingenious models have been proposed over the years, attempting to explain how the bacterial flagellum rotor generates torque. The most recent model can be summarised thus:
Vortices generated by the 6 rotating flagella of Escherichia coli as viewed from behind the cell looking forward along its direction of locomotion.
the 'circle' of flagella, driven by their CCW rotation, a CW
vortex would be established. At this low
Re we would expect no turbulence, so these are orderly vortices and the central CW vortex will spiral outward toward us (the central dot indicates the head of its velocity vector coming out of the page toward us). This core of fluid will be displaced, essentially drilled out of the viscous medium and the 6 flagella will close together to fill the void it leaves behind it (fluid flow from outside the flagella, passing in-between them to fill this void would not readily occur, since the flows of neighbouring vortices tend to cancel midway between them) and the flagella will bundle together. Fluid will continue to spiral CW around the bundle as it rotates CCW, propelling the cell.
We have to ask the question: do the flagella or the cell body generate most of the propulsive thrust? For a spherical or rod-shaped bacteria powered by a single CCW rotating flagellum the cell body will rotate CW. This rotation of the cell body will not, by itself, generate thrust since, as already discussed, in order to generate thrust in a high viscous fluid it is necessary to have time-non-reversible flow, which requires a helix of definite handedness. Perhaps the spiral S-layer of protein that encases many bacteria (forming an outer layer around the cell envelope) helps break the symmetry and generate thrust. In coma (vibrioid) and spiral forms, the cell body will indeed displace fluid in a non-reversible manner and so contribute to the thrust, effectively drilling its way through the medium.
So, what happens when the flagella rotate CW?
If the bundle remained intact then it would begin to pull the cell backwards by displacing a helix of fluid toward the cell body. However, the external fluid will flow towards the end of the flagellum bundle and perhaps this drives the bundle apart by forcing its way between the flagella. If this was so, might the cell be observed to reverse momentarily before tumbling? This raises the question as to how many flagella must rotate CW for the bundle to separate. If one flagellum only reverses direction, then its flow would
couple with the neighbouring flagellar vortices.
Consider two neighbouring vortices, one rotating CW and one rotating CCW, as shown below:
A pair of oppositely rotating vortices couple together and move downwards in this case.
vortex pair will drive fluid in one direction between them and
around them – the two vortices are coupled together. Viscosity
will cause the vortex pair to move in the direction of this
flow. Some of the pairs will try to move toward the center
(which they can not do since other flagella in the bundle will
block their path) and other pairs will tend to move away from
the bundle – the bundle will disperse.
In addition to changing rotation sense, the flagella also change pitch and this may also contribute to the decoupling of the flagella and dispersal of the bundle.
Most bacteria alternate between a single-celled stage and some form of multicellular stage during their life-cycle. Although bacteria rarely form true multicellular organisms, in which cells are in intimate contact and communicating directly with their neighbours through cell-to-cell junctions, most do form multicellular aggregates in which cells are separately embedded in a common slime layer, which is attached to a solid surface and forming a biofilm. Cells in the biofilm are often immotile and certainly lose their ability to swim as they shed their flagella. Swarmer cells, however, are a dispersive stage and these may float, swim, crawl or glide away from the biofilm before eventually settling down, adhering to a surface and establishing a new microcolony which may eventually develop into a biofilm. Sometimes the flagella assist in adhesion by sticking to a suitable surface they come into contact with. However, soon after adhering the swarmer cells lose their flagella and establish a microcolony. Bacteria in both the swarmer cell stage and the biofilm stage maintain a degree of individuality and both are capable of dividing and multiplying.
Biofilms also disperse by shedding slimy streamers, containing cells, downstream. Generally, dispersal is more efficient if the biofilm sits upstream of fluid flow. How do the swarmer cells, which are often released into the stream some 400 or so micrometres above the surface swim in these currents? Research has shown that Escherichia coli, at least, has a neat trick. In those strains under study it was found that swimming Escherichia coli (swarmer cells) preferentially circle to the left until they find a sheltered crevice in a surface, where flow is less, and they swim upstream to establish a new microcolony and biofilm!
Swarming and crawling
The normal swimming flagellated cells that we have considered so far are described as swarmer cells, or planktonic cells, distinguishing them from the non-flagellated cells that are integral residents in biofilms. However, ‘swarming’ is usually used for yet another, though closely related, phenomenon – the mass migration of colonies of bacteria across a solid surface. This swarming behaviour is regulated by quorum sensing and in Escherichia coli and Salmonella typhimurium involves a switch to a multinucleoid elongate filamentous hyperflagellated phenotype. These multinucleoid cells, called filaments, are up to 50 micrometres long and have several nucleoids, dispersed at intervals throughout the filament (nucleoid division and cell wall growth have continued in the absence of cell division). They move as a colony with the outer layers spiraling outward with the evacuated space inside the colony becoming filled with new cells. This can give rise to fast colony expansion at rates up to ~3 mm/s (1 cm/h). Spiral and 2D-branching patterns of colonial growth result and probably acts to optimize nutrient uptake on a solid substrate in which diffusion is limited (in a solid sheet that completely covers the surface, many cells may become starved of nutrients due to competition with their neighbours – similar considerations have shown to predict the branching growth of sponges in 3D in computer models utilizing the diffusion equation).
The hyperflagellated filaments use their many flagella to crawl over solid surfaces, such as agar in a petri dish. The flagella still bundle, but the cells do not tumble, instead when the flagella disperse the filament simply stops and the flagella may reform a bundle of the opposite sense resulting in a reversal of direction of movement. Synthesis of so many flagella appears to be triggered when the bacteria sense the presence of a highly viscous medium (such as the surface of an agar plate).
Update on Swarming
Research shows that bacteria only swarm when moving in groups and evidence suggests that they interlock their flagella to move as rafts. Cells may detach and reattach to the raft, but detached cells do not swarm but remain immotile instead. They can also secrete surfactant to reduce surface tension and facilitate gliding of the rafts. Research also suggests that both the flagella and pili can function as mechanoreceptors, sensing contact with a surface to trigger microcolony formation.
Above: a hyperflagfellated bacterial filament (this one is 4 times the normal length at about 8 micrometers and has 20 flagella). When fully developed this cell may have 100 flagella and measure about 50 micrometers in length!
articles on bacterial motility:
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!).
Learn about gliding and other alternative mechanisms of locomotion in bacteria.
Download an illustrated essay on bacterial motility and navigation in pdf format: Prokaryotes_motility.
How can something as complex as a bacterial flagellum evolve?
updated: 1/9/2013. Details about flagella bundles, swarming,
swarmer cells and protein structure added. Errata concerning the
nature of the stator and end-cap proteins applied.
Page updated: 24/8/2018. Recent discoveries on the mechanism of flagella rotation and the mechanism of swarming added.