The Mechanism Behind Bacterial/Prokaryotic Motility
Motility of the Flagella – General Features
Bacteria have evolved features that allow a bacterium to move in various environments, in particular the flagellum protein (locomotor appendages).
Locomotor appendages in bacteria defined:
- A rigid helix that rotates to move bacterium at 40-60 revolution per second
- Direction of rotation determines nature of movement (Clock wise and Anti clock wise)
- The motile bacteria can swim at up to 50um/sec, i.e 18cm/h
- The flagella cannot be seen with the light microscope
- Special stains can increase thickness to visualise flagellum
- Detailed structure of the flagella can be seen with the Electron Microscope.
Location of The Flagellum
The flagellum is a long, helical, rigid, proteinaceous and filamentous structure extending radially from the cell.
Enables the bacterial cell to swim through an aqueous medium via anti-clockwise rotation.
The flagellum is embedded in the cell membrane, running through the outer membrane (capsule), inner membrane, peptidoglycan and periplasmic space.
There is a slight difference in the ultra structure of the bacterial flagellum in comparison between gram-negative bacteria and gram-positive bacteria.
I have provided an image to the right to illustrate what I mean by the differences in the ultra structure of the flagella between the two different types of bacteria.
Flagellum Description for gram-negative enteric bacteria
Under the electron microscope the flagella is seen as a long, helical, rigid and filamentous structure.
There are four major components to a flagellum
- External Structure
- Motor-switch complex
- Basal Body
- Export apparatus
images added to help illustrate information, above.
The hook is connected to the rod, a straight hollow rube which forms the drive shaft.
Three rings surround the rod:
- MS ring
- P ring
- L ring
Chemical Composition of Flagella
All components of the flagellum are made of proteins
· Each specific protein is coded by a different gene
The protein components are synthesised in the cytoplasm and exported through the cell envelope and assembled. (self assembly)
How The Flagella is Assembled and Steps in Flagella Biosynthesis
The flagellar export apparatus (face of MS ring) secrets the rod building proteins, hook proteins and filament proteins in the exact sequence and amounts for them to diffuse through the hollow centre of the flagellum.
The proteins diffused through the flagellum and assemble at their assembly sites at the distal tip of the growing structure.
Flagellin sub-units travel through the flagellar core and attach to the growing tip. (image to the right)
- 20-30 Genes are involved in biosynthesis
- 10 Genes for hook and basal body
- Flagellin transported through flagellum
- Spontaneous self-aggregation
A thin, helical tube that serves as a propeller
- 10-20nm in diameter
- 15-20um long
- 2-2.5um wavelength
- Made from, helical arranged sub-units of the protein flagellin
A hollow, flexible structure which joins filament to the basal body.
-Functions as a universal joint and allows flexible coupling
The flagellum enables the bacterial cell to swim through an aqueous medium. An anti-clockwise movement of the flagellum is what causes a forward run.
A reversible motor powered by the transmembrane membrane proton motive force (PMF) drives flagella rotation (265H+ per revolution)
The motor-switch complex is mounted on the cytoplasmic face of the MS ring and comprises a bell-shaped structure known as the C ring.
The motor-switch complex contains three proteins (FliG, FliM & FLiN) involved in the generation of torque and switching of direction and rotate along with MS ring.
The Fli proteins act as a motor-switch, controlling the direction of rotation of the MS ring. FliG is the rotor interact with the ‘starter’, MotA. The other rings act as bearings and the rotation of the rod is transmitted through the hook to the filament.
Function of Anti-clockwise and Clockwise Rotation
Anti-clockwise rotation of the helical filament propels the cell through the aqueous environment, known as a “run”.
The clockwise rotation causes the cell to stop and re-orientate, known as a “tumble”.
Example: Image top right
Different Arrangements of Flagellum
Single polar, pole of cell
Over entire surface
Single at each pole
Cluster at one or both ends
Two types of chemotaxis: Positive and Negative
- Chemoreceptors: bind chemicals and transmit signals
- Absence of chemical causes a run and twiddle
- Presence of chemical cause more frequent runs
Movement towards attractants (such as amino acids and sugars) and away from repellents (harmful substances)
!Bacteria can respond to very low levels of attractants (about 10-8M).
Attractants and repellents are detected by chemoreceptors.
Chemoreceptors are proteins that bind to chemicals and transmit signals to the chemo-sensing system in the bacteria.
Examples of Taxis
Movement to response to environmental stimulus
- Phototaxis - Light
- Thermotaxis - Heat
- Magnetotaxis - Magnetic field
- Rheotaxis - Flow
- Chemotaxis - Chemical
Other Forms of Motility in Prokaryotes
- Gliding – surface contact e.g. Myxobacteria, Cyanobacteria
- Twitching – Similar to gliding, involving surface contact
A slow progressive movement of bacterial cells which requires surface contact. Gliding occurs only when the organism is in contact with a solid surface or gas-liquid interface.
Independent surface motility on solid media although relies on functional type of fimbriae or pili. The process of twitching motility is extremely rapid (0.6mm/h or more).
Bacterial movement is essential for survival; while some are better adapted to motility than others. Hopefully this hub contains the information needed for anyone to understand how prokaryotes, and in particular bacteria, move.
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