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
Microscopic Viewing
- 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.
Right: Gram-Negative Left: Gram-Positive
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 Base Diagram - Click to Enlarge
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
Basal Body
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
Growth of Flagellar Filament. Click to enlarge
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)
Example of Flagella Biosynthesis - Click to enlarge
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)
Flagella Biosynthesis
- 20-30 Genes are involved in biosynthesis
- 10 Genes for hook and basal body
- Flagellin transported through flagellum
- Spontaneous self-aggregation
Filament
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
Hook
A hollow, flexible structure which joins filament to the basal body.
-Functions as a universal joint and allows flexible coupling
Forward run, Tumble Example. Click to Enlarge
Flagella Functions
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
- Monotrichous
Single polar, pole of cell - Peritrichous
Over entire surface - Amphitrichous
Single at each pole - Lophotrichous
Cluster at one or both ends
Chemotaxis
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
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
Gliding
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.
Twitching Motility
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.
If you find my Hub interesting don't hesitate in leaving a comment, I would really appreciate it.
Thanks
Mike
Comments
Kristina on May 08, 2015:
I have a question. On that picture of location of the flagellum aren't gram-negative on the left side and gram-positive on the right?
whonunuwho from United States on April 15, 2012:
I wonder if most folks realize that bacteria are helpful and not harmful altogether. They may associate bacteria with diseases alone and don't really understand the purposes and positive effects. I wonder if genetic mutations and other outside causes may be responsible in preventing the bacteria and function of the flagellum to assist in the function of the immune system.I know that the formation of white blood cells is essential in alleviating such harmful viruses.The increase in their number is indicative of the harmful presence. Oh well, just keep on writing for others and in helping to crystallize data in your own mind.Thanks.
