Bacterial Flagella, Arrangement, ultra structure, Chemical composition, Synthesis, movement, Full notes
Full notes in Flagella |
Bacterial Flagella:
Flagella image from |
Flagella were first observed by Anton van Leeuwenhoek.
The majority of motile
bacteria move by the use of flagella.
Flagella are 20 –
15 um long. Flagella are so thin that they cannot be directly observed with
a bright-field microscope but must be stained with special techniques designed
to increase their thickness.
The detailed structure
of flagella can only be seen in the electron microscope.
More flagella occurs
in liquid preparation.
Flagella arrangement(Flagellation):
Bacterial species
often differ distinctively in their pattern of flagella distribution
1.
Monotrichous bacteria: have one flagellum
If it is located in the end it
is said to be Polar
Ex.
Pseudomonas
2.
Amphitrichous bacteria: Have a single flagellum located at each pole.
Ex.
Spirillum
3.
Lophatrichous bacteria: Have a cluster of flagella at both ends.
Ex.
Aqua spirillum
4.
Cephalotrichous bacteria: Have a cluster of flagella at end
5.
Peritrichous bacteria: Have a large number of flagella spread over the
whole surface.
Flagella ultrastructure:
Flagella |
Transmission electron
microscope studies have shown that the bacterial flagellum is composed of
three parts 1. Filament: The longest and most obvious portion
2.Basal body: Which
is embedded in the cell.
3.Hook: A short The curved segment links the filament to its basal body.
Hand draw of Flagella |
The hook and basal
bodies are quite different from the filaments. The hook is slightly wider than the
filament and made of different protein subunits. The basal body is the most
complex part of a flagellum.
In E.coli and most
gram-negative bacteria, the basal body having four rings connected with a
central rod.
The outer L and
P rings associated with the peptidoglycan layer and inner M and S ring connected with
plasma membrane.
Gram-positive
bacteria have only two basal body rings S and M and connected with
plasma membrane and outer one attached to the peptidoglycan.
Chemical composition of flagella:
Electron microscope
show that the flagella of bacteria consist of three parallel protein fibers
intertwining in a triple helical structure. These fibers composed of a protein called
flagellin.
The molecular weight
of flagellin is approximately in the range of 20,000 to 60,000.
An amino acid not
found elsewhere, has been identified in this protein compound is
E(epsilon) N-Methyl lysine.
Flagellar Synthesis:
The synthesis of
flagella is fairly complex process involving 20-30 genes. Besides the
gene for flagellin, 10 or more genes for hook and basal body proteins.
Other genes code for
control flagellar construction or function.
It is studied that
bacteria can be deflagellated and regenerate flagellar filament
Filament synthesis is
an excellent example of self-assembly.
Many structures from
spontaneously through the association of their component parts without the aid
of any special enzyme or other factors.
The information
required for filament construction is present in the structure of the flagellin
subunit itself.
The mechanism of Flagellar movement:
Prokaryotic
flagella are quite different from the eukaryotic flagella.
The filament is in the shape of the rigid helix and the bacteria moves when helix rotates.
Flagella act like
propellers of a boat.
If polystyrene latex
beads are attached to flagella the beads spin about the flagellar axis as a
result of rotation.
Flagella rotate at
rats as fast as 40-60 revolution per second(rpm)
The direction of
flagellar rotation determines the nature of bacteria.
Monotrichous; polar
flagella rotate counterclockwise when viewed from the outside the cell rotate
slowly clockwise.
Peritrichous
flagellate bacteria operate in a somewhat(slightly) similar way
In order to move
forward the flagella rotate in order to move forward, the flagella rotate
counterclockwise.
They band their hooks
to form a rotating bundle.
Clockwise rotation of
the flagella disrupt the bundle and cell tumbles.
According to one
hypothesis, a flagellum rotates because of the interaction between its S ring and M
ring. A rod or shaft extends from the hook and in the M ring which can rotate
freely in the plasma membrane.
It is believed
that S ring is attached to the cell and does not rotate.
The P and L rings
in gram, negative bacteria would act as bushings for the rotating rod.
Bacteria can swim from
20 to almost 90um/second. This is equivalent to traveling from 2 to over
100 cell lengths per second. In contrast, a very fast 6 ft human might be able
to run around 5 body lengths per second.
0 Comments