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.