Question

4. Describe the appendages of bacterial cells.
     a. How motility is accomplished in bacterial cells.
    b. What are pilin and the role of pili and fimbriae.
    c. Explain the structure of flagella.

Answer 1

4. Cell surface appendages (filamentous appendages) are proteinaceous tubular or fibrous structures found on the surface of bacterial cells. They extend from the surface of the bacterial cell wall and can have many functions such as locomotion, attachment, adhesion and assisting in genetic exchange.

• Three types of surface appendages can be found in the bacterial cells : 1) the flagella, which are organs of locomotion, and 2) pili (Latin hairs), 3) fimbriae (Latin fringes). The classification is based on their relative length ,composition and function.
• Flagella occur on both Gram-positive and Gram-negative bacteria, and their presence can be useful in identification and The main function of Flagella is locomotion
• Pili present on some gram -negative bacteria and they are comparitively shorter tham flagella and longer than fimbriae.
• Fimbriae present in both Grame- negativa and gram - positive bacteria and comparitively shorter in length than both Flagella and Pili. The main function is surface attachment.

a) The ability of an organism to move in the surface by itself is known as Motility. Motility is closely linked with chemotaxis, the ability to orientate along certain chemical gradients. Eucaryotic cells can move by means of different locomotor organelles such as cilia, flagella, or pseudopods. Procaryotes move by means of propeller-like flagella unique to bacteria or by special fibrils that produce a gliding form of motility.

Bacterial locomotion is of three types : Flagellar, Spirochaetal movement and gliding movement.

• Most motile bacteria move by use of flagella, thread like locomotor appendages extending outward from the plasma membrane and cell wall. Motile cells may have one flagellum, but most have several. Each flagellum has a very rigid, helical structure and actual motility results from the rotation of the flagellum in a manner similar to that of a boat propeller. Motility by means of flagella is particularly important for the identification of microorganisms.The positions at which flagella are inserted into the bacterial cell are characteristic for a genus.
• Flagella are the key structures concerned with the bacterial motility. Nevertheless, the bacteria that lacks flagella can still achieve motility. A type of gliding motility can be achieved by the flexible movement of the whole cell. This is mainly seen in a solid media whereas flagellar motion can be seen in liquid environment.
• Spirochaetal Movement : seen in all genera of bacterial group. They have flagella like axial filament buried in space between inner and outer menmbare of cell wall. Helps in swimming, spinning,creeping and flexing type movements.
• Gliding movement : Bacteria can move by gliding on the surface. Do not have flagellar like structure internally or externally but they secrete  a slimy substance  like snails during locomotion. The exact mechanism of gliding locomotion is unknow but some scientist suggests that they have fimbriae like appendages on the poles of glider cells.

b) Pilin and the role of Pili and Fimbriae.

Pilin : Pili are made up of proteins called Pilin Proteins .

Pili and Fimbriae :

• Pili are long haired, microfibre like structures and is tubular.
• Present only in some Gram - negative bacteria.
• Made od Pilin Proteins.
• Distributed randomly on cell surface.
• The main function of Pili are gene transfer and attachment.
• The formation of Pili is more controlled by the gene present in plasmids.

Fimbriae

• Fimbriae are bristle like short fibres.
• Present in both Gram positive and gram negative bacteria.
• Made of Fimbrillin protein.
• Evenly distributed in the cell surface.
• The formation of fimbriae is controlled by the genes in the nucletide region.
• Major function is surface attachment.
• Do not have any role in bacterial mobility and conjugation.

Both are able to stick bacteria to surfaces, but pili are typically longer and fewer in number than fimbriae.The short attachment pili or fimbriae are organelles of adhesion allowing bacteria to colonize environmental surfaces or cells and resist flushing.  Proteins associated with bacterial fimbriae and pili function as antigens and initiate adaptive immunity.

c) Structure of Flagella :

A flagellum is a whip-like structure that allows a cell to move. They are found in all three domains of the living world: bacteria, archaea, and eukaryota, also known as protists, plants, animals, and fungi.Some special flagella are used in few organisms as sensory organs that can sense changes in pH and temperature. While all three types of flagella are used for locomotion, they are structurally very different.

1. Archaeal flagella are nonhomologous.
2. Bacterial flagella are a coiled, thread-like structure, sharp bent, consisting of a rotary motor at its base and are composed of the protein flagellin.
3. Eukaryotic flagella are complicated cellular projections that pummel backwards and forward and are found in protist cells, plants, and animals.

Flagella structured into three parts :

1. Basal Body : attached to the cell membrane and cytoplasmic membrane.It consists of rings surrounded by a pair of proteins called Mot. The rings include:

• L-ring: Outer ring anchored in lipopolysaccharide layer and found in gram +ve bacteria.
• P-ring: Anchored in the peptidoglycan layer.
• C-ring: Anchored in the cytoplasm
• M-S ring: Anchored in the cytoplasmic membrane.

1. Hook : It is a broader area present at the base of the filament.Connects filament to the motor protein in the base.The hook length is greater in gram +ve bacteria.
2. Filament : Thin hair like structure arising from the hook.