Extra cellular structures

Extra cellular structures
*Capsule
The cell capsule is a very large structure this a gelatinous structure is present in some bacteria outside the cell wall. It is considered a virulence factor because it enhances the ability of bacteria to cause disease . The capsule may be polysaccharide as in pneumococci , meningococci or polypeptide as Bacillus anthracis or hyaluronic acid as in streptococci . The capsule is antigenic. The capsule has antiphagocytic function so it determines the virulence of many bacteria. It also plays a role in attachment of the organism to mucous membranes

Demonstration of Capsule
1. India ink staining: the capsule appears as a clear halo around the bacterium as the ink can t penetrate the capusule.
2. Serological methods: Capsular material is antigenic and can be demonstrated by mixing it with a specific anticapsular serum. When examined under the microscope, the capsule appears swollen due to an increase in its refractivity. This phenomenon is called as capsule swelling reaction or Quellung phenomenon.
3. Special capsule staining: These techniques employ copper salts as mordants for staining the capsule .
For Vaccination capsular material is effective against some organisms (e.g., H. influenzae type b and S. pneumoniae).

*Flagella
Flagella are the organelles of cellular mobility. They arise from cytoplasm and extrude through the cell wall. They are long and thick thread-like appendages, protein in nature called flagellin. Three types of arrangement are known: monotrichous (single polar flagellum), lophotrichous (multiple polar flagella), and peritrichous (flagella distributed over the entire cell). staining with basic fuchsin makes the flagella visible in the light microscope

*Fimbriae (pill)
They are short and thin hair like filaments, formed of protein called pilin (antigenic). Fimbriae are responsible for attachment of bacteria to specific receptors of human cell (adherence). There are special types of pili called (sex pili) involved in conjunction.

*Cilia
Cilia are especially notable on the single-celled protozoans. They are composed of extensions of the cell membrane that contain microtubules. When present in humans they are typically found in large numbers on a single surface of the cells, where rather than moving cells, they move materials like these present in respiratory system that consists of mucus-secreting cells lining the trachea and bronchi, and ciliated epithelial cells that move the mucus ever-upward .
*Cell Wall
The bacterial cell wall is strength layer composed of a substance variously referred to as murein, mucopeptide, or peptidoglycan (all are synonyms). In addition to giving osmotic protection, the cell wall plays an essential role in cell division as well as serving as a primer for its own biosynthesis. Various layers of the wall are the sites of major antigenic determinants of the cell surface, and one component—the lipopolysaccharide of gram-negative cell walls—is responsible for the nonspecific endotoxin activity of gram-negative bacteria.
The Peptidoglycan Layer
Peptidoglycan is a complex polymer consisting of three parts: a backbone, composed of alternating N-acetylglucosamine and N-acetylmuramic acid; a set of identical tetrapeptide side chains attached to N-acetylmuramic acid; and a set of identical peptide cross-bridges .
The tetrapeptide side chains of all species, however, have certain important features in common. Most have L-alanine at position 1 (attached to N-acetylmuramic acid), D-glutamate or substituted D-glutamate at position 2, and D-alanine at position 4. Position 3 is the most variable one: Most gram-negative bacteria have diaminopimelic acid at this position, to which is linked the lipoprotein cell wall component. Gram-positive bacteria usually have L-lysine at position 3; however, some may have diaminopimelic acid or another amino acid at this position.
Diaminopimelic acid is a unique element of bacterial cell walls. It is never found in the cell walls of Archaea or eukaryotes. Diaminopimelic acid is the immediate precursor of lysine in the bacterial biosynthesis of that amino acid The fact that all peptidoglycan chains are cross-linked means that each peptidoglycan layer is a single giant molecule. In gram-positive bacteria, there are as many as 40 sheets of peptidoglycan, comprising up to 50% of the cell wall material; in gram-negative bacteria, there appears to be only one or two sheets, comprising 5–10% of the wall material.
Special Components of Gram-Positive Cell Walls
Most gram-positive cell walls contain considerable amounts of 1-teichoic acid containing glycerophosphate or ribitol phosphate residues. These polyalcohols are connected by phosphodiester linkages and usually have other sugars and D-alanine attached .There are two types of teichoic acids: wall teichoic acid (WTA), covalently linked to peptidoglycan, and membrane teichoic acid(MTA), covalently linked to membrane glycolipid. Because the latter are intimately associated with lipids, they have been called lipoteichoic acids (LTA) and 2- teichuronic acid, which may account for up to 50% of the dry weight of the wall and 10% of the dry weight of the total cell. In addition .The teichuronic acids are similar polymers, but the repeat units include sugar acids (such as N-acetylmannosuronic or D-glucosuronic acid) instead of phosphoric acids. They are synthesized in place of teichoic acids when phosphate is limiting. some gram-positive walls may contain polysaccharide molecules.