BACTERIAL MORPHOLOGY CONTD...

STRUCTURE EXTERNAL TO CELL WALL :

2).  Pili or Fimbriae :   They are thin,soft, hair like appendages that extends from cytoplasmic membrane of Gram negative bacteria . They are shorter, straighter & numerous than flagella. They are composed of pilin protein, but don't function as motility.

Advantages: 

                                                     Fig: Prokaryotic cell structure showing Pili.

fig:  Schematic drawing of bacterial conjugation. 1- Donor cell produces pilus. 2- Pilus attaches to recipient cell, brings the two cells together. 3-The mobile plasmid is nicked and a single strand of DNA is then transferred to the recipient cell. 4- Both cells recircularize their plasmids, synthesize second strands, and reproduce pili; both cells are now viable donors.

• Helps in attachment ie allows pathogenic bacteria to attach epithelial cells, linings of respiratory tract, intestinal tract etc. So, Pili  helps in colonization & prevents bacteria from being washed away by the flow of mucus or body fluid.
• .F-Pilis:  It acts as port of entry of genetic material during bacterial matting(conjugation).      

3). Capsule: Capsule is a viscous substance forming  a covering layer around the cell wall. If this layer is visualized by light microscopy using special staining techniques, termed as Capsule. If the layer is too thin to be visualized by light microscopy,is called Micro capsule.  If it is so abundant that  many cells are embedded in a common matrix, the material is called  Slime. Most of bacterial capsule are composed of polysaccharides whereas some are polypeptide in nature. Eg: Bacillus anthracis, Streptococcus mutans , Klebsiella pneumoniae .

             

            Glycocalyx:    Glycocalyx is a general term referring to extra cellular polymeric material (glycoprotein) produced by some bacteria, epithelia and other cells. For instance, the slime on the outside of a fish is considered to be a glycocalyx. The term was initially applied to the polysaccharide matrix excreted by epithelial cells forming a coating on the surface of epithelial tissue, but its functions have been discovered to go well beyond that. Most animal epithelial cells have a fuzzy coat called the glycocalyx on the external surface of its plasma membrane. This coating consists of several carbohydrate moieties of membrane glycolipids and glycoproteins, which serve as backbone molecules for support. Further research has shown that the glycocalyx, which is located on the apical surface of endothelial cells, is composed of a negatively charged network of proteoglycans, glycoproteins, and glycolipids. Generally, the carbohydrate portion of the glycolipids found on the surface of plasma membranes helps these molecules contribute to cell-cell recognition, communication, and intracellular adhesion.

The glycocalyx is a type of identifier that the body uses to distinguish between its own healthy cells and transplanted tissues, diseased cells, or invading organisms. Included in the glycocalyx are cell-adhesion molecules that enable cells to adhere to each other and guide the movement of cells during embryonic development. The glycocalyx plays a major role in endothelial vascular tissue, including the modulation of red blood cell volume in capillaries, as well as many other functions of the vascular system.

Glycocalyx in Bacteria and Nature

A glycocalyx, literally meaning "sugar coat" (glykys = sweet, kalyx = husk), is a network of polysaccharides that project from cellular surfaces of bacteria, which classifies it as a universal surface component of a bacterial cell, found just outside the bacterium cell wall. A distinct, gelatinous glycocalyx is called a Bacterial capsule, whereas an irregular, diffuse layer is called a slime layer. This coat is extremely hydrated and stains with ruthenium red.

Bacteria growing in natural ecosystems, such as in soil, bovine intestines, or the human urinary tract, is surrounded by some sort of glycocalyx-enclosed micro colony. It serves to protect the bacterium from harmful phagocytes by creating capsules or allowing the bacterium to attach itself to inert surfaces, like teeth or rocks, via biofilms (e.g. Streptococcus pneumoniae attaches itself to either lung cells, prokaryotes, or other bacteria which can fuse their glycocalyxes to envelop the colony).  

                                           Fig: Streptococcus mutans                                  

                                      Fig: Bacillus anthracis

                                               

                                           

                                         Fig: Klebsiella pneumoniae

Advantages Of Capsule:

• Provides protection against temporary dryness by binding with water molecules.
• Blocks attachment  by Bacteriophages. 
• It acts as antiphagocytic  & thus contributes for virulence of bacteria,
• Promotes attachment of bacteria to surface. May promotes stability of bacteria ie prevents aggregation.
• Slime layers causes binding of non pathogenic bacteria to solid forming a thick layer of cells called Biofilms.
• It acts as antigen & helps in identification of disease.