GLYCOCALYX (Capsule and Slime Layer)

 GLYCOCALYX

·   Some bacterial cells have a gelatinous, sticky substance that surrounds the outside of the cell. This substance is known as a Glycocalyx (Plural: Glycocalyces), which literally means “Sweet cup” or “Sugar coat”.

·  Glycocalyx may be composed of Polysaccharides or Polypeptides, or both Polysaccharides and Polypeptides. Its chemical composition varies widely with the species. These chemicals are produced inside the cell and are extruded onto the cell’s surface.

·    When the Glycocalyx of a bacterium is composed of organized repeating units of organic chemicals firmly (strongly) attached to the cell’s surface, the Glycocalyx is called a Capsule. In contrast, a loose, water-soluble Glycocalyx is called a Slime layer.

·  Glycocalyx is the currently accepted term used to refer to all Polysaccharide-containing substances found external to the cell wall, from the thickest Capsules to the thinnest Slime layers.

·       Capsules is not present in all bacteria but all bacteria have at least a thin Slime layer.

CAPSULE

·      The Glycocalyx is considered a Capsule when the Polysaccharides are more firmly (strongly) attached to the cell wall.

·       Capsules have a gummy, slimmy and sticky consistency layer which is present outside the Cell wall of bacteria.

·      Generally, bacterial capsules are made of polymer of Polysaccharides but some bacteria such as Bacillus anthracis use other materials such as Poly-D-glutamic acid.

·     The thickness of the Capsule is ranging from 0.1 µ to 10 µ. Based on its size, Capsules are classified into two types. They are

a)     Macrocapsules (Thickness more than 0.2 µ)

b)     Microcapsules (Thickness less than 0.2 µ)

·       Capsule is thicker and highly rigid than Slime layer.

·       Capsules are Hydrophilic in nature.

·       Capsule is antigenic in nature and involved in Bacterial virulence.

·       Capsule is an Organized layer.

·       Capsule is firmly (strongly) attached to the Cell wall and not removed easily from Cell wall.

·       Bacteria with Capsule produce mucoid colonies in Agar medium.

·   Capsules can be visualized by Negative staining techniques using Nigrosine or India ink under the microscope. The capsule will appear as clear halos surrounding the bacterial cells. Some examples of encapsulating bacteria are Klebsiella pneumoniae and Streptococcus pneumoniae.

Functions of Capsules

·      The Adhesive power (Attachment) of Capsules is also a major factor in the initiation of some bacterial diseases (Bacterial virulence).

·      Capsules prevents the bacteria from Phagocytosis process (engulfment by WBCs) and act as a Virulence factor of Bacteria.

·       Capsules offers protection to Bacteria from a variety of different threats such as

a)     Desiccation (Extreme dryness)

b)     Hydrophobic toxic materials (Detergents)

c)     Bacterial viruses (Bacteriophages)

SLIME LAYER

·       A Glycocalyx is considered a Slime layer when the Glycoprotein molecules are loosely associated with the cell wall.

·       Slime layer is composed of Exopolysaccharides, Glycoproteins and Glycolipids.

·       Slime layer is thinner and less rigid than Capsule.

·       Slime layer is an Unorganized layer.

·       Slime layer loosely adheres to the Cell wall and easily removable by Centrifugation.

Functions of Slime layer

·       Slime layer offers protection to Bacteria from various threats such as

a)     Desiccation (Extreme dryness)

b)     Loss of nutrients

c)     Antibiotics

d)     Cell wall degrading enzymes

·       Slime layer can also allow for Adherence (attachment) of the bacteria to the Solid surfaces.