Glycocalyx

Definition
The glycocalyx is the outermost layer of a bacterial cell. It lies outside the cell wall and is considered extra-cellular, which means that it is not an integral part of the cell. The clycocalyx a non-living and non-essential portion of the cell, similar to the hair on humans.

There are two basic types of glycocalyx are the slime layer and a capsule.

Slime layer
The slime layer is a thin, sticky layer that serves to transport bacteria but is easily washed away. Staphylococcus aureus has a slime layer.

Capsule
A capsule is a thicker and larger glycocalyx that is not easily removed. Organisms that have capsules include:
 * 1) Klebsiella pneumoniae
 * 2) Streptococcus pneumoniae
 * 3) Streptococcus pyogenes
 * 4) Bacillus anthracis
 * 5) Hamaphilus influenza

Chemistry
The glycocalyx is made of proteins or polysaccharides or a combination of the two. Typically a capsule looks like mucous because of the consistency of the starches and proteins.

Genetic
The genes for expressing a slime layer or capsule must be in a bacteria for it to be able to produce a glycocalyx. Some bacteria lack the gene for either.

Environmental
The bacteria is more likely to produce a glycocalyx if it is growing on a suitable, nutrient rich medium. The Klebsiella pneumoniae we grow in lab has a small capsule because it is not in its natural environment, while a sample taken directly from a host will have a larger and more defined capsule.

Colonial Appearance
The appearance of a macroscopic colony, usually seen on an agar plate.

Slime layers look wet and shiny, almost like shiny pants. Observable in Staphylococcus colonies.

Capsules look like mucus on the plate, almost as if someone sneezed on it. Klebsiella pneumoniae will have this appearance.

Microscopic Appearance
The glycocalyx is made of proteins and polysacchardes that repel conventional dyes. In order to see the capsules, India ink (carbon particles in suspension) is used to stain the background.



The organism is first introduced to a pool of dye, which is then physically spread across the glass slide. A crystal violet dye is then used to stain the bacteria. This way, the capsule is not dyed, but everything else is. The capsule will look like a halo around the microorganism.

Protection
Capsules help form microbe colonies because bacteria stick together in groups. They form biofilms that help provide protection, especially for the bacteria that are on the inside. These biofilms can protect the bacteria from drying, chemicals, and phagocytosis.

Streptococcus pyogenes will produce a high amount of hyluronic acid in its capsule to disguise itself as human cells to avoid being attacked by macrophages.

Adhesion and Transport
The capsule and slime layers allow bacteria to adhere to inanimate objects and hosts alike. If the organism has a capsule, it is virulent, or pathogenic. Likewise, if the organism has no capsule, it is considered avirulent or non-pathogenic.