Most bacterial capsules are composed of polysaccharide,[4] but some species use other materials, such as poly-D-glutamic acid in Bacillus anthracis.
To visualize encapsulated bacteria using a microscope, a sample is treated with a dark stain, such as India ink.
When viewed,JB bacterial capsules appear as a bright halo around the cell on a dark background.
Capsules allow adhesion to surfaces and help enable the bacteria to evade the host immune system.
The capsule is considered a virulence factor because it enhances the ability of bacteria to cause disease (e.g. prevents phagocytosis).
The outer red layer in this diagram is the capsule, which is distinct from the cell envelope. This bacterium is
gram-positive
, as its cell envelope comprises a single
cell membrane
(orange) and a thick
peptidoglycan
-containing cell wall (purple).
A bacterial capsule has a semi-rigid border that follows the contour of the cell. The capsule excludes India Ink when dyed. A slime layer is a non-rigid matrix that is easily deformed and is not able to exclude India Ink. Biofilms are composed of many cells and their outer barriers. The primary functions of both capsules and slime layers are for protection and adhesion.
