The bacterial capsule is considered as the most important virulence factor among all factors that are present within the bacteria. 

Some examples of a capsulated strain are Neisseria meningitidis, Streptococcus pneumoniae, Klebsiella pneumoniae, Haemophilus influenzae, Escherichia coli, and group B streptococci al of them are virulent. While the Nonencapsulated mutated strain of these organisms is avirulent.

What is a bacterial capsule?

Capsules are a types of Gelatinous covering layer of entire bacterium which are well organized and not easily washed off.

• In most cases, the capsule is made of polysaccharides, but some of them are constructed of other materials such as Bacillus anthracis contain a proteinaceous capsule which is made of poly D-glutamic acid.
• It is lies outside the cell envelope.
• Both gram-negative and gram-positive bacteria contain capsules. It differs from the bacterial outer membrane which is made of lipopolysaccharides and lipoproteins and mainly found in gram-negative bacteria.
• Capsules can be observed under a light microscope by applying negative stains or specific capsule stains on them.
• It protect the pathogenic bacteria from phagocytosis by host phagocytes. For example, the non-capsulated form of Streptococcus pneumoniae is phagocytosed easily and does not cause disease. While the capsulated form quickly kills mice.
• Capsules also contain water which protects the cells from desiccation.
• They exclude viruses and most hydrophobic toxic materials such as detergents.
• The Slime layer is differe from a capsule it is a zone of diffuse, unorganized material that can be removed easily. It is normally made of polysaccharides still is not as easily seen by light microscopy.

Compositions of capsules 

• Most of the bacterial Capsules are made of polysaccharides but in some species it is made of other materials, For example, the capsule of Bacillus anthracis is made of poly-D-glutamic acid.
• The capsules are tightly packed layers, therefore it is difficult to stain them because most standard stains cannot penetrate the capsule. Therefore the capsule is treated with a specialized dark stain called India ink and then observed under a microscope.

Function of Bacterial Capsule

1. Virulence Factor: Capsule is an important virulence factor for pathogenic bacteria. They are anti-phagocytic, which means they protect the bacterial cell from the phagocytosis. The negative charge and smooth nature of the capsule help to prevent the adhering to and engulfment of bacterial cells by the phagocytes. Non-capsulated bacteria has no ability to cause disease.
2. Prevent the cell from desiccation and drying: The bacterial capsule contains a significant amount of water which prevents the desiccation of the cell and protects the cell from benign dry.
3. Repulsion: Those bacterial cells contain the same charged capsule they repel each other.
4. Protection: Capsule defends the cell from physical pressure and chemical damage.
5. Adhesion/Attachment: In pathogenic bacteria, the capsule helps them to get attached to a solid surface or on a host body surface. For example, Streptococcus mutants get attached to the teeth surface with the help of a capsule and causes dental carries.
6. Prevent attachment: They also prevent the bacteriophage from being attached to the cell surface.
7. Bacterial Identification: Capsulated bacterial cells can be identified by using specific antiserum against capsular polysaccharides such as Quellung reaction. It also helps in characterization of bacterial colonies. The capsule containing bacteria forms smooth colonies while non-capsulated bacteria forms rough (R) colonies.
8. Nutrition supply: When the nutrient supply is low in the cell it functions as a source of nutrition.
9. Vaccine development: In certain vaccine bacterial Capsular polysaccharides is used as an antigen such as Polyvalent (23 serotypes) polysaccharide vaccine of Streptococcus pneumoniae capsule, Polyvalent (4 serotypes) vaccine of Neisseria meningitidis capsule.
10. Capsule protects the anaerobic bacteria from oxygen toxicity.
11. Capsule prevents complement-mediated bacterial cell lysis.

Types of capsule

There are two types of capsule such as

Macro-capsule

• The thickness of the Macro-capsule is 0.2µm or more.
• Macro-capsule are visible under a light microscope.

Microcapsule

• The thickness of the Microcapsule is 0.2µm.
• Macro-capsule is visible under Electron microscope.

Demonstration of capsule

India ink staining 

Capsule can be visualized by using India ink staining. In this staining method, the capsule formed a clear halo around the bacterium as the ink can’t penetrate the capsule.

Maneval’s capsule stain

In this staining method, the capsule develops a clear halo between the pink-stained (acid fuschin) bacterium and the bluish-grey stained background (acidic stain Congo red).

Serological methods

In this method, a specific anticapsular serum is mixed with the capsule because of its antigenic nature. Under the microscope, this capsule appears ‘swollen’ due to an improvement in its refractivity. This method is also known as quellung reaction.

Differences between Bacterial Capsule and Slime layer

Characters	Capsule	Slime Layer
Definition	A glycocalyx layer, consisting of firmly associated polysaccharide molecules with the cell wall is called the capsule.	A glycocalyx layer that consists of loosely associated glycoprotein molecules is called the slime layer.
Binding	It is tightly bound to the cell wall.	It is loosely bound to the cell wall.
Organization	well-organized layer	unorganized layer
Washed off	it is difficult to be washed off.	it can be easily washed off.
Role	Acts as a virulence factor that helps to evade phagocytosis.	It mainly aids in the adherence. It also protects the cell from dehydration and nutrient loss.
Thickness	Thicker than the slime layer	It is a thin glycocalyx layer.
Composition	Composed of polysaccharides.	composed of exopolysaccharides, glycoproteins, and glycolipids.

Example of Capsule

Gram-Positive Bacteria	Gram-negative Bacteria
Bacillus anthracis	Salmonella
Streptococcus pneumoniae	Pseudomonas aeruginosa
Streptococcus pyogenes	Neisseria meningitidis
Streptococcus agalactiae	Haemophilus influenzae
Bacillus megaterium	Klebsiella pneumoniae
Staphylococcus epidermidis	Escherichia coli (in some strains)

Cryptococcus neoformans is a yeast which contain Polysaccharide capsule.

References

• https://www.britannica.com/science/bacteria/Capsules-and-slime-layers
• https://www.atmph.org/article.asp?issn=1755-6783;year=2015;volume=8;issue=4;spage=151;epage=153;aulast=Kandi;type=3
• http://textbookofbacteriology.net/structure_4.html
• https://microbeonline.com/bacterial-capsule-structure-and-importance-and-examples-of-capsulated-bacteria/
• https://www.onlinebiologynotes.com/bacterial-capsule-structure-function-examples-capsulated-bacteria/