The cell capsule is a very large structure of some prokaryotic cells, such as bacterial cells. It is a layer

Layer

Layer may refer to:* A layer of archaeological deposits in an excavation* A layer hen, a hen raised to produce eggs* Stratum, a layer of rock or soil with internally consistent characteristics...

 that lies outside the cell wall

Cell wall

The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...

 of bacteria. It is a well organized layer, not easily washed off, and it can be the cause of various diseases.

Composition

It usually consists of polysaccharides, but can be composed of other materials (e.g., polypeptide in B. anthracis). Because most capsules are water soluble , they are difficult to stain using standard stains because most stains do not adhere to the capsule. For examination under the microscope, the bacteria and their background are stained darker than the capsule, which doesn't stain. When viewed, bacterial cells as well as the surface they are on, are stained dark, while the capsule remains pale or colorless and appears as a ring around the cell.

Function

The capsule is considered a virulence factor

Virulence factor

Virulence factors are molecules expressed and secreted by pathogens that enable them to achieve the following:* colonization of a niche in the host...

 because it enhances the ability of bacteria to cause disease (i.e. prevents phagocytosis). The capsule is slippery and fragile, so when a phagocyte

Phagocyte

Phagocytes are the white blood cells that protect the body by ingesting harmful foreign particles, bacteria, and dead or dying cells. Their name comes from the Greek phagein, "to eat" or "devour", and "-cyte", the suffix in biology denoting "cell", from the Greek kutos, "hollow vessel". They are...

 tries to phagocytose the bacteria, it can slip away. A capsule-specific antibody may be required for phagocytosis to occur. Capsules also contain water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 which protects bacteria against desiccation

Desiccation

Desiccation is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic substance that induces or sustains such a state in its local vicinity in a moderately sealed container.-Science:...

. They also exclude bacterial viruses and most hydrophobic toxic materials such as detergents. There are 14 different capsule types, which each impart their own specific antigenicity. Immunity to one capsule type does not result in immunity to the other types. Capsules also help cells adhere to surfaces.

Diversity

The capsule is found most commonly among Gram-negative bacteria:

• Escherichia coli
  Escherichia coli
  Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...
  
  
• Klebsiella pneumoniae
  Klebsiella pneumoniae
  Klebsiella pneumoniae is a Gram-negative, non-motile, encapsulated, lactose fermenting, facultative anaerobic, rod shaped bacterium found in the normal flora of the mouth, skin, and intestines....
  
  
• Haemophilus influenzae
  Haemophilus influenzae
  Haemophilus influenzae, formerly called Pfeiffer's bacillus or Bacillus influenzae, Gram-negative, rod-shaped bacterium first described in 1892 by Richard Pfeiffer during an influenza pandemic. A member of the Pasteurellaceae family, it is generally aerobic, but can grow as a facultative anaerobe. H...
  
  

However, some Gram-positive bacteria may also have a capsule:

• Bacillus megaterium
  Bacillus megaterium
  Bacillus megaterium is a rod-shaped, Gram-positive, endospore forming, species of bacteria used as a soil inoculant in agriculture and horticulture.Bacterium is arranged into the streptobacillus form....
  
  for example, synthesizes a capsule composed of polypeptide and polysaccharides.
• Streptococcus pyogenes
  Streptococcus pyogenes
  Streptococcus pyogenes is a spherical, Gram-positive bacterium that is the cause of group A streptococcal infections. S. pyogenes displays streptococcal group A antigen on its cell wall. S...
  
  synthesizes a hyaluronic acid capsule.

- Streptococcus agalactiae produces a polysaccharide capsule of nine antigenic types that all contain sialic acid (Ia, Ib, II, III, IV, V, VI, VII, VIII).

The yeast

Yeast

Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with 1,500 species currently described estimated to be only 1% of all fungal species. Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding...

 Cryptococcus neoformans

Cryptococcus neoformans

Cryptococcus neoformans is an encapsulated yeast that can live in both plants and animals. Its teleomorph is Filobasidiella neoformans, a filamentous fungus belonging to the class Tremellomycetes. It is often found in pigeon excrement....

, though not a bacterium, has a similar capsule.

Capsules too small to be seen with an ordinary microscope, such as the M protein

M protein

M protein may refer to:* M protein ', a virulence factor of the bacterium Streptococcus pyogenes* Another name for paraprotein, an abnormal protein in the urine or blood, often seen in multiple myeloma or MGUS...

 of Streptococcus pyogenes

Streptococcus pyogenes

Streptococcus pyogenes is a spherical, Gram-positive bacterium that is the cause of group A streptococcal infections. S. pyogenes displays streptococcal group A antigen on its cell wall. S...

, are called microcapsules.

Mnemonic

A common mnemonic used to remember some encapsulated pathogens is:

"Even Some Killers Have Pretty Nice Capsules"

Escherichia coli

Escherichia coli

Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...

, Streptococcus pneumoniae

Streptococcus pneumoniae

Streptococcus pneumoniae, or pneumococcus, is Gram-positive, alpha-hemolytic, aerotolerant anaerobic member of the genus Streptococcus. A significant human pathogenic bacterium, S...

, Klebsiella pneumoniae

Klebsiella pneumoniae

Klebsiella pneumoniae is a Gram-negative, non-motile, encapsulated, lactose fermenting, facultative anaerobic, rod shaped bacterium found in the normal flora of the mouth, skin, and intestines....

, Haemophilus influenzae

Haemophilus influenzae

Haemophilus influenzae, formerly called Pfeiffer's bacillus or Bacillus influenzae, Gram-negative, rod-shaped bacterium first described in 1892 by Richard Pfeiffer during an influenza pandemic. A member of the Pasteurellaceae family, it is generally aerobic, but can grow as a facultative anaerobe. H...

, Pseudomonas aeruginosa

Pseudomonas aeruginosa

Pseudomonas aeruginosa is a common bacterium that can cause disease in animals, including humans. It is found in soil, water, skin flora, and most man-made environments throughout the world. It thrives not only in normal atmospheres, but also in hypoxic atmospheres, and has, thus, colonized many...

, Neisseria meningitidis

Neisseria meningitidis

Neisseria meningitidis, often referred to as meningococcus, is a bacterium that can cause meningitis and other forms of meningococcal disease such as meningococcemia, a life threatening sepsis. N. meningitidis is a major cause of morbidity and mortality during childhood in industrialized countries...

, and the yeast

Yeast

Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with 1,500 species currently described estimated to be only 1% of all fungal species. Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding...

 Cryptococcus neoformans.

Demonstration of Capsule

1. India ink staining: the capsule appears as a clear halo around the bacterium as the ink can't penetrate the capusule.
2. Serological methods: Capsular material is antigenic and can be demonstrated by mixing it with a specific anticapsular serum. When examined under the microscope, the capsule appears 'swollen' due to an increase in its refractivity. This phenomenon is called as capsule swelling reaction or Quellung phenomenon.
3. Special capsule staining: These techniques employ copper salts as mordants for staining of the capsule.

Use in vaccination

Vaccination

Vaccination

Vaccination is the administration of antigenic material to stimulate the immune system of an individual to develop adaptive immunity to a disease. Vaccines can prevent or ameliorate the effects of infection by many pathogens...

 using capsular material is effective against some organisms (e.g., H. influenzae type b and S. pneumoniae).

See also

• Bacterial cell structure
  Bacterial cell structure
  Bacteria, despite their simplicity, contain a well-developed cell structure which is responsible for many of their unique biological properties. Many structural features are unique to bacteria and are not found among archaea or eukaryotes...
  
  
• Quellung reaction
  Quellung reaction
  The Quellung reaction is a biochemical reaction in which antibodies bind to the Bacterial capsule of Streptococcus pneumoniae, Klebsiella pneumoniae, Neisseria meningitidis and Haemophilus influenzae and thus allow them to be visualized under a microscope...
  
  , a method to visualize capsule under a microscope