Bacterial capsule
The bacterial capsule is a large structure common to many bacteria.[1] It is a polysaccharide layer that lies outside the cell envelope, and is thus deemed part of the outer envelope of a bacterial cell. It is a well-organized layer, not easily washed off, and it can be the cause of various diseases.[2][3]
The capsule—which can be found in both gram negative and gram-positive bacteria—is different from the second lipid membrane – bacterial outer membrane, which contains lipopolysaccharides and lipoproteins and is found only in gram-negative bacteria. When the amorphous viscid secretion (that makes up the capsule) diffuses into the surrounding medium and remains as a loose undemarcated secretion, it is known as a slime layer. Capsule and slime layer are sometimes summarized under the term glycocalyx.
Composition
Most bacterial capsules are composed of
Function
The bacterial capsule serves as a shield, giving protection from toxins, and from drying out. Capsules allow adhesion to surfaces and help enable the bacteria to evade the host immune system.[6] The water content in the capsule gives the protection against drying out. The capsule is considered a
Diversity
The capsule is found most commonly among gram-negative bacteria:
- Escherichia coli (in some strains)
- Neisseria meningitidis[9][10][11]
- Klebsiella pneumoniae[12][13][14]
- Haemophilus influenzae[15]
- Pseudomonas aeruginosa[16]
- Salmonella[17]
- Acinetobacter baumannii[18][19]
However, some gram-positive bacteria may also have a capsule:
- Bacillus megaterium for example, synthesizes a capsule composed of polypeptide and polysaccharides.
- Bacillus anthracis
- Streptococcus pyogenes synthesizes a hyaluronic acid capsule.
- Streptococcus pneumoniae[20] has at least 91 different capsular serotypes.[21] These serotypes are the basis for the pneumococcal vaccines.
- Streptococcus agalactiae produces a polysaccharide capsule of nine antigenic types that all contain sialic acid (Ia, Ib, II, III, IV, V, VI, VII, VIII).
- Staphylococcus epidermidis
- Staphylococcus aureus
- Lactococcus garvieae synthesizes capsular gene clusters and some time synthesizes a hyaluronic acid capsule.[22]
The yeast Cryptococcus neoformans,[23] though not a bacterium, has a similar capsule.[24][25]
Capsules too small to be seen with an ordinary microscope, such as the M protein of Streptococcus pyogenes, are called microcapsules.
Demonstration of capsule
- India ink staining: the capsule appears as a clear halo around the bacterium as the ink can't penetrate the capsule.[26]: 87
- Maneval's capsule stain: the capsule appears as a clear halo between the pink-stained bacterium and the bluish-grey stained background. The background stain is the acidic stain Congo red (which changes color to bluish-grey due to the pH), and the pink stain is fuchsine.
- 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 the basis of quellung reaction.
Use in vaccination
See also
- Bacterial cell structure
- Quellung reaction, a method to visualize capsule under a microscope
References
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