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[edit]

Most bacterial capsules are composed of polysaccharide,^[4] but some species use other materials, such as poly-D-glutamic acid in Bacillus anthracis. Because most capsules are so tightly packed, they are difficult to stain because most standard stains cannot penetrate the capsule. To visualize encapsulated bacteria using a microscope, a sample is treated with a dark stain, such as India ink. The structure of the capsule prevents the stain from penetrating the cell. When viewed, bacterial capsules appear as a bright halo around the cell on a dark background.^[5]

Function[edit]

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 virulence factor because it enhances the ability of bacteria to cause disease (e.g. prevents phagocytosis). The capsule can protect cells from engulfment by eukaryotic cells, such as macrophages.^[7] A capsule-specific antibody may be required for phagocytosis to occur. They also exclude bacterial viruses and most hydrophobic toxic materials such as detergents.^{[citation needed]} Immunity to one capsule type does not result in immunity to the other types. Capsules also help cells adhere to surfaces. As a group where the capsule is present they are known as polysaccharide encapsulated bacteria or encapsulated bacteria.^[8]

Diversity[edit]

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[edit]

1. India ink staining: the capsule appears as a clear halo around the bacterium as the ink can't penetrate the capsule.^[26]: 87
2. 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.
3. 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[edit]

Vaccination using capsular material is effective against some organisms (e.g., H. influenzae type b,^[27][28] S. pneumoniae, and N. meningitidis^[29]). However, polysaccharides are not highly antigenic, especially in children, so many capsular vaccines contain polysaccharides conjugated with protein carriers, such as the tetanus toxoid or diphtheria toxoid. This stimulates a much more robust immune response.^[30]

See also[edit]

• Bacterial cell structure
• Quellung reaction, a method to visualize capsule under a microscope

References[edit]