Bacterial capsule

The bacterial capsule is a large structure common to many bacteria. 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.

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.

Most bacterial capsules are composed of polysaccharide, 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.

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. 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. 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. In many bacterial species capsule can have a negative effect on biofilm formation (especially in bacteria with very thick or "hypermucoviscous" capsules). A thick biofilm can interfere with the function of adhesins (like pili or fimbriae) necessary for the initial attachment to a surface, which is a critical step for biofilm formation. As a group where the capsule is present they are known as polysaccharide encapsulated bacteria or encapsulated bacteria.

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

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

The yeast Cryptococcus neoformans, though not a bacterium, has a similar capsule.

Capsules too small to be seen with an ordinary microscope, such as the M protein of Streptococcus pyogenes, are called microcapsules.