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Bacterial capsule
The cell capsule is a very large structure of some prokaryotic cells, such as bacterial cells. It is a polysaccharide layer that lies outside the cell envelope of bacteria, 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 bacteria and Gram-positive bacteria—should not be confused with the second lipid membrane (or 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 slime layer.
== Composition ==
It usually consists of polysaccharides, but can be composed of other materials (e.g., polypeptide(D-glutamic acid) in ''B. anthracis''). Because most capsules are so tightly packed, they are difficult to stain using standard stains because most stains cannot 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, or halo, around the cell.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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