Bacterial capsule

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Capsules are a common feature of many bacteria. The capsule is a tough, organized layer made mostly of sugars that sits outside the cell wall, forming part of the outer envelope. It is not easily washed away and can help bacteria cause disease. Capsules are found in both Gram-negative and Gram-positive bacteria; they are different from the outer membrane found in Gram-negative bacteria, which contains lipopolysaccharides and lipoproteins. When capsule material diffuses into the surroundings as a loose layer, it is called a slime layer; together these forms are referred to as the glycocalyx. Most capsules are polysaccharides, but some bacteria use other substances, such as poly-D-glutamic acid in Bacillus anthracis. Capsules are very hard to stain, so special methods such as India ink staining are used to visualize them under a microscope, which shows a bright halo around the cell. The capsule acts as a shield, protecting bacteria from drying out and from certain toxins, and it helps them stick to surfaces. It is also a key virulence factor because it helps bacteria evade the host immune system and can prevent engulfment by immune cells like macrophages. Antibodies against the capsule can boost phagocytosis, but immunity to one capsule type does not protect against others. Capsules can influence biofilm formation; very thick capsules can sometimes hinder the initial attachment steps needed for biofilms. Many bacteria with capsules are called polysaccharide-encapsulated bacteria. Vaccines targeting capsule material work well against some organisms such as Haemophilus influenzae type b, Streptococcus pneumoniae, and Neisseria meningitidis, but capsule polysaccharides are not very antigenic on their own, so they are often connected to a protein carrier to improve the immune response.