Bacterial capsule

A bacterial capsule is a specialized outer layer found on many types of bacteria. It sits outside the cell envelope, which includes the cell membrane and cell wall. This layer is highly organized and stays firmly attached to the cell. Because it is part of the outer envelope, it is a key part of the cell's structure.

Scientists often group capsules and slime layers together under the term glycocalyx. A capsule is a thick, well-organized structure with a semi-rigid border. In contrast, a slime layer is a loose, amorphous secretion that diffuses into the surrounding environment. While both provide protection, their physical structures are quite different.

The composition of these layers varies between different bacterial species. Most capsules are made of polysaccharides, which are complex sugar molecules. However, some bacteria use different materials to build their shields. For example, the bacterium Bacillus anthracis uses poly-D-glutamic acid instead of sugars. Some species, like Bacillus megaterium, create a capsule using both polypeptides and polysaccharides. Others, such as Streptococcus pyogenes and Lactococcus garvieae, produce a capsule made of hyaluronic acid. These chemical differences are important for how the bacteria interact with their environment.

Capsules function as a vital shield for the bacterium. One primary role is protection against toxins and bacterial viruses. They also act as a barrier against hydrophobic toxic materials, such as detergents. Because capsules contain a high water content, they prevent the cell from drying out. Beyond protection, capsules enable adhesion, which allows bacteria to stick to various surfaces. However, very thick or "hypermucoviscous" capsules can actually have a negative effect on biofilm formation. This happens because a thick capsule can interfere with adhesins, like pili or fimbriae, that are needed for initial attachment.

In the context of medicine, the capsule is considered a virulence factor. This means it increases the ability of bacteria to cause disease. One way it does this is by helping the bacteria evade the host immune system. Specifically, the capsule can prevent phagocytosis, which is the process where immune cells like macrophages engulf and destroy bacteria. To overcome this, a host might require a capsule-specific antibody to allow phagocytosis to occur. Because capsules vary so much, immunity to one capsule type does not provide immunity to other types.

Because capsules are so tightly packed, they are difficult to see with standard laboratory stains. Most dyes cannot penetrate the dense structure of the capsule. To visualize them, scientists use a technique called India ink staining. The dark ink is unable to enter the capsule, so the capsule appears as a bright, clear halo around the cell against a dark background. Another method is the Maneval's capsule stain. This uses fuchsine to stain the bacterium pink and Congo red to stain the background. Due to the pH, the Congo red background appears bluish-grey, leaving a clear halo between the pink cell and the background.

There is great diversity among encapsulated bacteria across different groups. Many gram-negative bacteria possess capsules, including Escherichia coli, Neisseria meningitidis, and Klebsiella pneumoniae. Other examples include Haemophilus influenzae and Pseudomonas aeruginosa. Some gram-positive bacteria also have them, such as Streptococcus pneumoniae and Staphylococcus aureus. Streptococcus pneumoniae is particularly complex, possessing at least 91 different capsular serotypes. These serotypes are the specific basis used for developing pneumococcal vaccines. Some capsules, like the M protein in Streptococcus pyogenes, are so small they are called microcapsules.

Understanding these structures is essential for creating effective vaccines. Vaccines can be made using the material from the capsule to protect against organisms like Haemophilus influenzae type b. However, polysaccharides alone are not highly antigenic, especially in young children. This means they do not trigger a strong enough immune response on their own. To solve this, scientists use conjugation. They join the polysaccharides with protein carriers, such as diphtheria toxoid or tetanus toxoid. This process stimulates a much more robust immune response from the body, providing better protection.