A capsule is a large, well‑organised layer that surrounds the cell envelope of many bacteria. It is usually made of polysaccharide although some species, such as Bacillus anthracis, produce a polypeptide capsule. Capsules adhere tightly to the cell surface and are not easily washed off, distinguishing them from the looser slime layers of the glycocalyx.
Explanation
Capsules serve as a protective barrier between bacteria and their environment. They shield cells from desiccation, protect against predation by protozoa and resist uptake and destruction by phagocytic immune cells. The dense matrix can also block antibiotics and detergents. Encapsulation aids adhesion to host tissues and inert surfaces, facilitating the formation of biofilms. Most capsules are composed of polysaccharide chains, but a minority use other polymers; for example, the capsule of Bacillus anthracis consists of poly‑D‑glutamic acid. Because capsule material excludes dyes such as India ink, encapsulated bacteria appear as bright halos under a microscope; special staining methods such as negative staining or the Quellung reaction are used to visualize them. In pathogenic species, the capsule is a critical virulence factor. Streptococcus pneumoniae, Neisseria meningitidis and Haemophilus influenzae type b all rely on their capsules to evade complement and phagocytosis in the host. Capsules are antigenic, and the chemical composition of capsular polysaccharides defines serotypes used in epidemiology.
Clinical and Environmental Significance
Encapsulated bacteria are associated with serious infections such as pneumonia, meningitis and septicemia. Vaccines against pneumococcal and meningococcal disease are based on purified capsular polysaccharides conjugated to proteins, stimulating immune responses against the capsule. In soil and aquatic environments, capsules help bacteria retain moisture and nutrients and protect against bacteriophages. Capsule production can vary with environmental conditions, and the ability to produce a capsule often enhances survival in both hosts and nature.
Capsules illustrate how a simple extra layer can dramatically influence bacterial interactions with their surroundings. Understanding capsule composition and function informs vaccine development and strategies to prevent biofilm formation.
Related Terms: glycocalyx, slime layer, virulence factor, biofilm, polysaccharide