Glossary Item
Capsulated refers to bacteria or other cells that are enclosed by a capsule, a dense and well-defined layer outside the cell envelope. This protective covering is typically made of polysaccharides or sometimes polypeptides and adheres firmly to the cell surface, distinguishing capsulated organisms from those with loose slime layers.
Explanation
Encapsulated bacteria have evolved a thick external layer that confers several advantages. Capsules shield against desiccation, harmful chemicals and immune defenses. The polysaccharide matrix impedes phagocytosis by host immune cells and masks underlying antigens, making it harder for antibodies to bind. Some capsules also inhibit complement activation. Composition varies between species; most consist of repeating sugars such as glucose, galactose or rhamnose, while a few, like Bacillus anthracis, use poly-d-glutamic acid. Capsules are highly hydrated and can help cells retain water and nutrients. Laboratory staining techniques such as negative staining with India ink or the Quellung reaction are used to visualize and type these structures. Serotyping based on capsular antigens is important in epidemiology and vaccine design because specific capsule types correlate with virulence and disease patterns.
Notable Encapsulated Organisms
Several medically important bacteria owe much of their pathogenicity to their capsules. Streptococcus pneumoniae uses its capsule to resist phagocytosis and cause pneumonia, meningitis and otitis media. Neisseria meningitidis and Haemophilus influenzae type b have capsular polysaccharides that enable them to invade the bloodstream and cerebrospinal fluid. Klebsiella pneumoniae produces a prominent mucoid capsule that contributes to severe pneumonia and urinary tract infections. Bacillus anthracis, the agent of anthrax, has a poly-d-glutamic acid capsule that prevents detection by the immune system. Vaccines targeting these capsular antigens, such as the pneumococcal conjugate vaccine, provide protection by inducing antibodies that promote opsonization and clearance.
Capsulated organisms illustrate how a surface layer can significantly influence interactions with hosts and the environment. Understanding the composition and function of capsules informs clinical diagnosis, vaccine development and strategies to prevent biofilm formation.
Related Terms: capsule, glycocalyx, virulence factor, biofilm, polysaccharide