Bacterial Capsules and Evasion of Immune Responses


The bacterial surface contains various structures that are capable of activating the host defences and consequently inducing host immune responses. Bacterial capsules are one of the most external structures on the bacterial surface, which may completely surround all the antigenic molecules or may be coexpressed with other bacterial antigens. They are involved in a wide range of biological processes, such as prevention of desiccation, adherence and resistance to nonspecific and specific host immunity.

Gram‐negative and Gram‐positive capsular polysaccharides contribute to the bacterial resistance of host immune responses by different mechanisms. Usually, capsular polysaccharides that mask the underlying cell surface structures activate weakly or not at all the immune system, whereas bacterial capsules coexpressed with other bacterial antigens activate the immune system but mask opsonines and prevent complement attack complex formation, as well as phagocytosis.

Key Concepts

  • Bacterial capsules completely surround bacterial cell and enhance the ability of bacteria to cause disease.
  • Polysaccharides are polymeric carbohydrate structures, formed of one or more sugar residues repeating units joined together by glycosidic bonds.
  • Antigenic variation modifies microorganism's surface structures and allows them to overcome the immune response.
  • Phagocytosis is the process by which certain cells called phagocytes engulf and digest a solid particle as bacteria.
  • The complement system is a part of the innate immune system and consists of a number of plasma small proteins which circulating as inactive precursors and after their activation cascade are able to serve as opsonins.

Keywords: bacterial capsules; polysaccharides; antigenic variation; avoidance of phagocytosis; complement resistance

Figure 1. Models of capsule assembly. (a) Biosynthesis, export and translocation pathway for group 1 capsules. The capsular linkage to the lipid A core in the outer membrane is only presented in KLPS capsules. (b) Biosynthesis, export and translocation pathway for group 2 capsules.
Figure 2. Immunoelectromicroscopy of whole cells of Aeromonas hydrophila serogroup O:11 (O+:S+) with (a) anti‐S layer serum and (b) anti‐O:11 lipopolysaccharide (LPS) serum, and of the isogenic S layer mutant (O+:S) with (c) anti‐S layer serum and (d) anti‐O:11 LPS serum. Bar, 0.4 mm.
Figure 3. Scheme of complement activation and evasion mechanisms associated with the bacterial surface structures. LPS, lipopolysaccharide; MBL, mannose‐binding lectin and MAC, membrane attack complex.


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Merino, Susana, and Tomás, Juán M(Jul 2015) Bacterial Capsules and Evasion of Immune Responses. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000957.pub4]