Capsule in bacterial cell helps in preventing the cell from desiccation and drying as capsular polysaccharide binds significant amount of water, thus making cells resistant to drying. It also helps in protection from mechanical injury, temperature etc. They also aid in the attachment of microbial cell to the various surfaces E.g. Streptococcus mutants that cause dental carries attach on teeth surface by its capsule.
Resisting the phagocytosis of microbial cells by macrophages and other phagocytic cells is also an important feature of capsular protection. Some of the capsulated cells repel each other due to the charge present in capsules. Attachment of bacteriophages are prevented as well as facilitated depending upon circumstances as capsules are also noted as attachment sites for some bacteriophages.
Capsules are also the virulence determinants of the bacterial cells as they are anti-phagocytic thus, limits the phagocytosis. The smooth nature and negative charge of the capsule prevents the phagocyte from adhering to and engulfing the bacterial cell. If a pathogenic bacteria lose capsule (by mutation), they won’t be able to cause disease (i.e. loses disease causing capacity).
Bacterial capsule saves engulfed bacteria from action of neutrophils as they prevents the direct access of lysosome contents with the bacterial cell, preventing their killing. They also prevents complement-mediated bacterial cell lysis.
Some major Functions of Capsule are:
- Capsule protects bacterial cell against harmful agents such as chemicals, toxin or antibiotics as they act as barrier for their entry.
- Protection from injury & attachment of bacteriophage. [i.e. they may block the attachment of bacteriophage, thus lysis of bacteria can be prevented by preventing attachment of bacteriophage. Bacteriophage can’t get receptor on cell wall to attach].
- Capsule increases virulence. It resists phagocytosis; which is a defensive process in which WBC or macrophages digest foreign bodies ie; it has anti-phagocytic features.
- Due to the presence of capsule lysozyme may not degrade cell. These organisms can attach to host cell due to its viscous nature and increases pathogenesis.
- The capsule acts as an antigenic agent or antigen which induced the formation of antibodies in the host.
- It helps to attach on smooth solid surfaces as well as various surfaces in its natural environments in order to survive.eg Streptococcus mutans (imp. cause of dental caries) adhere to the smooth surface of teeth by glycocalyx.
- Through attachment bacteria can survive in natural environments due to its sticky property (viscous). After attachment they can grow in diverse surfaces such as plant roots, human teeth, tissues rocks in fast moving streams, medical implants, water pipes etc.
- Use as a source of nutrient. [ mutans may use its capsule as a source of nutrition by breaking it down & utilizing the sugars when energy stores are low.]
- A glycocalyx also protects cell against dehydration and its viscosity may inhibit the movement of nutrients out of the cell. i.e; it prevents the loss of nutrients from the cell.
- Capsule contains water which protects bacteria against desiccation and drying.
Besides, these capsules can be helpful in following:
Identification of bacteria
Capsules can be helpful in identification of bacteria:
- Using specific antiserum against capsular polysaccharide. E.g. Quellung Reaction – A biochemical reaction in which antibodies binds in bacterial capsule thus, detecting the presence of particular bacterium. This test is gold standard for serotyping Streptococcus pneumoniae and helps in detection of number of capsulated microbes such as Haemophilus influenza, Neisseria meningitides etc.
- Colony characteristics in culture media: Bacteria with capsules form smooth (S) colonies while those without capsules form rough (R) colonies. A given bacterial species may undergo a phenomenon called S-R variation whereby the cell loses the ability to form a capsule. Some capsules are very large and absorb water; bacteria with this type of capsule (e.g., Klebsiella pneumoniae) form mucoid (M) colonies.
Capsular polysaccharides can be used as the antigens in certain vaccines. For examples:
- Polyvalent (23 serotypes) polysaccharide vaccine of Streptococcus pneumoniae
- Polyvalent (4 serotypes) vaccine of Neisseria meningitidis
- A monovalent vaccine made up of capsular material from Haemophilus influenzae.
Initiation of infection
Capsules helps the organism to adhere to host cells. It also facilitates and maintains bacterial colonization of biologic (e.g. teeth) and inanimate (e.g. prosthetic heart valves) surfaces through formation of biofilms.
Download Link: Bacterial Capsule (Functions). Pdf