Encapsulated bacteria is the term refers to bacteria that covered externally with a polysaccharide capsule. This capsule can be found in both gram-positive & gam negative bacteria. The capsule is different from the outer membrane of bacteria as in the case of gram-negative bacteria that contain lipopolysaccharide (LPS) and lipoproteins.
Examples of such bacteria include Haemophilus influenza, Streptococcus pneumonia, Klebsiella, Neisseria meningitis & Pseudomonas aeruginosa
A capsule is a gelatinous outer layer that is the gelatinous layer that is decreased by the cell & that surrounds & adheres to the cell wall. Itis an uncommon characteristics of the microorganism
Composition of bacterial capsule
The capsule composition & its thickness varies with individual bacterial species. Generally, it is composed of polysaccharides, polypeptides & glycoproteins. Capsule & slime layer is usually categorized under the category of glycocalyx ( Glycoprotein & glycolipid)
Both capsule & slime layers consist of polysaccharides with the main difference in their attachment with the bacterial cell wall. In capsule, polysaccharides are tightly & firmly attached to the cell wall of bacteria. While in the slime layer, there is a loose attachment of glycoproteins with bacterial cell wall
Pathogenic bacteria with a thick capsule will be more violent than bacteria with no capsule. Capsules serve to increase the virulence of bacteria & protect it from phagocytotic activity. Capsules help bacteria continue their survival & reproduction in the host body by facilitating many ways.
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- First of all, it helps bacteria to resist phagocytosis
- It protects bacteria against desiccation
- It serves as a food reservoir when organic compounds are in excess
- It helps to exclude hydrophobic toxic substance such as detergents
- It also aids bacterial attachment to the surface of solid objects in the aquatic environment or tissue surface in plant & animal host, thus helps to make biofilms
- It protects anaerobes from oxygen toxicity
An organism’s virulence is increased by the presence of a capsule since the capsule protects the organism from phagocytosis by WBCs & inhibits antibody to form a complex. The water-soluble polysaccharide or polypeptide composition of the bacterial capsule makes staining difficult.
Moreover, bacteria become too difficult to detect during antimicrobial susceptibility testing. Gram-positive bacteria that form capsules include bacillus anthracis & streptococcus pneumonia. In comparison, gram-negative bacteria that form capsules include hemophilic influenza & Klebsiella pneumonia.
Vaccination against encapsulated bacteria
Infection that is caused by encapsulated bacteria can be prevented by using vaccination. These vaccines are prepared with capsular polysaccharides either alone or may be conjugated to a protein carrier. A vaccine based on conjugated polysaccharide-protein helps to induce memory response that is true T call dependent. This memory response generates antibodies on exposure to the bacterial strain in children below two years of age. These vaccines, however, don’t work in infants & adults.
How does spleen work to remove encapsulated bacteria?
In the spleen, macrophages are present, known as splenic macrophages. In the case of asplenia ( when the spleen is no longer present in the body due to its removal), Immunoglobin IgG & complimentary proteins like C3b are still there to bound to bacteria; however, that cannot be removed throughout the body from blood circulation due to absence of splenic macrophages. So bacteria are free to cause infection in the host.
Points to be considered
Capsule staining is used when a bacterial infection is not being cleared or responding to antibiotic therapy as expected, so staining of an isolated organism is performed to determine a capsule’s presence.