Cell Wall Components and Antigenic Structure - Streptococcus pneumonia

Cell Wall Components and Antigenic Structure

The cell wall of S. pneumoniae consists of the following components (Fig. 25-2):

·           Capsule: The polysaccharide capsule surrounds the cellwall. The capsule is essential for virulence, its role being to protect the cocci from phagocytosis.

·           Cell wall: It consists of peptidoglycan, teichoic acid, andproteins.

·           Peptidoglycan: Peptidoglycan confers rigidity to the cellwall.

·           Teichoic acid: The teichoic acid present in cell wall is of twotypes: (a) C polysaccharide and (b) Forssman or F antigen.

      C polysaccharide: It is present on the surface of cellwall. The exposed part of teichoic acid, which is linked to peptidoglycan layer and extends through the capsule, is known as C polysaccharide or C antigen. This is a spe-cies-specific antigen and in no way related to the group-specific C carbohydrate antigen found in beta-hemolytic streptococci. The C polysaccharide present in the cell wall of pneumococci precipitates with C reactive protein (CRP), a serum globulin.

      C-reactive protein: The CRP is an acute-phase substancesynthesized in the liver. It is not produced specifically against C antigen of pneumococci. It is present in low concentrations in healthy individuals, but the concen-tration increases in inflammation, malignancies, and bacterial infections. CRP increases during pneumonia and disappears during convalescence, hence can be used as a prognostic tool. CRP is used as an index of the treat-ment in rheumatic fever and certain other conditions.

        F antigen: F antigen is the other type of teichoic acid, whichis covalently bound to the lipids in cytoplasmic membrane. It is so called because it cross-reacts with the Forssman sur-face antigen of the mammalian cells. F proteins mediate the binding of pneumococci to the host cell surface.

·           M protein: M protein is a type-specific protein similarto the M protein of Streptococcus pyogenes, but is immu-nologically distinct. Antibodies against M proteins are not protective, as they do not inhibit phagocytosis of pneumococci.

◗ Serotyping of pneumococci

Capsular polysaccharide is antigenic in humans and rabbits. The capsular polysaccharide is also called specific soluble sub-stance (SSS) as it diffuses into the culture medium or infective pus or host tissues. S. pneumoniae are classified into more than 90 different serotypes (1–90) based on the antigenic structure of the capsular polysaccharide. Of these, only 23 serotypes are associated with pneumococcal diseases. Serotyping of S. pneu-moniae is not carried out routinely and is done only for epide-miological studies. There are three typing methods:

·           Agglutination of the bacteria with type-specific antiserum by co-agglutination (Co-A) test. The Co-A test is a rapid slide agglutination test, uses less antiserum, and is in complete agreement with Quellung test.

·           Precipitation of the specific soluble substances with specific serum by counter-current immunoelectrophoresis (CIEP). This is also a rapid test used to serotype pneumococci.

·           Capsular swelling reaction with type-specific antiserum by “Quellung reaction”. This is called capsular swelling reac-tion due to swelling (Latin quellung: swelling) of the capsule observed in this test. This reaction was first described by Neufeld in the year 1902. In this test, a drop of type-specific antiserum is added to a drop of suspension of pneumococci on a glass slide along with a drop of methylene blue solution. The capsule, in the presence of the specific homologous anti-serum, becomes apparently swollen, clearly delineated, and refractile.