Streptococcus (first part)

Hi, after long time,

Today, we will explain Streptococcus with its structure and classification, so let’s begin:

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*Pyogenes cocci are those in which the hallmarks (special characteristic) of the infective process results in formation of pus. And they are classified into Gram(+ve) pyogenes such as Streptococcus and staphylococcus tend to cause diseases by exotoxins while Gram (-ve) pyogenes such as Neisseria and Haemophilus causes diseases depending on host’s response to lipopolysaccharides (LPS) (endotoxin) as LPS cause releasing of inflammatory cytokines. *

Streptococcus are aerobic and facultative anaerobic Gram-positive cocci, arranged in pairs (diplococci) such as Streptococcus pneumonia or chains such as Streptococcus pyogenes.

They are nonmotile, non-sporing, have hyaluronic acid capsules and catalase negative by which they are distinguished from staphylococci.

They are classified based on:

1)     Hemolysis in blood agar (bacteriologically):

Brown classified these aerobic streptococci into three groups on the basis of their growth in 5% horse blood agar:

a)     Alpha-hemolytic streptococci: These cocci produce colonies surrounded by a narrow zone (greenish zone) of hemolysis with persistence of some partially lysed RBCs such as Virdians streptococci and Streptococci Pneumonia.

b)     Beta-hemolytic streptococci: These cocci produce a clear, colorless zone of hemolysis around the colonies such as Group A (S. pyogenes), Group B (S. agalactiae), Group C (S. equismilis, S. equi, S. zooepidemicus) and Group D.

c)     Gamma-hemolytic streptococci: These do not produce any hemolysis or discoloration on blood agar such as Enterococcus and Non-enterococcus.

2)     Antigenic structure: (immunologically):

Lancefield classification is a serological classification of beta-hemolytic streptococci. This classification is based on detection of group-specific carbohydrate antigen (C-antigen) (it will be explained later) on the cell wall.

Beta-hemolytic streptococci are classified into 21 serological groups (A to W, except I and J).

Note: S.viridans and S. pneumonia don’t possess C- carbohydrate thus not serogrouped.

Based on the M, T, and R protein antigens ((it will be explained later) present on the cell surface, S. pyogenes have been further classified into 80 serotypes. This classification is known as Griffith typing.

Structure of Streptococcus:

We will talk about cell wall components and antigenic structure in addition to virulence factors, So focus!!!!!

First: cell wall components and antigenic:

1)     Group-Specific Carbohydrate:

The cell wall contains a group-specific polysaccharide that forms approximately 10% of the dry weight of the cell. It is a polymer of N-acetylglucosamine and rhamnose.

2)     Type-Specific Protein:

The cell wall of S. pyogenes has three major proteins, M, T, and R proteins.

a)     M proteins: is the important protein and is considered chief virulence factor of cocci as it inhibits phagocytosis and facilitates attachment of cocci to epithelial cells.

M protein is alpha helix consisting of carboxyl terminus and amino terminus. This carboxyl terminus is attached to cytoplasmic membrane and is highly conserved while amino terminus is present from cell wall to cell surface of host cell and is highly variable so S. pyogenes is divided into more than 80 (l–60) serotypes based on M-protein.

Amino terminus binds to fibrinogen which masks C3b binding site (as C3b binds to surface of the pathogen and attracts phagocytes to pathogen) that facilitates by Lipoteichoic acid and F proteins and binds to Factor H that can inhibit C3 convertase and C5 convertase (alternate pathway for innate immunity).

M-protein is responsible for ARF as there is amino acid sequence inside M protein which is non-immunogenic as it is similar to that in cardiac muscle protein (tropomyosin) but sometimes our bodies can generate immune response to this amino acid sequence so antibodies generated bind to cardia muscle protein causing ARF.

So: M-protein confers type specific immunity while F protein and LTA don’t.

Second: Virulence factors:

Notes: 

M-like proteins bind to Fc portion of IgG and IgA molecules and binds to alpha 2 macroglobulin that can inhibit PMN proteases (enzymes that catalyzes proteolysis).

Plasminogen binding proteins which bind to plasminogen activators such as tPA (Tissue plasminogen activator), urokinase and streptokinase. These activators convert plasminogen to plasmin (proteases) that may aid in dissolution of host protein structures so facilitating spread of infection.

60kD Rheumatic Fever associated Antigen (RFAA): this protein has been implicated in generation of antibodies that cross react with cardiac tissue proteins that lead ARF.

Wait for another post in Streptococcus types!!!!!!