Streptococci - Morphology And Virulence Factors

MORPHOLOGY

If you do a gram stain of GAS from agar you’ll see short chains of round things attached to each other. If you do it from a throat swab, you’ll see long chains. This is the only Strep with long chains, so it’s immediately recognizable under the microscope.

Strep produce adhesion pili on their surface. In cross section these look like fuzz going out of the bacteria and sticking to the cell. What is this fuzz layer?Polyglycerophosphate linked to fatty acid. There are many types of adhesins, such as LTA and F protein.

What makes GAS so virulent?

It has lots of virulent factors, including two anti-phagocyte factors, which is rare in a bacterium. These are:

1. Capsule  made up of polyhiuronic acid (a self-antigen). This means that the body can’t recognize it as foreign and eat it.

2. Protein M  prevents compliment from acting.

A summary of other virulence factors:

1) F protein  mediates adhesion to fibronectin on epithelial cells

2) Lipoteichoic acid (LTA)  mediates adhesion to many type of cells

3) Pyrogenic Exotoxins  responsible for scarlet fever (Spe A), and for Streptococcal septic shock. Enhances susceptibility to LPS, stimulates a subset of T lymphocytes bearing defined V beta elements, and suppresses B lymphocyte function.

4) Streptolysin S  Cytolytic pore forming toxin, lyses many type of cells including leukocytes and erythrocytes. Nonimmunogenic.

5) Streptolysin O  Nonactive in oxygen. Cytolytic pore forming toxin. Lyses many type of cells, including platelets and erythrocytes. When injected into animals there is preference for injury of the heart tissue.Immunogenic and determination of serum antibody titer is of diagnostic value.

6) Streptokinase  Lyses blood clots; facilitates spread of bacteria in tissues.

7) Dnase  Depolymerizes cell-free DNA in purulent material.

Category: Microbiology Notes