What is Streptococcus? Streptococcus pyogenes, Diseases it Causes, and its Virulence Factors

Page content

What is Streptococcus?

Streptococcus is a group of related bacteria (with similar characteristics) all included in a biological genus. They are all spherical and gram-positive bacteria (that is they react positively to the Gram test). Streptococcus bacteria divide in a particular way: they all divide along a single axis forming a chain. In contrast, other bacteria divide along multiple axes.

How are Streptococci Bacteria Classified?

Basically, there are two ways to classify the vast array of streptococcus bacteria. One approach is to classify them by the way they behave in a special culture media that contains blood. In this respect, there are:

  • Non-hemolytic streptococcus - that is they do not break blood cells - also called gamma-hemolytic
  • Beta - hemolytic - they completely break red cells
  • Alpha-hemolytic - they are only able to reduce the red cell hemoglobin.

Another way to classify these bacteria is by antigen groups present on the cell surface. Specific proteins on the cell surface differentiate particular types of streptococci. There are several groups (Lancefield classification, in honor of scientist Rebecca Lancefield) and they are designated letters from A to O - so there is Group A Streptococcus (GAS), Group B Streptococcus (GAS), etc.

Streptococcus pyogenes

Streptococcus pyogenes, also known as Group A Streptococcus (GAS) is a beta-hemolytic type streptococcus. It is one of the most common human pathogens. S. pyogenes can be present within the human bacteriological flora without showing signs of infection. But under certain circumstances it can overgrow and cause illnesses and diseases. This bacterium is responsible for diseases such as pharyngitis (strep throat), scarlet fever (rash), impetigo (infection of the superficial layers of the skin) and cellulitis (infection of the deep layers of the skin).

Streptococcus pyogenes virulence factors

When a S. pyogenes infection occurs, the bacteria are able to rapidly grow and avoid the natural host defenses using some clever tricks. The cell surface of S. pyogenes is very complex and has many proteins that help this bacterium colonize host cells and avoid phagocytosis (the presence of a hyaluronic acid capsule inhibits phagocytosis). In addition, the cytoplasm of S. pyogenes has many proteins similar to those of humans (cardiac, skeletal, and smooth muscle, heart valve fibroblasts, and neuronal tissues). The net result is a sort of “molecular mimicry” that allows the host cell “to tolerate” the invasion. Some strains release potent toxins that can destroy cells as well as possessing enzymes which can protect the bacterium from being trapped by neutrophils.


Samuel Baron. Medical Microbiology, 4th edition. (https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mmed.chapter.800)

Streptococcus (bacterium) – Britannica Online Encyclopedia

Todars Online textbook of Bacteriology).