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Streptococcal Infections (cont.)

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What is group A Streptococcus (GAS)?

Group A Streptococcus is defined as a gram-positive bacterial genus composed of Streptococcus pyogenes strains. Group A Streptococcus strains have a similar surface antigen recognized by specific laboratory tests, termed the Lancefield group A antigen. Lancefield groups (there are about 18 Lancefield groups) are composed of different Streptococcus species groups that have specific antigens and are distinguished by specific antibody tests. In addition, group A Streptococcus strains are beta-hemolytic (beta-hemolytic means the bacteria lyse red blood cells suspended in agar plates with secreted substances, see for example, Fig. 3). These tests are mentioned because they are frequently used to distinguish group A Streptococcus bacteria from group B, group C, and other Streptococcus groups. Group A Streptococcus bacteria appear as pairs and chains when gram-stained (see Fig. 1); these bacteria are also termed "beta strep, GAS, and GABHS." Although these bacteria can harmlessly colonize people on their throat and skin, sometimes they can cause mild to serious diseases. GAS bacteria have been causing diseases in humans probably since humans first developed.

Streptococcus pyogenes (GAS) bacteria have many components that contribute to the pathogen's ability to cause disease:

  • Lipoteichoic acid on its surface helps the GAS bacteria to bind to epithelial cell membranes.
  • M proteins (over 100 types on the GAS bacterial strains) help the bacteria resist immunologic host defenses.
  • Exotoxins (for example, DNAses A, C and D, streptolysin S, proteinase, streptokinase, and pyrogenic exotoxins [A-D])
  • Human immune system stimulators (for example, streptolysin O, DNAse B, and hyaluronidase)

Exotoxins cause the scarlet fever rash, damage organs, may cause shock, and inhibit the human immune system, while the human immune system stimulators may stimulate the immune system to produce antibodies that likely play a role in the development of autoimmune responses that can lead to glomerulonephritis or acute rheumatic fever. S. pyogenes (GAS) has over 100 serotypes that may vary somewhat in their ability to produce the above components that contribute to the pathogenicity of each strain of the bacteria. In 2015, a new GAS strain, known as genotype emm89, was noted in the United Kingdom; the genetic changes seemed to enhance the strain's ability to cause disease and seems to be replacing older GAS genotypes.

Photograph of Streptococcus pyogenes (GAS).
Figure 1: Photo of Streptococcus pyogenes (GAS). SOURCE: CDC
Medically Reviewed by a Doctor on 2/19/2016


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