Streptococcus Pyogenes (GAS): Common characteristics; virulence Factors, Diseases and Key Tests

Popularly known as “flesh eating bacteria” , Streptococcus pyogenes is one of the pathogenic gram positive cocci.  Streptococcus pyogenes, or Group A streptococcus (GAS) is mostly known for streptococcal sore throat (strep throat). It is a gram positive cocci; mostly occurs as chains and occasionally in pairs.

It is the causative agents for acute pharyngitis, impetigo, erysipelas, necrotizing fasciitis (flesh eating bacteria) and myositis, bacteremia with potential infection in any of several organs, pneumonia, scarlet fever, streptococcal toxic shock syndrome.

Rheumatic fever and acute post-streptococcal glomerulonephritis are two prominent diseases that result due to streptococcal infections.

β-hemolysis by S.pyogenes  Image source: Linda Johansson et al. Clin Infect Dis. 2010;51:58-65
β-hemolysis by S.pyogenes
Image source: Linda Johansson et al. Clin Infect Dis. 2010;51:58-65

Major Characteristics of S. Pyogenes (GAS)

  1. Gram positive cocci
  2. Non-motile
  3. Non-sporing.
  4. Fastidious organism; grows well in 5% sheep blood agar (producing β-hemolysis) and Chocolate agar.
  5. Catalase negative (this test helps to differentiate Streptococcus spp from Staphylococcus spp).
  6. Group A Streptococci: β-Hemolytic streptococcus are arranged into groups A-U (Known as Lancefield groups) on the basis of antigenic differences in C carbohydrate.
  7. Bacitracin sensitive: the growth of S. pyogenes is inhibited by Bacitracin, which is an important diagnostic criterion.

Antigen detection methods are used as screening test. Detection of S. pyogenes antigen in throat specimen is possible by using latex agglutination test, co-agglutination, or ELISA technologies.

Virulence factors of Streptococcus pyogenes and their roles  

Streptococcus pyogenes possesses a wide variety of virulence factors and can cause severe invasive infections.
They are:

  1. Hemolysins
  2. Pyorogenic Exotoxin (Erythrogenic toxins)
  3. Spreading factors
  4. Deoxyribonucleases (StreptodornaseDNase)
  5. Streptokinase (Fibrinolysin)
  6. Hyaluronidase:
  7. Other enzymes


Diseases caused by S.pyogenes 

Noninvasive disease

  1. Streptococcal sore throat (strep throat)
  2. Impetigo

    Streptococcal sore throat (Strep throat) features.
    Streptococcal sore throat (Strep throat) features.

Invasive disease

  1. Necrotizing fasciitis (NF)
  2. Streptococcal toxic shock syndrome (STSS)
  3. Cellulitis
  4. Bacteremia
  5. Pneumonia
  6. Puerperal sepsis

Nonsuppurative sequelae

  1. Acute rheumatic fever (ARF)
  2. Post-streptococcal glomerulonephritis (PSGN)

 Key Tests that are used to identify S. pyogenes:

The sample for the isolation/identification of S. pyogenes is  either pharyngeal exudates, pus, blood, tissue, or body fluids depending on the site and nature of infection.

Key tests that are commonly employed in diagnostic laboratory  for this purpose are:

  1. Gram Staining (Gram positive cocci in chains)
  2. Catalase Test (Negative)
  3. Culture on Blood Agar ( β-Hemolysis)
    • Grown anaerobically, 100% of strains are beta-hemolytic
    • Grown aerobically, 85% of strains are beta hemolytic (15% are non-hemolytic)

two hemolysins (streptolysins) – O and S

  • O = encoded by 100% of strains; O2 labile
  • S = encoded by 85% of strains; O2 stable
  1. Bacitracin sensitivity test (Sensitive)
  2. Pyrrolidonyl Arylamidase (PYR) Test (+ve)
  3. Anti-Streptolysin O (ASO) Test: ASO titer is not done for the diagnosis of Streptococcal sore throat but for sequelae (complications) that results due to previous infections with Streptococcus pyogenes.
  • Rheumatic fever
  • Post streptococcal glomerulonephritis (PSGN)
  • Scarlet fever
  • Erysipelas

Do you have any queries? Please leave me in the comments section below. I will be happy to read your comments and reply.