Streptococcus Pyogenes (GAS): Characteristics, Diseases, Tests

By Acharya Tankeshwar •  Updated: 06/25/21 •  3 min read

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 that mostly occurs as chains and occasionally in pairs.

It is the causative agent of acute pharyngitis, impetigo, erysipelas, necrotizing fasciitis (flesh-eating bacteria), and myositis. Other infections caused by this organism are bacteremia with potential infection in any of several organs, pneumonia, scarlet fever, and streptococcal toxic shock syndrome.

Rheumatic fever and acute post-streptococcal glomerulonephritis are two prominent diseases (sequelae) that result due to previous 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 Streptococcus 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 streptococci 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 a 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 possess 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

Find detail information about virulence factors of S.pyogenes and their roles here

Diseases caused by S.pyogenes 

Mnemonic: Diseases caused by Streptococcus pyogenesNIPPLES:
Necrotising fasciitis and myositis
Erysipelas and cellulitis
Scarlet fever/ Streptococcal TSS

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 sites 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. 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)
  3. Two hemolysins (streptolysins) – O and S
    • O = encoded by 100% of strains; O2 labile
    • S = encoded by 85% of strains; O2 stable

Biochemical tests

Following biochemical tests are useful for the identification of Streptococcus pyogenes.

Name of the testS. pyogenes
Catalase testNegativeUseful to differentiate staphylococci from streptococci
Bacitracin sensitivity testSensitive  Presumptive identification of group A streptococci (GAS)
Pyrrolidonyl-β-naphthylamide (PYR) test PositivePresumptive identification of GAS and enterococci
CAMP testNegativeGBS (S. agalactiae) is CAMP test positive.

Anti-Streptolysin O (ASO) Test: ASO titer is not done for the diagnosis of Streptococcal sore throat but for sequelae (complications) that result due to previous infections with Streptococcus pyogenes.

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. I am working as an Asst. Professor and Microbiologist at Department of Microbiology and Immunology, Patan Academy of Health Sciences, Nepal. If you want me to write about any posts that you found confusing/difficult, please email at

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