Streptococcus Pyogenes (GAS): Characteristics, Diseases, 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 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.

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 pyogenes: NIPPLES:
Necrotising fasciitis and myositis
Impetigo
Pharyngitis
Pneumonia
Lymphangitis
Erysipelas and cellulitis
Scarlet fever/ Streptococcal TSS

Noninvasive disease

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

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 test	S. pyogenes
Catalase test	Negative	Useful to differentiate staphylococci from streptococci
Bacitracin sensitivity test	Sensitive	Presumptive identification of group A streptococci (GAS)
Pyrrolidonyl-β-naphthylamide (PYR) test	Positive	Presumptive identification of GAS and enterococci
CAMP test	Negative	GBS (S. agalactiae) is CAMP test positive.
Hippurate hydrolysis test	Negative	Streptococcus agalactiae is 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.

• Rheumatic fever
• Post streptococcal glomerulonephritis (PSGN)
• Scarlet fever
• Erysipelas

References

1. Madigan Michael T, Bender, Kelly S, Buckley, Daniel H, Sattley, W. Matthew, & Stahl, David A. (2018). Brock Biology of Microorganisms (15th Edition). Pearson.
2. Spellerberg B, Brandt C. Laboratory Diagnosis of Streptococcus pyogenes (group A streptococci) 2016 Feb 10. In: Ferretti JJ, Stevens DL, Fischetti VA, editors. Streptococcus pyogenes : Basic Biology to Clinical Manifestations [Internet]. Oklahoma City (OK): University of Oklahoma Health Sciences Center; 2016-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK343617/