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)
- Gram-positive cocci
- Fastidious organism; grows well in 5% sheep blood agar (producing β-hemolysis) and chocolate agar.
- Catalase negative (this test helps to differentiate Streptococcus spp from Staphylococcus spp).
- 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.
- Bacitracin sensitive: the growth of S. pyogenes is inhibited by bacitracin, which is an important diagnostic criterion.
Virulence factors of Streptococcus pyogenes and their roles
Streptococcus pyogenes possess a wide variety of virulence factors and can cause severe invasive infections.
- Pyorogenic Exotoxin (Erythrogenic toxins)
- Spreading factors
- Deoxyribonucleases (StreptodornaseDNase)
- Streptokinase (Fibrinolysin)
- 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
Erysipelas and cellulitis
Scarlet fever/ Streptococcal TSS
- Streptococcal sore throat (strep throat)
- Necrotizing fasciitis (NF)
- Streptococcal toxic shock syndrome (STSS)
- Puerperal sepsis
- Acute rheumatic fever (ARF)
- 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:
- Gram Staining (Gram-positive cocci in chains)
- 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
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