Streptococcus pyogenes: Introduction, Morphology, Culture Characteristics, Virulence Factor, Lab Diagnosis, Treatment and Keynotes

Streptococcus pyogenes: Introduction, Morphology, Culture Characteristics, Virulence Factor, Lab Diagnosis, Treatment and Keynotes

Introduction of Streptococcus pyogenes

Streptococcus pyogenes is a group A streptococcus that is responsible for a wide array of manifestations ranging from mild localized infections to life-threatening invasive infections.  Ineffective treatment of S. pyogenes infections can result in the postinfectious consequences of acute rheumatic fever and post-streptococcal glomerulonephritis. Besides, it causes invasive infections like necrotizing fasciitis and toxic shock syndrome that are associated with and high morbidity and mortality.

Morphology of Streptococcus pyogenes 

  • Ovoid to spherical in shape
  • Size of 0.5 to 1 µm
  • Gram-positive cocci
  • Arranged in the chains but also may be in pairs and singles.
  • Capsulated ( hyaluronic acid)
  • Non – motile
  • Non – sporing

Culture Characteristics of Streptococcus pyogenes

  • Aerobes and facultative anaerobes
  • Optimum temperature 37°C
  • Optimum pH: 72
  • Enriched media for growth
  • Blood agar:  Small (0.5-1mm), circular, semi-transparent colonies which Produce a wide zone of β- hemolysis, and growth and hemolysis are promoted by 5-10% CO2.  Virulent strains, on fresh isolation form lesions, produce a ‘matt’ (finely granular) colony while avirulent strains form ‘glossy’ colonies. Mucoid colonies are formed by strains that produce large capsules.
  •   Liquid media- Glucose or serum broth medium is needed in which growth occurs as granular turbidity with a powdery deposit and no pellicle is formed.

Biochemical Reactions of Streptococcus pyogenes

Catalase Test : Negative

Coagulase test: Negative

Ferment sugars like glucose, maltose, and lactose: Only acid production

Bile insoluble

PYR test positive

Hydrolyze pyrrolidonyl-beta-napthylamide (PYR) due to the presence of peptidase, the resulting napthylamide produces a red color upon the addition of 0.01% cinnamaldehyde reagent

Faliure to ferment ribose

Bile insoluble

Antigenic Structure

Beta-hemolytic streptococci- 20 types ( Lancefield grouping)

Hemolytic streptococci-Group A

Based on M, T, R proteins present on the cell wall surface (Griffith typing)-65 types

M- antigen

  • Virulent factor
  • Inhibits phagocytosis
  • Antibodies to it are protective
  • Over 60 types in Group A

T- antigen

  • Antibodies to ti are not protective
  • No relationship with virulence
  • It permits differentiation of certain types of Streptococci


R- antigen

  • Present in Group A streptococci ( serotypes 2,3,28 and 48 and also in some strains of group B and C.
  • No relationship with virulence.

Toxins and Enzymes

  • Hemolysin
  1. Streptolysin O (antigenic, virulent cardiotoxic)
  2. Streptolysin S (non-antigenic, nephrotoxic)
  • Erythrogenic toxin ( pathogenesis of scarlet fever)
  • Streptokinase (spread of infection)
  • Deoxyribonuclease (liquefaction of thick pus)
  • Diphosphopyridin nucleotidase (leucotoxic)
  • Hyaluronidase  ( spread of infection – Intercellular space)

Pathogenicity of Streptococcus pyogenes

  • Meningitis
  • Sinusitis
  • Septic tonsilitis
  • Sore throat
  • Pulmonary abscess
  • Rheumatic fever and rheumatic fever and rheumatic carditid
  • Septicemia
  • Puerperal abscess
  • Skin infection spreading with cellulitis
  • Empyema
  • Osteomyelitis
  • Adenitis
  • Otitis media and mastoiditis
  • Acute glomerulonephritis (serotype 4,12 ,49, 2, 52, 57)

Laboratory Diagnosis


  • Throat swab
  • Nasopharyngeal swab
  • Pus swab
  • Sputum
  • Cerebrospinal fluid


Pinpoint beta-hemolytic  colonies on blood agar

0.04 Unit Bacitracin Sensitivity Testing

Streptococcus pyogenes: Sensitive

Serological Tests

  • Antidtreptolysin O (ASO)

The titer of 1:286 or above suggestive of streptococcal (rheumatic) activity

  • Anti DNase
  • Anti-hyaluronidase
  • Anti-streptokinase
  • Anti- M type-specific antibodies

Dick Test (Skin Test)

0.2 ml erythrogenic toxin is injected intradermally in the forearm. Observe for bright red rash in 6 hours.  Bright red rash maximum in 24 hours and then fades away.  The test is positive if no immunity scarlet fever persists.


The recommended drug for the treatment of bacterial pharyngitis is penicillin. This treatment is cost-effective and has a narrow spectrum of activity. In case of penicillin allergy, macrolides and first-generation cephalosporins can be applied. However, some strains of Streptococcus pyogenes have developed resistance to macrolides, and macrolides are used as a third-line of treatment for Streptococcal throat infection. Vancomycin or clindamycin is used in severe invasive infections. Intravenous antibiotic therapy and surgery for the removal of necrotic tissue are suggested in the case of soft tissue skin infection.


Further Readings

  1. Ananthanarayan R. & Paniker C. J. (2009) Textbook of Microbiology. Hyderabad: Universities Press (India) Private Limited.
  2. Harvey Richard A. et al. Lippincott’s Illustrated Reviews Microbiology, Wolters Kluwer (India) Pvt. Ltd.,2nd edition.
  3. Chakraborty P. (2009). A Textbook of Microbiology. Kolkata: New Central Book Agency (P) Ltd. 4. Murray et all. Medical microbiology, Elsevier Saunders, 7th edition.


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