I. General Features
- Streptococcus pyogenes is a Gram-positive, non-motile, non-spore forming coccus arranged in chains or pairs under the microscope.
- It is catalase-negative, distinguishing it from Staphylococci.
- Grows best on blood agar, where it produces β-hemolysis — complete clearing around colonies due to lysis of red blood cells.
- It is a facultative anaerobe, meaning it can grow with or without oxygen.
- Classified as Group A Streptococcus based on Lancefield serotyping, which detects group-specific carbohydrate antigen in the cell wall.
- PYR test positive — helps differentiate it from S. agalactiae.
The β-hemolysis helps in lab identification. Catalase negativity helps separate it from Staphylococcus. The Lancefield group A antigen is specific to S. pyogenes.
III. Epidemiology and Transmission
- Humans are the only reservoir.
- Transmitted by:
- Respiratory droplets → pharyngitis
- Skin contact or contaminated surfaces → skin infections
- More common in children (ages 5–15 years).
- Asymptomatic carriage is common, especially in the nasopharynx.
IV. Clinical Diseases of S. pyogenes
A. Suppurative (pus-forming) Infections
These involve direct bacterial invasion and local tissue destruction.
- 1. Pharyngitis (Strep Throat)
- Sudden sore throat, fever, swollen lymph nodes, tonsillar exudates.
- Complication risk: rheumatic fever if untreated.
- 2. Scarlet Fever
- Occurs when the strain produces pyrogenic exotoxins.
- Presents with:
- Sandpaper-like rash
- Strawberry tongue
- Circumoral pallor
- Often follows untreated pharyngitis.
- 3. Impetigo (non-bullous)
- Honey-crusted skin lesions, usually on face.
- Spread via contact, especially in children.
- 4. Erysipelas
- Superficial skin infection with raised, red, demarcated edges.
- Often affects face or lower limbs.
- 5. Cellulitis
- Deeper skin/subcutaneous infection. Less defined borders.
- 6. Necrotizing Fasciitis
- “Flesh-eating disease.” Rapid destruction of muscle and fascia.
- Requires surgical debridement and IV antibiotics.
- Caused by highly virulent strains producing SpeB exotoxin.
- 7. Streptococcal Toxic Shock Syndrome (STSS)
- Systemic illness with shock, multiorgan failure, rash.
- Caused by superantigenic exotoxins (esp. SpeA).
B. Non-suppurative (Immune-mediated) Complications
Occur after pharyngeal infection, due to immune cross-reactivity.
- 1. Acute Rheumatic Fever (ARF)
- Autoimmune response, 2–3 weeks after pharyngitis.
- Affects heart, joints, skin, brain.
- JONES criteria: Joint (arthritis), O (heart), Nodules, Erythema marginatum, Sydenham chorea.
- Due to cross-reactive antibodies against M protein and cardiac tissue.
- 2. Acute Post-Streptococcal Glomerulonephritis (APSGN)
- Occurs after pharyngitis or skin infection.
- Immune complex deposition in glomeruli → hematuria ("cola-colored urine"), edema, hypertension.
- Type III hypersensitivity reaction.