Streptococcus pyogenes

• Also commonly referred to as Group A Streptococcus

Microbiology

• Gram-positive coccus, typically in short chains
• Non-motile, non–spore forming, catalase-negative, and facultatively anaerobic
• β hemolytic on blood agar (complete hemolysis)

Pathophysiology

Virulence factors

• Capsular hyaluronic acid is similar to human
• M protein is the main factor imparting virulence
  • M protein differences given S. pyogenes its serotypes (about 150)
  • Confers resistance to phagocytosis by modulating host immune response
  • Impairs granulocyte maturation even if it is phagocytosed
  • It is also an adhesin
• F protein binds to fibronectin, helps with adhesion
• Hemolysins, include streptolysin O and S, confers red and white cell lysis
  • O means oxygen-labile, so will only grow in anaerobic environment, while S is stable in oxygen
• DNAse (streptokinase) disrupts coagulation and the body's ability to prevent the bacteria from spreading

Antibiotic resistance

• The PBP is extremely stable, so low mutation rate, and essentially always susceptible to penicillin
• Macrolides
  • Inducible in the presence of erythromycin (D test)
  • Efflux pump

Clinical Presentation

• Streptococcal pharyngitis
• Scarlet fever
• Skin and soft tissue infections, including cellulitis, erysipelas, impetigo, and necrotizing fasciitis
• Upper and lower respiratory tract infections
• Bacteremia without a focus
• Septic arthritis and osteomyelitis
• Myositis
• Pelvic infections, including postpartum endometritis (puerperal fever)
• Toxic shock syndrome
• Less commonly:
  • Pneumonia, typically post-viral and associated with military recruits, and often complicated by empyema and 38% mortality
  • Lymphangitis
  • Meningitis
  • Endocarditis

Classification of Invasive Disease

• As per the Public Health Agency of Canada National case definition: Invasive group A streptococcal disease

Confirmed case

• Laboratory confirmation of infection with or without clinical evidence of invasive disease, requiring isolation of group A streptococcus (Streptococcus pyogenes) from a normally sterile site
  • Blood, CSF, pleural fluid, pericardial fluid, peritoneal fluid, deep tissue specimen taken during surgery (e.g. muscle collected during debridement for necrotizing fasciitis), bone or joint fluid excluding the middle ear and superficial wound aspirates (e.g. skin and soft tissue abscesses).

Probable case

• Clinical evidence of invasive disease in the absence of another identified aetiology and with non-confirmatory laboratory evidence of infection:
  • Isolation of group A streptococcus from a non-sterile site
  • or
  • Positive group A streptococcus antigen detection

Clinical evidence

• Streptococcal toxic shock syndrome, which is characterized by hypotension (systolic blood pressure ≤ 90 mm Hg in an adult and < 5 percentile for age for children) and at least two of the following signs:
• Renal impairment (creatinine level ≥ 177 μmol/L for adults)
• Coagulopathy (platelet count ≤ 100,000/mm3 or disseminated intravascular coagulation)
• Liver function abnormality (SGOT, SGPT, or total bilirubin ≥ 2x upper limit of normal)
• Adult respiratory distress syndrome
• Generalized erythematous macular rash that may desquamate
• Soft-tissue necrosis, including necrotizing fasciitis, myositis or gangrene
• Meningitis

Prognosis

• Highest risk of streptococcal toxic shock syndrome (TSS) or death
  • Necrotizing fasciitis (50% and 50%)
  • Pneumonia (30% and 30%)
  • Bacteremia (15% and 25%)