Streptococcus pyogenes Group A
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ℹ️ About
- Lancefield Group A beta-haemolytic streptococci: Known as Streptococcus pyogenes, these bacteria are a significant cause of human infections.
- They are the main streptococcal pathogen, producing various toxins and enzymes that contribute to their virulence.
Characteristics
- Gram-positive, beta-haemolytic, catalase-negative: Characterized by complete hemolysis on blood agar and lack of catalase enzyme.
- Facultative anaerobe: Capable of fermentation even in the presence of oxygen.
- Lancefield classification: Identified by Lancefield antibodies which detect group-specific carbohydrates in the bacterial cell wall using surface antigens. Group A is the most significant.
Virulence Factors
- M protein: Major virulence factor that impedes phagocytosis. Over 100 types exist, which contribute to the variety of infections. It also cross-reacts with human tissues, contributing to post-infectious syndromes like rheumatic fever.
- Fimbriae (pili): Facilitate bacterial adherence to epithelial cells, particularly in the pharynx.
- Capsular antigens: Antibodies to the capsular antigen help identify serotypes of Streptococcus pyogenes.
- Lipoteichoic acid: Contributes to adherence to epithelial cells and inflammatory responses.
Toxins
- Streptokinase: Breaks down fibrin clots, helping the spread of infection.
- Hyaluronidase: Breaks down hyaluronic acid in connective tissues, facilitating bacterial spread.
- DNAses (types A, B, C, D): Break down host DNA, aiding the bacteria in escaping immune responses.
- NADase: Destroys white blood cells, reducing host immune defenses.
- Haemolysins: Streptolysins (S and O) that lyse red blood cells, white blood cells, and platelets, contributing to tissue damage and immune evasion.
- Erythrogenic toxins: Cause fever and rash in diseases like scarlet fever. Superantigens (A and C) lead to massive immune activation and tissue damage.
Source
- Commensal of the human nasopharynx , especially in asymptomatic carriers.
Pathogenicity
- Tonsillitis and pharyngitis: Presents as a sore throat with inflamed tonsils, exudate, fever, and nausea. Can lead to complications like peritonsillar abscesses.
- Mastoiditis and otitis media: Infections of the middle ear or mastoid region, commonly following pharyngitis.
- Scarlet fever: Caused by erythrogenic toxins, leading to fever, a characteristic rash, and T-cell proliferation.
- Impetigo: Presents as honey-crusted skin lesions. Infection with the M12 serotype can lead to post-infectious glomerulonephritis.
- Skin/soft tissue infections: Includes wound infections, cellulitis, and erysipelas.
- Necrotizing fasciitis: A life-threatening infection involving deep tissue destruction caused by kinases, hyaluronidase, and erythrogenic toxins. Urgent medical and surgical management is required.
- Osteomyelitis: Bone infection that can arise following soft tissue infection or hematogenous spread.
- Toxic shock syndrome (TSS): A systemic illness caused by toxin release, leading to shock and multi-organ failure.
- Puerperal sepsis: Postpartum infection of the reproductive tract, historically a leading cause of maternal death.
- Post-streptococcal syndromes:
- Acute rheumatic fever: An autoimmune response triggered by cross-reactivity between M proteins and host tissues.
- Acute glomerulonephritis: Can follow infection by the M12 serotype (impetigo or pharyngitis), leading to kidney inflammation and dysfunction.
🔎 Investigations
- Blood agar culture: Grows well on blood agar with beta-hemolysis, enhanced by 10% carbon dioxide. Colonies may appear mucoid due to capsule production.
- Gram staining and microscopy: Reveals Gram-positive cocci arranged in chains.
- Rapid antigen test: Detects group A streptococcal antigens from throat swabs.
- ASO titre: Measures anti-streptolysin O antibodies, indicative of a recent streptococcal infection.
- Bacitracin sensitivity: Group A strep are inhibited by bacitracin in culture, helping distinguish them from other beta-hemolytic streptococci.
Resistance
- Some strains have developed resistance to erythromycin , though this remains relatively low.
Sensitivity
- Benzylpenicillin: First-line treatment for most infections caused by Streptococcus pyogenes.
- Clindamycin: Can be used in severe infections like necrotizing fasciitis, as it inhibits toxin production.
- Erythromycin: Alternative to penicillin in penicillin-allergic patients, although some resistance exists.
- Necrotizing fasciitis: Requires urgent surgical debridement in addition to antibiotic therapy.
