Morphology and Culture Characteristics
Bacteria of the genus Streptococcus are slightly elongated spherical cells measuring 0.5–2.0 µm in diameter. They are non-motile (lack flagella), non-spore-forming, and many strains possess a capsule. The cell wall is three-layered, consisting of peptidoglycan, polysaccharides, and a protein layer. A notable feature is the ability to form cell wall-deficient L-forms.
Functionally, they are facultative anaerobes (some strains are capnophilic). Optimal growth conditions include a temperature of 35–37 °C and a pH of 7.2–7.4. These microorganisms are fastidious and require enriched media supplemented with blood, serum, and carbohydrates.
On solid media, they form small, grayish-translucent colonies. Colony morphology correlates with virulence: matte colonies contain abundant surface fibrillae (M protein) and are more virulent, whereas glossy colonies are less virulent. Biochemically, the organism ferments glucose, lactose, and other sugars with the production of lactic acid; it is bile-esculin negative and does not reduce nitrates.
Virulence Factors of Group A Streptococcus
The pathogenicity of S. pyogenes is mediated by the production of over 20 extracellular products and structural components of the cell wall.
Surface Factors:
- Capsule and M protein: Provide anti-phagocytic activity (immune evasion).
- Lipoteichoic acids: Mediate adhesion to host epithelial cells.
Enzymes of Invasion:
- Streptokinase (fibrinolysin): Converts host plasminogen to plasmin, degrading fibrin and dissolving blood clots to facilitate bacterial spread.
- Hyaluronidase: A "spreading factor" that breaks down the extracellular matrix of connective tissue.
- Streptodornase (DNase): Liquefies purulent exudates.
- C5a peptidase: Inhibits the recruitment of phagocytes to the site of infection.
Exotoxins:
- Pyogenic exotoxin (erythrogenic toxin): Produced by lysogenized strains, acts as a superantigen. Responsible for scarlet fever and toxic shock syndrome.
- Streptolysin O: Oxygen-labile, lyses erythrocytes and other cells, highly immunogenic.
- Streptolysin S: Oxygen-stable, cytotoxic, but non-immunogenic.
Pathogenesis and Clinical Manifestations
Streptococci can infect virtually any tissue (lack of strict organ tropism). Infection spreads via lymphatic channels (causing lymphadenitis) or hematogenously, potentially leading to septicopyemia. The pathogenesis consists of three main syndromes:
- Infectious syndrome: Local inflammation (purulent, serous, or necrotic) driven by direct tissue invasion.
- Toxic syndrome: Fever, malaise, and vomiting mediated by exotoxins.
- Allergic syndrome: Development of hypersensitivity to bacterial components.
This specific sensitization (allergic syndrome) triggers diffuse connective tissue diseases, such as acute glomerulonephritis and rheumatic fever. Chronic persistence (chronic tonsillitis, recurrent erysipelas) is often associated with the formation of L-forms.
Diagnosis and Treatment
The gold standard for microbiological diagnosis is culture. Clinical specimens (pus, blood, throat swabs) are plated on blood agar, followed by Lancefield serogrouping and biochemical identification. Direct microscopy has only presumptive value.
Serological testing plays an important role by measuring antibody titers against streptococcal toxins. The primary marker is Antistreptolysin O (ASO).
Treatment relies on antibiotics (penicillins, macrolides), to which most strains remain highly susceptible. The main therapeutic challenge is not antimicrobial resistance, but managing severe immunopathological sequelae that require immune modulation (particularly in rheumatic fever).