Morphological and Staining Features
The microorganism has a characteristic morphology that aids in initial microscopy. They are Gram-positive (Gram+) bacteria, typically appearing as ovoid cocci arranged in pairs or short chains in clinical specimens.
Key negative features: the bacteria are non-spore-forming and non-motile. When entering a host or when grown on enriched media, they form a distinct polysaccharide capsule closely associated with the peptidoglycan cell wall.
Cultural Characteristics
In terms of respiration, these bacteria are facultative anaerobes. A specific environmental requirement is capnophilia — they require an enriched carbon dioxide (CO₂) atmosphere for optimal growth.
Optimal cultivation conditions:
- Incubation temperature: strictly 36–37 °C.
- Medium pH: slightly alkaline, around 7.6–7.8.
- Culture media: enriched media supplemented with blood or serum are required.
On blood agar, the bacteria form small colonies. A hallmark feature is the formation of a zone of beta-hemolysis (complete hemolysis) around the colonies, which is critical for presumptive identification. Growth in liquid media produces uniform turbidity.
Biochemical and Antigenic Identification
The pathogen possesses specific enzymatic activity that serves as the basis for laboratory differentiation.
Saccharolytic activity includes the fermentation of glucose, trehalose, and lactose, while differing from other streptococci by being CAMP-test positive and hydrolyzing sodium hippurate.
Regarding respiratory enzymes, tests are negative: the bacterium produces neither catalase nor oxidase.
The antigenic structure and classification of the microorganism are based on the Lancefield grouping (Group B carbohydrate antigen) and variations in its polysaccharide capsule.
Pathogenicity Factors
The bacterium features a robust complex of factors that ensure tissue invasion and robust protection against the host immune response. They can be divided into structural components and secreted substances.
Structural components:
- Capsule: The primary virulence factor. Its main function is the strong inhibition of phagocytosis.
- Surface proteins: Mediate adherence to host epithelial cells and extracellular matrix components.
Enzymes and toxins:
- CAMP factor — a protein that acts synergistically with the Staphylococcus aureus beta-hemolysin to produce enhanced hemolysis.
- Hyaluronidase — a classic "spreading factor" that breaks down hyaluronic acid in the extracellular matrix, paving the way for bacterial deeper tissue penetration.
- Proteases and nucleases — contribute to tissue destruction and immune evasion.