Morphological and tintorial features
The microorganism has a very characteristic morphology that aids in initial microscopy. These are Gram-positive (G+) bacteria, measuring approximately 1 µm in size.
In shape, they are oval or lancet-shaped cocci. Visually, their outlines are often compared to a candle flame. In smears, the cells are located predominantly in pairs (diplococci), but can also form short chains.
Important negative features: bacteria do not form spores and are completely non-motile. Upon entering a macroorganism or when grown on special nutrient media, they form a pronounced capsule. This capsule is of polysaccharide nature, and its polysaccharides are tightly bound to the peptidoglycan layer of the cell wall.
Cultural properties
According to the type of respiration, these bacteria are facultative anaerobes. A special requirement for environmental conditions is capnophilia—they require an increased carbon dioxide (CO₂) content for optimal multiplication.
Optimal conditions for cultivation:
- Incubation temperature: strictly 36–37 °C.
- Medium reaction (pH): slightly alkaline, within 7.6–7.8.
- Nutrient media: complex formulations are required, necessarily with the addition of native blood or serum.
On blood agar, bacteria form small colonies. They can be gray, transparent, or slightly matte. A characteristic feature is the formation of a α-hemolysis zone around the colonies, which visually manifests as a greening of the nutrient medium. When grown in liquid media, uniform diffuse turbidity of the broth is observed.
Biochemical and antigenic identification
The pathogen possesses specific enzymatic activity, which forms the basis of its laboratory differentiation.
Saccharolytic activity manifests in the ability to break down glucose, maltose, and galactose. However, for an accurate diagnosis, microbiologists rely on three key differential-diagnostic signs:
- Ability to ferment inulin.
- Bile solubility (cells lyse in the presence of bile).
- High sensitivity to optochin (pronounced inhibition of microbial growth is observed).
As for respiratory enzymes, the test results here are negative: the bacterium produces neither catalase nor oxidase.
The antigenic structure and classification of the microorganism are entirely based on differences in the structure of its polysaccharide capsule. To date, microbiologists have succeeded in isolating about 90 different serotypes (serovars).
Pathogenicity factors
The bacterium possesses a powerful complex of factors that ensure its tissue invasion and reliable protection against the host immune response. They can be divided into structural components and secreted substances.
Structural components:
- Capsule: Serves as the main virulence factor. Its primary function is the stringent suppression of the phagocytosis process.
- Substance C (C-polysaccharide): This is a specific component of cell wall teichoic acids. Its role lies in the ability to activate the complement system via the alternative pathway.
Enzymes and toxins:
- IgA protease — an enzyme that selectively destroys secretory immunoglobulin A (IgA) on mucous membranes, thereby depriving the body of local immune protection.
- Neuraminidase — reduces the viscosity of protective mucus, which is critical for successful colonization and subsequent invasion.
- Lysozyme (muramidase) — acts as an additional factor in the colonization of the respiratory tract mucous membranes.
- Hyaluronidase — a classic "spreading factor". By breaking down hyaluronic acid in the extracellular matrix, the enzyme paves the way for bacteria deeper into tissues.
- Pneumolysin (β-hemolysin) — an aggressive cytotoxin that destroys erythrocytes. An important nuance: it is capable of causing true β-hemolysis exclusively under anaerobic conditions.
Briefly: answers to topical questions
Which serological groups are distinguished in Streptococcus agalactiae based on antigenic structure?
Based on its antigenic structure, Streptococcus agalactiae belongs to serological group B.
According to the Lancefield classification, streptococci are divided into groups based on cell wall polysaccharides. Antigens of a polysaccharide nature determine the bacterium's belonging to a specific serogroup. Within the species Streptococcus agalactiae itself, there is an additional division: based on differences in the structure of the polysaccharide capsule, about 90 serovars (serotypes) are distinguished. The capsular polysaccharides are thereby bound to the peptidoglycan of the microorganism's cell wall.
What clinical diseases does Streptococcus agalactiae cause in newborns?
In newborns, Streptococcus agalactiae causes pneumonia, meningitis, and sepsis. In the perinatal period, it ranks among the pathogens of purulent meningitis and is also one of the causative agents of non-lactational mastitis in infants.
What is the role of Streptococcus agalactiae in the pathology of pregnant women?
Colonization with Streptococcus agalactiae (Group B Streptococcus) in pregnant women increases the risk of preterm birth.
What are the methods for preventing the transmission of Streptococcus agalactiae to newborns during childbirth?
The main method for preventing the transmission of Streptococcus agalactiae to newborns is intrapartum antibiotic prophylaxis.
It is indicated upon detection of group B streptococcus carriage in the mother. Administration of the drug is initiated simultaneously with the onset of labor or at any point during it. For effective protection, at least 4 hours of systemic antibiotic action are required before the child's birth, although a decrease in colonization is observed as early as 2 hours. If the threat of preterm birth is halted and subsequently recurs, prophylaxis is repeated. Prophylaxis is not performed if the bacterial culture test was negative within the last 5 weeks.
To memorize the triad of distinctive tests, use the word "I-B-O" (in Russian): I — Inulin (ferments), B (Zhelch) — Bile (dissolves), O — Optochin (susceptible).
Class and exam questions
Why is the capsule considered the primary virulence factor of this microorganism?
The capsule is polysaccharide in nature and effectively disguises the bacterium from immune cells, suppressing phagocytosis and allowing the pathogen to survive within the host organism.
How does the bacterium overcome local immunity on mucous membranes?
To do this, it produces a special enzyme — IgA protease, which specifically destroys secretory immunoglobulin A (IgA), serving as the first line of defense for mucous membranes.
What causes the greening of blood agar during cultivation?
The greening of the medium is associated with α-hemolysis — the partial destruction of erythrocytes resulting in the formation of greenish hemoglobin breakdown products.
What else to study on this topic
- Mechanism of alternative complement pathway activation via substance C
- Features of pneumolysin action depending on the presence of oxygen
- The role of hyaluronidase in the degradation of the extracellular matrix
- Principles of bacterial serotyping based on capsular antigens
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