Epidemiology and Survival
Humans are the sole reservoir of infection. Pneumococcus is a normal resident of the oropharyngeal mucosa (serving as the portal of entry); carriage occurs in 40–70% of people.
Transmission occurs via airborne droplets. Small children, the elderly, and patients with weakened immunity, alcoholism, drug addiction, or pulmonary congestion are at particularly high risk. Nosocomial infection is also possible, such as the development of severe pneumonia following surgery or inhalation anesthesia.
The bacterium has low persistence in the external environment:
- It is rapidly killed by standard disinfectants.
- At 60 °C it is destroyed in 30 minutes, and upon boiling—instantaneously.
- However, in sputum, which acts as a protective colloid, it can persist for up to 10 days.
Pathogenicity factors and pathogenesis
The infectious process begins with adhesion and colonization of the epithelium. Pneumococcus produces enzymes that destroy the cilia of the ciliated epithelium, blocking mucociliary clearance. The pathogen also activates the complement system via the classical pathway and directly damages neutrophils, monocytes, epithelium, and endothelium.
Depending on the direction of spread, three scenarios are distinguished:
- Pulmonary route (pneumonia): the bacterium moves down the bronchial tree into the alveoli. Here, the enzyme pneumolysin plays a key role by promoting active reproduction of the pathogen.
- ENT pathology: colonization of the middle ear and paranasal sinuses causes otitis media, frontal sinusitis, and sinusitis.
- Generalization: with profound suppression of immunity, the invasion enzyme hyaluronidase helps breach tissue barriers. The pathogen enters the bloodstream (bacteremia) and, crossing the blood-brain barrier, causes purulent meningitis.
Diagnostics, treatment, and prevention
Research specimens include sputum, pus, blood, or cerebrospinal fluid (CSF), depending on the clinical form. Post-infection immunity is species-specific, but its strength and duration are variable.
Diagnostic methods:
- Microscopic: the smear is Gram-stained. The typical picture shows Gram-positive oval or lancet-shaped diplococci surrounded by a thick capsule.
- Bacteriological (primary): inoculation on blood agar. Identification is based on biochemical properties (tests for optochin and bile sensitivity are important) and serology.
- Biological: infection of white mice (rarely used, only in complex cases).
Due to frequent multidrug resistance of pneumococcus to antibiotics, treatment (usually with $\beta$-lactams) is prescribed strictly based on antibiotic susceptibility testing results. To protect risk groups, a specific chemical vaccine consisting of capsular polysaccharides is used: the pediatric version contains 7 antigens, and the adult version contains 23.
Briefly: answers to topical questions
What pathogenicity factors does pneumococcus produce?
Pneumococcus (Streptococcus pneumoniae) produces a complex of structural components, enzymes, and toxins that ensure invasion and evasion of the immune response.
- Capsule — inhibits phagocytosis and is the main virulence factor.
- C-substance (C-polysaccharide) — a teichoic acid component that activates complement via the alternative pathway.
- IgA protease — degrades secretory IgA on mucous membranes.
- Neuraminidase — reduces mucus viscosity, promoting colonization.
- Lysozyme (muramidase) — a respiratory tract colonization factor.
- Hyaluronidase — a spreading factor that cleaves hyaluronic acid in the extracellular matrix.
- Pneumolysin ($\beta$-hemolysin) — a cytotoxin that destroys erythrocytes.
What type of hemolysis does pneumococcus produce when cultured on blood agar?
When cultured on blood agar, pneumococcus produces $\alpha$-hemolysis or $\beta$-hemolysis depending on environmental conditions. Under standard conditions, $\alpha$-hemolysis (incomplete hemolysis) is observed, in which a green zone forms around small colonies due to the production of methemoglobin. However, the pathogen produces pneumolysin ($\beta$-hemolysin), which is capable of causing $\beta$-hemolysis, but this occurs exclusively under anaerobic conditions.
What antigens are part of the antigenic structure of pneumococcus?
The antigenic structure of pneumococcus is complex and includes several polysaccharide and protein components. It comprises capsular polysaccharide antigens used to produce specific chemical vaccines. It also contains C-substance (C-polysaccharide), a component of teichoic acids. Additionally, a protein-target antigen is localized directly within the bacterial cell itself.
What is the mechanism of the cytotoxic action of pneumolysin?
The mechanism of the cytotoxic action of pneumolysin involves the formation of transmembrane pores in target cells. This toxin belongs to the group of pore-forming cytolysins that damage eukaryotic cell membranes. Pore formation leads to a disruption of selective ion transport, ultimately causing cell lysis. Specifically, pneumolysin destroys erythrocytes, acting as a $\beta$-hemolysin.
Remember the main enzymes and their pathways: Pneumolysin targets the Lungs (P–L [in Russian: Pnevmolizin — Lyogkiye]), and Hyaluronidase ensures Generalization and Purulent meningitis (H–G–M [in Russian: Gialuronidaza — Generalizatsiya — Gnoyny meningit]).
Class and exam questions
What does Streptococcus pneumoniae look like on smear microscopy?
These are Gram-positive diplococci with a lancet or oval shape. A characteristic feature is the presence of a thick capsule surrounding the cells.
Which tests are used to identify pneumococcus in bacteriological diagnostics?
Upon inoculation on blood agar, the pathogen is identified using biochemical tests for optochin and bile sensitivity.
To which antibiotics does pneumococcus show natural resistance?
The bacterium possesses natural resistance to polymyxin and many drugs from the aminoglycoside group.
What else to study on this topic
- Mechanism of the destruction of ciliated epithelial cilia by pneumococcus
- Features of complement activation via the classical pathway in pneumococcal infection
- Biological diagnostic method: technique of laboratory mouse infection
- Mechanisms of multidrug resistance development in Streptococcus pneumoniae
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