Epidemiology and Survival
Humans are the only reservoir for the infection. Streptococcus pneumoniae is a normal inhabitant of the oropharyngeal mucosa (serving as the primary portal of entry), with carriage rates ranging from 40% to 70% in the general population.
Transmission occurs via airborne droplets. High-risk groups include young children, the elderly, and immunocompromised patients, as well as individuals with alcohol use disorder, substance use disorders, or pulmonary congestion. Nosocomial infection is also possible, such as the development of severe postoperative or post-inhalational anesthesia pneumonia.
The bacterium has low environmental persistence:
- Rapidly killed by standard disinfectants.
- Destroyed within 30 minutes at 60 °C, and instantaneously upon boiling.
- However, it can survive for up to 10 days in sputum, which acts as a protective colloid.
Virulence Factors and Pathogenesis
The infectious process begins with adhesion to and colonization of the epithelium. S. pneumoniae produces enzymes that damage the cilia of respiratory epithelial cells, impairing mucociliary clearance. Additionally, the pathogen activates the complement system via the classical pathway and directly damages neutrophils, monocytes, epithelial, and endothelial cells.
Depending on the route of spread, three main clinical scenarios are distinguished:
- Pulmonary route (pneumonia): the bacteria advance down the bronchial tree into the alveoli. The enzyme pneumolysin plays a key role here by promoting active bacterial replication.
- ENT pathology: colonization of the middle ear and paranasal sinuses leads to otitis media, frontal sinusitis, and other forms of sinusitis.
- Dissemination: in the setting of severe immunosuppression, the invasion enzyme hyaluronidase helps breach tissue barriers. The pathogen enters the bloodstream (bacteremia) and crosses the blood-brain barrier, resulting in purulent meningitis.
Diagnostics, Treatment, and Prevention
Diagnostic specimens include sputum, pus, blood, or cerebrospinal fluid (CSF), depending on the clinical presentation. Post-infection immunity is species-specific, but its strength and duration vary.
Diagnostic Methods:
- Microscopy: Gram-stained smears reveal characteristic oval or lancet-shaped Gram-positive diplococci surrounded by a thick capsule.
- Culture (primary method): inoculation on blood agar. Identification is based on biochemical properties (testing for optochin sensitivity and bile solubility) and serology.
- Animal inoculation: infection of white mice (rarely used, reserved for complex cases).
Due to frequent multidrug resistance to antibiotics, treatment (typically with $\beta$-lactams) must be prescribed strictly based on antimicrobial susceptibility testing. To protect high-risk groups, a specific capsular polysaccharide conjugate/polysaccharide vaccine is used: the pediatric formulation contains 7 antigens, while the adult formulation contains 23.