Streptococcus pneumoniae: Morphology and Identification, Antigenic Structure, Enzymes and Toxins, Pathogenesis, Pathology, Clinical Findings,

Question 1

Morphology and identification of Streptococcus pneumoniae

Answer 1

Streptococcus pneumoniae is a Gram-positive, lancet-shaped diplococcus. It is encapsulated, nonmotile, and facultatively anaerobic. It is alpha-hemolytic on blood agar (greenish discoloration), optochin-sensitive, and bile soluble. Colonies may have a mucoid appearance due to the polysaccharide capsule. Identification involves Gram stain, optochin disk test, and bile solubility.

Question 2

Antigenic structure of Streptococcus pneumoniae

Answer 2

The major antigen is the polysaccharide capsule, which determines the serotype and is essential for virulence. More than 90 serotypes exist. The capsule inhibits phagocytosis. Other important antigens include C-polysaccharide (teichoic acid), which reacts with C-reactive protein (CRP), and surface proteins like Pneumococcal surface protein A (PspA) and adhesins that aid in colonization and immune evasion.

Question 3

Enzymes and toxins of Streptococcus pneumoniae

Answer 3

1) Pneumolysin: A cytotoxin that damages ciliated epithelial cells, inhibits immune responses, and activates complement. 2) Autolysin (LytA): Promotes cell lysis and release of pneumolysin. 3) Hyaluronidase: Breaks down connective tissue, aiding spread. 4) Neuraminidase: Modifies host cell surfaces, facilitating adherence and invasion. These enzymes contribute to inflammation, tissue damage, and immune evasion.

Question 4

Pathogenesis of Streptococcus pneumoniae

Answer 4

Colonization begins in the nasopharynx. The capsule prevents phagocytosis, while adhesins help attach to respiratory epithelium. Pneumolysin damages tissues and inhibits immune responses. Aspiration of organisms into the lungs can lead to pneumonia. Hematogenous spread can cause meningitis or sepsis. Risk factors include impaired immunity, viral infections, smoking, asplenia, and extremes of age.

Question 5

Pathology of Streptococcus pneumoniae

Answer 5

In pneumonia, alveoli become filled with neutrophils and exudate, leading to consolidation. The infection typically affects a lobe (lobar pneumonia). In meningitis, inflammation of the meninges causes increased intracranial pressure, cerebral edema, and neuronal damage. Otitis media and sinusitis involve mucosal inflammation and pus formation. In septicemia, systemic inflammation leads to multi-organ dysfunction.

Question 6

Clinical findings of Streptococcus pneumoniae infections

Answer 6

1) Pneumonia: Sudden onset of fever, chills, productive cough with rusty sputum, pleuritic chest pain, and shortness of breath. 2) Meningitis: Headache, neck stiffness, photophobia, vomiting, and altered mental status. 3) Otitis media: Ear pain, fever, irritability in children. 4) Sinusitis: Facial pain, nasal discharge, congestion. 5) Septicemia: Fever, hypotension, organ failure. Infections are more severe in immunocompromised and asplenic patients.

Question 7

Diagnostic laboratory tests for Streptococcus pneumoniae

Answer 7

1) Gram stain of sputum or CSF: Shows lancet-shaped Gram-positive diplococci. 2) Culture: Alpha-hemolytic colonies on blood agar; optochin-sensitive and bile soluble. 3) Quellung reaction: Capsule swelling with specific antisera. 4) Urinary antigen test: Detects pneumococcal C-polysaccharide antigen. 5) PCR: For pneumococcal DNA, especially in CSF or blood. 6) Blood cultures: Useful in pneumonia and sepsis.

Question 8

Treatment of Streptococcus pneumoniae infections

Answer 8

First-line treatment includes penicillin G or amoxicillin if the strain is sensitive. Due to rising resistance, third-generation cephalosporins (ceftriaxone), vancomycin, or fluoroquinolones may be used for serious infections like meningitis. Macrolides (e.g., azithromycin) and doxycycline are alternatives for mild infections. Antibiotic susceptibility testing is essential due to variable resistance patterns.

Question 9

Epidemiology of Streptococcus pneumoniae

Answer 9

S. pneumoniae is a leading cause of bacterial pneumonia, meningitis, and otitis media worldwide. It colonizes the nasopharynx in up to 40% of healthy children and fewer adults. Spread occurs via respiratory droplets. High-risk groups include children <5 years, elderly, immunocompromised, and individuals with chronic diseases or asplenia. Pneumococcal disease is more common in winter months and after viral respiratory infections.

Question 10

Prevention and control of Streptococcus pneumoniae

Answer 10

Two main vaccines are used: 1) PCV13 (13-valent pneumococcal conjugate vaccine): Recommended for infants and immunocompromised adults; induces strong T-cell-dependent immunity. 2) PPSV23 (23-valent pneumococcal polysaccharide vaccine): For adults ≥65 years and high-risk patients; induces T-cell-independent immunity. Preventive measures include vaccination, hand hygiene, and early treatment of respiratory infections. Antibiotic stewardship is crucial to prevent resistance.