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Answer shortly in reference to apurba sastry microbiology
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I have all the information needed. Here are concise answers to all 5 questions, based on standard medical microbiology content aligned with Apurba Sastry's Microbiology:
Clinical Vignette 39 - Pneumococcal Pneumonia
Organism: Streptococcus pneumoniae
1. Clinical Diagnosis
Pneumococcal pneumonia (community-acquired lobar pneumonia caused by Streptococcus pneumoniae). The clues are: elderly alcoholic male (predisposed to aspiration), high-grade fever, productive cough, right lower lobe infiltrates on X-ray, and Gram-positive cocci in pairs (lancet-shaped diplococci) in sputum.
2. Virulence Factors
| Factor | Role |
|---|---|
| Polysaccharide capsule | Most important - antiphagocytic, basis of 90+ serotypes |
| Pneumolysin | Cytotoxin - lyses RBCs/leukocytes, activates complement, causes local inflammation |
| IgA protease | Cleaves secretory IgA, facilitates colonization of mucosal surfaces |
| Surface adhesins (PspA, PspC) | Aid colonization of nasopharynx/oropharynx |
| Teichoic acid / C polysaccharide | Triggers inflammatory response; contains phosphorylcholine |
| Autolysin (amidase) | Releases cell wall fragments that amplify inflammation |
3. Laboratory Diagnosis
Specimen: Sputum (lower respiratory), blood (bacteremia), CSF (meningitis)
- Gram stain of sputum: Gram-positive, lancet-shaped diplococci surrounded by a capsule; numerous neutrophils
- Culture: Blood agar (5% sheep blood) - alpha-hemolytic (green/partial hemolysis), small mucoid colonies with draughtsman/coin appearance; incubated in 5-10% CO2
- Identification tests:
- Optochin (ethylhydrocupreine) sensitivity - zone of inhibition ≥14 mm (distinguishes from viridans streptococci which are resistant)
- Bile solubility test - positive (colonies dissolve in 10% sodium deoxycholate)
- Catalase negative
- Quellung (capsular swelling) reaction: Capsule swells visibly when mixed with type-specific antisera - used for serotyping
- Urine antigen test: Detects C polysaccharide (useful in adults with pneumonia)
- Blood culture: Positive in 20-30% cases in early illness
4. Prophylaxis
- PCV13 (Pneumococcal Conjugate Vaccine - 13 valent): Recommended for all children <2 years and immunocompromised adults; polysaccharide conjugated to carrier protein - produces T-cell dependent response
- PPSV23 (Pneumococcal Polysaccharide Vaccine - 23 valent): Recommended for adults >65 years, asplenic patients, alcoholics, COPD, diabetes, and other high-risk groups; T-cell independent response
- Alcoholics like this patient are specifically at high risk due to impaired splenic function and ciliary clearance - they should receive PPSV23
5. Other Organisms Causing Pneumonia
Community-acquired pneumonia (other causes):
- Haemophilus influenzae (elderly, COPD)
- Staphylococcus aureus (post-influenza)
- Klebsiella pneumoniae (alcoholics, diabetics - currant jelly sputum)
- Mycoplasma pneumoniae (atypical/walking pneumonia - young adults)
- Legionella pneumophila (atypical - water-borne, AC systems)
- Chlamydophila pneumoniae (atypical)
- Viruses: Influenza, RSV, SARS-CoV-2
Note on this case: Alcoholics are also prone to Klebsiella pneumoniae (upper lobe, thick sputum) and aspiration pneumonia (anaerobes). The Gram-positive diplococci here confirm pneumococcus.
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