A 55-year-old alcoholic man develops pneumonia with 'currant-jelly' sputum, consolidation predominantly in the upper lobes and early cavitation on CXR. Blood and sputum cultures grow mucoid, string-test positive (mucoviscosity) lactose-fermenting gram-negative rods. What is the organism and its primary virulence mechanism in pulmonary infection?

• A Pseudomonas aeruginosa — mucoid colonies in alcoholics cause necrotising pneumonia with string-test positivity
• B Klebsiella pneumoniae — hypermucoviscous strains produce a thick polysaccharide capsule (K1/K2 serotypes) that inhibits phagocytosis and complement-mediated killing, causing necrotising 'Friedlander's pneumonia' with early abscess formation ✓
• C Streptococcus pneumoniae — capsulated gram-negative rod causing lobar pneumonia with rusty sputum
• D Acinetobacter baumannii — common in alcoholics; causes cavitary lobar disease with currant-jelly sputum

Explanation

Klebsiella pneumoniae is the classic cause of 'Friedlander's pneumonia' in alcoholics and debilitated patients. Characteristics: mucoid lactose-fermenting gram-negative rod, mucoviscous string test positive (hypercapsulate strains), currant-jelly sputum (haemorrhagic mucoid), upper lobe predilection with early cavitation and abscess. Virulence: the thick polysaccharide capsule (especially hypervirulent K1 and K2 serotypes) inhibits phagocytosis and complement deposition; endotoxin causes systemic inflammatory response. Hypervirulent K. pneumoniae (hvKP) also causes liver abscess with metastatic spread (Taiwanese strain). Rusty sputum is pneumococcal. Pseudomonas is not string-test positive and rarely causes upper lobe primary pneumonia in alcoholics.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

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Written and medically reviewed by the StethoPrep medical team.