A 65-year-old man with chronic bronchitis comes to the emergency department due to 2 days of fever, chills, chest pain, and productive cough. He has been hospitalized on several occasions for exacerbations of chronic bronchitis. The patient has been advised to stop smoking but continues to smoke cigarettes. He drinks 6–10 beers daily. Temperature is 40.2 C (104.4 F), blood pressure is 100/70 mm Hg, pulse is 104/min, and respirations are 28/min. On examination, he is in mild respiratory distress. There are crackles and decreased breath sounds in the right upper lobe. His chest x-ray shows right upper lobe consolidation. Sputum microscopy reveals encapsulated gram-negative bacilli. The bacteria grow pink, mucoid colonies on MacConkey agar. Which of the following organisms is the most likely cause of this patient's condition?
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Gram-negative bacilli are the leading cause of nosocomial pneumonia but are a relatively rare (<5%) cause of community-acquired pneumonia. However, patients with underlying immunosuppression (especially neutropenia) or impaired host defenses (eg, infants, the elderly, patients with alcohol use disorder) are at increased risk. The most common pathogens are Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Enterobacter, Serratia, Proteus, and Acinetobacter.
Klebsiella is a gram-negative bacillus that turns MacConkey agar pink due to its ability to ferment lactose. The presence of a thick capsule is seen as a clear zone on Gram stain and also causes the characteristic mucoid growth in culture. Klebsiella pneumonia classically affects patients with alcohol use disorder. Most cases arise due to colonization of the oropharynx followed by microaspiration of upper airway secretions. Because the aspiration usually occurs while supine (during sleep), the posterior segments of the upper lobes (particularly the right lung) are classically affected. The hallmarks of Klebsiella pneumonia are thick, mucoid, blood-tinged sputum (currant jelly sputum) and liquefying necrosis of the lung with early abscess formation.
(Choice A) Haemophilus influenzae is a leading cause of community-acquired pneumonia and bacterial meningitis. It is a gram-negative coccobacillus that can form mucoid colonies but does not ferment lactose.
(Choice C) Legionella pneumophila is a gram-negative bacillus that causes atypical pneumonia, generally following aerosolization from a contaminated water supply. However, Legionella is not an encapsulated organism and does not produce mucoid colonies.
(Choice D) Moraxella catarrhalis is a gram-negative diplococcus that is a common cause of upper respiratory infections (eg, bronchitis, laryngitis, sinusitis) and otitis media. It can also cause bronchopneumonia in patients with COPD. However, this pathogen does not ferment lactose.
(Choice E) Mycoplasma pneumoniae is a common cause of atypical pneumonia. However, this pathogen has no cell wall so it is not visible on Gram stain. It also does not grow in conventional culture.
(Choice F) Pseudomonas aeruginosa is a common nosocomial pathogen that may cause pneumonia in patients with structural lung disease or impaired immunity. It is an encapsulated, gram-negative bacillus that can appear mucoid in culture. However, it does not ferment lactose.
(Choice G) Streptococcus pneumoniae is the most common cause of community-acquired pneumonia. However, this pathogen is a gram-positive, lancet-shaped cocci usually found in pairs.
Educational objective:
Klebsiella is an encapsulated, lactose-fermenting, gram-negative bacillus that appears mucoid in culture. It causes pneumonia in individuals with impaired host defenses, especially patients with alcohol use disorder. Klebsiella pneumonia is characterized by tissue necrosis and early abscess formation with production of thick, mucoid, blood-tinged sputum (currant jelly sputum).