A 48-year-old man presents to the hospital with acute onset fever (39.2°C), productive cough with greenish sputum, and dyspnea. Chest X-ray shows bilateral lower lobe infiltrates. Sputum culture grows a Gram-negative bacillus that is oxidase-positive, non-motile, and produces a characteristic blue-green pigment on blood agar. Which of the following is the most likely organism?

Explanation

## Identification of Pseudomonas aeruginosa ### Clinical Presentation The patient presents with community-acquired pneumonia with bilateral lower lobe infiltrates and productive cough with greenish sputum. This clinical picture is highly suggestive of Pseudomonas aeruginosa, which is known for its association with respiratory tract infections, particularly in hospitalized patients and those with underlying lung disease. ### Microbiological Features The key diagnostic clues are: | Feature | Finding | Significance | |---------|---------|---------------| | Gram stain | Gram-negative bacillus | Confirms bacillary morphology | | Oxidase test | Positive | Pseudomonas is oxidase-positive | | Motility | Non-motile | Indicates non-flagellated strain | | Pigment | Blue-green (pyoverdine) | Pathognomonic for Pseudomonas aeruginosa | | Culture medium | Blood agar | Produces characteristic pigmentation | **Key Point:** The combination of oxidase-positivity and blue-green pigment production is virtually pathognomonic for Pseudomonas aeruginosa among Gram-negative bacilli. ### Pathophysiology 1. Pseudomonas aeruginosa is an aerobic, Gram-negative bacillus 2. It produces multiple virulence factors including exotoxins (A and U), elastase, and lipopolysaccharide 3. The blue-green pigment (pyoverdine) is an iron-chelating siderophore that aids in pathogenesis 4. It is commonly found in moist environments and is a leading cause of nosocomial infections ### Clinical Significance **High-Yield:** Pseudomonas aeruginosa is the most common Gram-negative aerobic bacillus causing respiratory infections in hospitalized patients and is a major concern in cystic fibrosis patients. **Clinical Pearl:** The greenish sputum is a classic clinical sign of Pseudomonas aeruginosa infection and is due to the production of pyoverdine and pyocyanin pigments. ### Differential Considerations While other Gram-negative bacilli can cause pneumonia, none produce the characteristic blue-green pigment or demonstrate the same oxidase-positive, non-motile phenotype in this clinical context. [cite:Jawetz, Melnick & Adelberg's Medical Microbiology 28e Ch 19]