Pus aspirated from an empyema showed Gram-negative bacilli and colonies producing a soluble green pigment; the most likely organism is?

• A. Chromobacterium violaceum
• B. Legionella pneumophila
• C. Pseudomonas aeruginosa
• D. Serratia marcescens

Answer: (C)

Pigment production helps distinguish this organism. Pseudomonas aeruginosa is a Gram-negative rod known for making pigments, notably a soluble green pigment called pyoverdin (and also a blue-green pigment, pyocyanin). The presence of a soluble green pigment in colonies is a classic clue pointing to Pseudomonas aeruginosa, especially in pus from a pleural empyema where Gram-negative bacilli are seen.

Chromobacterium violaceum would produce a purple pigment (violacein), not green. Serratia marcescens typically produces a red pigment (prodigiosin) under many conditions. Legionella pneumophila is a Gram-negative bacillus that does not characteristically show a soluble green pigment in routine culture.