The presence of a soluble green pigment in culture from empyema is most consistent with which organism?

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

Answer: (C)

A blue-green, soluble pigment in culture is a classic clue for Pseudomonas aeruginosa. This organism produces pyocyanin, a blue-green pigment that dissolves in the medium and often gives colonies a distinctive green color (and it can fluoresce under UV light due to another pigment, pyoverdine). This pigment pattern helps differentiate it from others: Chromobacterium violaceum makes a violet pigment, Serratia marcescens usually appears red due to prodigiosin (often at room temperature), and Legionella pneumophila does not exhibit a characteristic green pigment in routine culture. So, the green soluble pigment points most strongly to Pseudomonas aeruginosa.