Clinical microbiology focus: Pseudomonas aeruginosa produces a characteristic diffusible blue-green pigment that aids presumptive identification in the laboratory. What is the name of this diagnostic pigment?

Introduction / Context:
Pseudomonas aeruginosa is a non-fermenting, oxidase-positive Gram-negative bacillus and a major opportunistic pathogen. In clinical microbiology, certain pigments produced by this organism help with rapid, presumptive identification. Among these, the blue-green pigment pyocyanin is especially iconic and often visible on routine media or even in wound dressings.

Given Data / Assumptions:

• The question asks for the specific pigment used for diagnosis.
• Culture conditions are typical (e.g., on nutrient agar, cetrimide agar).
• P. aeruginosa can make multiple pigments, but only one is the classic blue-green compound.

Concept / Approach:
P. aeruginosa may synthesize several pigments: pyocyanin (blue-green, a phenazine compound), pyoverdin (yellow-green fluorescent siderophore), pyomelanin (brown), and occasionally pyorubrin (red). Of these, pyocyanin most strongly associates with the pathogen’s traditional “blue-green” appearance and the characteristic sweet, grape-like odor noted on plates. While pyoverdin contributes a greenish fluorescence under UV, pyocyanin remains the key pigment for classic visual recognition.

Step-by-Step Solution:
List known pigments of P. aeruginosa (pyocyanin, pyoverdin, pyomelanin, pyorubrin). Identify which pigment produces a blue-green coloration on media. Confirm that the diagnostic hallmark most widely taught is pyocyanin. Select “Pyocyanin” as the correct answer.

Verification / Alternative check:
Growth on cetrimide agar enhances pigment production; observing a blue-green hue along with oxidase positivity supports a presumptive identification pending further testing (e.g., growth at 42°C, biochemical panels, MALDI-TOF).

Why Other Options Are Wrong:

• Pyoverdin: Yellow-green fluorescent siderophore; important but not the classic blue-green pigment.
• Pyomelanin: Brown pigment; not diagnostic for the classic appearance.
• Pyorubrin: Red pigment; uncommon and not the standard indicator.
• Prodigiosin: Red pigment of Serratia marcescens, not P. aeruginosa.

Common Pitfalls:
Confusing green fluorescence (pyoverdin) with blue-green pigment (pyocyanin) or attributing prodigiosin to Pseudomonas instead of Serratia.

Final Answer:
Pyocyanin.