Which organism typically produces a green discoloration of the medium in which it is grown?

• A. Burkholderia cepacia
• B. Pseudomonas aeruginosa
• C. Staphylococcus aureus
• D. Escherichia coli

Answer: (B)

Green coloration of the medium around the colonies is a classic sign of pigment production by Pseudomonas aeruginosa. This organism makes two key pigments: pyocyanin, which is blue-green and diffuses into the agar to give a green hue, and pyoverdin, a yellow-green pigment that can fluoresce under UV light. The combined diffusion of these pigments into the surrounding medium is what you typically observe as a green discoloration.

Other organisms listed do not produce this characteristic green diffusion. Staphylococcus aureus has a golden-yellow pigment in its colonies, not green. Escherichia coli is generally colorless on many media and only shows a green sheen in specific media like EMB due to dye interactions, which is a different phenomenon from a diffuse green color of the medium. Burkholderia cepacia may produce pigments in some strains, but the distinct blue-green pigment diffusion seen with Pseudomonas aeruginosa is the hallmark.

That pigment production profile is why Pseudomonas aeruginosa is the organism most typically associated with a green discoloration of the growth medium.