A patient with a postoperative urinary tract infection shows an oxidase-positive organism that produces a diffusible green pigment; which organism is most likely?

• A. Escherichia coli
• B. Klebsiella pneumoniae
• C. Proteus mirabilis
• D. Pseudomonas aeruginosa

Answer: (D)

Think about distinguishing features: oxidase positivity and pigment production. Pseudomonas aeruginosa is an oxidase-positive, non-lactose–fermenting Gram-negative rod that characteristically makes a diffusible blue-green pigment, pyocyanin, which gives colonies a green coloration. This pigment clue is classic for Pseudomonas and helps separate it from other common UTI organisms. E. coli and Klebsiella are oxidase negative and do not produce a diffusible green pigment, and Proteus, while also a gram-negative rod associated with UTIs, is oxidase negative and lacks this pigment pattern. In a postoperative UTI with a diffusible green pigment, the best-match organism is Pseudomonas aeruginosa.