Which microorganisms produce a positive phenylalanine deaminase reaction?

Which microorganisms produce a positive phenylalanine deaminase reaction?

• A. Citrobacter freundii
• B. Klebsiella pneumoniae
• C. Providencia stuartii
• D. Yersinia enterocolitica

Answer: (C)

The test looks for phenylalanine deaminase enzyme activity. If the organism can deaminate phenylalanine, it produces phenylpyruvic acid, which then reacts with ferric chloride to give a green color. This positive color change is characteristic of certain genera, notably Providencia, Morganella, and Proteus. Providencia stuartii is known to express phenylalanine deaminase, so it yields a positive result with this test, turning the medium green after the ferric chloride reagent is added. The other organisms listed—Citrobacter freundii, Klebsiella pneumoniae, and Yersinia enterocolitica—do not typically produce phenylalanine deaminase, so they do not produce the green color and are considered negative for this reaction. This distinction helps differentiate PAD-positive members of the Proteae tribe (and related genera) from PAD-negative enteric bacteria during routine identification.

The test looks for phenylalanine deaminase enzyme activity. If the organism can deaminate phenylalanine, it produces phenylpyruvic acid, which then reacts with ferric chloride to give a green color. This positive color change is characteristic of certain genera, notably Providencia, Morganella, and Proteus.

Providencia stuartii is known to express phenylalanine deaminase, so it yields a positive result with this test, turning the medium green after the ferric chloride reagent is added. The other organisms listed—Citrobacter freundii, Klebsiella pneumoniae, and Yersinia enterocolitica—do not typically produce phenylalanine deaminase, so they do not produce the green color and are considered negative for this reaction. This distinction helps differentiate PAD-positive members of the Proteae tribe (and related genera) from PAD-negative enteric bacteria during routine identification.