Phenylalanine Deaminase Test: Principle, Procedure, Results

Phenylalanine deaminase test also known as phenylpyruvic acid (PPA) test is used to test the ability of an organism to produce enzyme deaminase. This enzyme removes the amine group from the amino acid phenylalanine and produces phenylpyruvic acid (PPA) and ammonia i.e. oxidative deamination of phenylalanine. Phenylpyruvic acid reacts with ferric iron (10% ferric chloride is added to the medium) producing a visible green color.

Phenylalanine agar, also known as phenylalanine deaminase medium which contains DL-phenylalanine and nutrients is used as a test medium.

Phenylalanine Deaminase Test

Media and Reagents

Phenylalanine agar medium is prepared and poured as a slant into a tube. The composition of the medium is as follows:

  • DL-Phenylalanine: 2 gm
  • Yeast extract: 3 gm
  • Sodium chloride: 5 gm
  • Disodium phosphate: 1 gm
  • Agar: 12 gm
  • Distilled water: 1 L
  • pH: 7.3

Yeast extract serves as carbon and nitrogen source. Meat extracts or protein hydrolysates cannot be used because of the varying natural content of phenylalanine.


  • Ferric chloride: 12 gm
  • Concentrated HCL: 2.5 mL
  • Distilled water: 100 mL


  1. Take or prepare a phenylalanine deaminase agar medium
  2. Inoculate the phenylalanine slant (with a loop on the surface) with a test organism.
    Note: If you are using the test medium i.e. phenylalanine agar for the first time use positive (Proteus vulgaris) and negative control (Escherichia coli) to check the efficacy of the test medium.
  3. Incubate the test medium at 37°C for overnight.
  4. Add 4-5 drops of 10% aqueous ferric chloride (FeCl3) solution to the slant. After addition of the reagent, rotate the tube to dislodge the surface colonies.
Phenylalanine deaminase test results
Phenylalanine deaminase test results (Source: microbugz)


  1. Positive test: Production of green color (Phenylpyruvic acid thus formed reacts with ferric chloride producing a green colored compound thus turning the medium dark green). Proteus sp., Morganella sp., Providenica sp give positive PPA test.
  2. Negative: No color change (medium remains straw/yellow color; no PPA to react with ferric chloride).

Uses of Phenylalaine Deaminase Test

Phenylalaine deaminase test is used to differentiate members of the genera Proteus, Morganella, and Providencia (+ve)  from other members of Enterobacteriaceae which give negative results.

Quality Control

Each new batch of medium or reagent must be tested with positive and negative control.

  • Positive control: Proteus species
  • Negative control: Escherichia coli


  1. Color Atlas and Textbook of Diagnostic Microbiology, Koneman, 5th edition
  2. Microbucz. Phenylalanine Deaminase Test.  

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. As an asst. professor, I am teaching microbiology and immunology to medical and nursing students at PAHS, Nepal. I have been working as a microbiologist at Patan hospital for more than 10 years.

3 thoughts on “Phenylalanine Deaminase Test: Principle, Procedure, Results

  1. Hello sir, want to ask you that in stool wet mount if we see cyst or trophozoites of entamoeba shall we report it as cyst or trophozoites of entamoeba sip. Or we can report as entamoeba history tics as books mention that intracellular structures are not well appreciated in wet mount. Also can cyst form & trophozoites form can be seen in same stool.

    1. Hello Varsha
      Only one species of Entamoeba i.e. E.histolytica is of medical importance as it causes amoebiasis. Finding other species of Entamoeba (harmless commensals) in the stool do not help in the diagnosis. Cyst of E.histolytica can be distinguished from others looking at the number of nuclei, whereas if you see the trophozoite, you have to look for characterstics diagnostic feature (presence of RBCs) and nature of nucleus. Generally, cyst are found in the formed/semi formed stools, and trophozoites in watery stools, but sometime both can be present. Find more here

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