Phenylalanine Deaminase Agar
A bench guide to Phenylalanine Deaminase Agar, used to differentiate Proteus, Morganella, and Providencia (positive) from other Enterobacteriaceae (negative) by adding ferric chloride.
Phenylalanine slants A-B
4 to 5 drops of 10% ferric chloride are added to incubated slants. A green color shift indicates phenylalanine deaminase activity.
Slant turns a deep forest green color immediately after adding 10% ferric chloride. Phenylpyruvic acid formed a colored complex with ferric iron.
Slant remains original amber/yellow color after adding 10% ferric chloride. Ferric chloride creates a temporary yellow puddle but no green complex.
Slant turns a deep forest green color immediately after adding 10% ferric chloride. Phenylpyruvic acid formed a colored complex with ferric iron.
Slant remains original amber/yellow color after adding 10% ferric chloride. Ferric chloride creates a temporary yellow puddle but no green complex.
What to look for
| Visual clue | Meaning | Student shorthand |
|---|---|---|
| Forest green slant | Phenylpyruvic acid (from phenylalanine deamination) binds with ferric chloride | Positive result |
| Amber/Yellow slant | Phenylalanine remains intact; no phenylpyruvic acid produced | Negative result |
| Reagent addition | Requires adding 10% ferric chloride (FeCl3) post-incubation; color fades within 10-15 minutes | Read immediately |
Deaminase Interpretive Matrix
| Pattern | What to call it | How to think about it |
|---|---|---|
| Deep green color on slant | Positive | Organism produces phenylalanine deaminase (e.g., Proteus mirabilis). |
| No color change (amber/yellow) | Negative | Organism lacks phenylalanine deaminase (e.g., Escherichia coli). |
- Phenylalanine deaminase removes the amine group (NH2) from phenylalanine, yielding phenylpyruvic acid and ammonia.
- Ferric chloride reacts directly with phenylpyruvic acid to form a green coordination compound.
- This test is a classic tool for identifying the Proteus-Morganella-Providencia tribe within Enterobacteriaceae.
P is for Proteus (and Providencia/Phenylalanine). Green = Positive. Yellow = Negative (like E. coli).