Decarboxylase Tests (Moeller's Method)
An original Learn Microbes bench guide for reading amino acid decarboxylation (Lysine, Ornithine, Arginine) reactions using Moeller's broth and mineral oil.
Decarboxylase tubes A-D
Always read the specific amino acid tube alongside the control tube (no amino acid). A valid test requires the control tube to be acid (yellow).
Organism fermented glucose (acid/yellow) first, then decarboxylated the amino acid, releasing alkaline amines and shifting the pH back to purple.
Organism fermented glucose to produce acid (yellow), but lacked the specific decarboxylase enzyme, so the broth remains yellow.
Control broth contains glucose but no amino acid. Must turn yellow (acid) to validate glucose fermentation. If the control is purple, the test is invalid!
Uninoculated Moeller's broth sits at pH 6.0 and exhibits an orange-amber color.
Organism fermented glucose (acid/yellow) first, then decarboxylated the amino acid, releasing alkaline amines and shifting the pH back to purple.
Organism fermented glucose to produce acid (yellow), but lacked the specific decarboxylase enzyme, so the broth remains yellow.
Control broth contains glucose but no amino acid. Must turn yellow (acid) to validate glucose fermentation. If the control is purple, the test is invalid!
Uninoculated Moeller's broth sits at pH 6.0 and exhibits an orange-amber color.
What to look for
| Visual clue | Meaning | Student shorthand |
|---|---|---|
| Purple broth | Alkaline reaction (decarboxylation positive) | The specific decarboxylase enzyme is active |
| Yellow broth in amino acid tube | Acid reaction (decarboxylation negative) | The specific decarboxylase is inactive; glucose was fermented |
| Yellow broth in control tube | Valid control test | Proves glucose was fermented and organism grew successfully |
| Purple/Orange broth in control tube | Invalid test result | Either the organism did not grow or failed to ferment glucose; do not report results |
| Translucent oil overlay | Sterile mineral oil barrier | Creates anaerobic conditions required for active decarboxylation |
Coagulase study readout
| Pattern | What to call it | How to think about it |
|---|---|---|
| Test (+) / Control (-) | Decarboxylase Positive | The organism grew, fermented glucose (yellow), and decarboxylated the amino acid (purple). |
| Test (-) / Control (-) | Decarboxylase Negative | Both tubes turned yellow; glucose was fermented, but the specific amino acid was not decarboxylated. |
| Test (+) / Control (+) | Invalid Test Result | Both tubes are purple. Either no growth occurred, glucose was not fermented, or the organism is nonfermenting. |
| Uninoculated | Baseline Orange-Amber | Broth sits at pH 6.0 prior to inoculation. Do not interpret as a patient test. |
- Differentiates decarboxylase-producing Enterobacteriaceae from other Gram-negative rods.
- Measures enzymatic ability to decarboxylate an amino acid (Lysine, Ornithine, or Arginine) to form alkaline amines.
- Requires an anaerobic environment created by overlaying with sterile mineral oil.
A yellow control tube is your reference validation. A purple control tube voids the entire test run.