Decarboxylases are a group of substrate-specific enzymes that are capable of reacting with the carboxyl (COOH) portion of amino acids, forming alkaline-reacting amines and byproduct carbon dioxide. Increased pH of the medium is detected by a color change of the pH indicators bromocresol purple and cresol red.
Bromcresol purple turns purple at an alkaline pH and turns yellow at an acidic pH.
Each decarboxylase enzyme is specific for an amino acid. Lysine, ornithine, and arginine are the three amino acids routinely tested in the identification of Enterobacteriaceae.
The specific amine products are:
- Lysine- Cadaverine
- Arginine- Citrulline
These byproducts are sufficient to raise the pH of the media so that the broth turns purple. The medium used is arginine dihydrolase broth which is a nutrient broth supplemented with 0.5% arginine.
Arginine is hydrolyzed to ornithine (arginine is first converted to citrulline via dihydrolase reaction, in which NH2 group is removed from arginine. Citrulline is next converted to ornithine). Ornithine then undergoes decarboxylation to form putrescine. The production of amine, putrescine, elevates the pH, and the pH indicator Bromo cresol purple gives purple color in the medium (alkaline condition).
If the inoculated medium is yellow, or if there is no color change, the organism is decarboxylase-negative for that amino acid. If the medium turns purple, the organism is decarboxylase-positive for that amino acid.
- Moeller decarboxylase base-4 tubes with lysine, ornithine and agrinine hydrochloride 1% and control
- Inoculate the test medium, overlaid with sterile paraffin oil (Inoculated tubes must be protected from air to avoid false alkalinization at the surface of the medium)
- Incubate and read daily for four days
- Purple color-Positive decarboxylation
- Yellow color-Negative i.e. no decarboxylation
- Lysine: Klebsiella pneumoniae
- Ornithine: Enterobacter cloacae
- Arginine: Enterobacter cloacae
- Lysine: Enterobacter cloacae
- Ornithine: Klebsiella pneumoniae
- Arginine: Klebsiella pneumonia
Uses of decarboxylation Tests:
Arginine decarboxylase Test: It aids in differentiating enteric bacteria with closely related physiological characteristics.
Lysine Decarboxylase Test (LDC): To assist in the identification of Salmonellae (+ve) and Shigellae (-ve). Bacteria that are Lysine Decarboxylase (LDC) positive are: