Lysine decarboxylase test,
negative (left) & positive (right)
Decarboxylation Tests
DESCRIPTION
Decarboxylation test is used to determine bacteria's ability to decarboxylate amino acids. Most used  amino acids are arginine,  lysine
and  ornithine. To induce the decarboxylase enzymes both the substrate (amino acids) and acidic conditions are required.
MEDIUM PREPARATION
Composition: peptone 5 g, meat extract 5 g, bromcresol purple 1.6% (0.625 ml);
cresol red 0.2% (2.5 ml), glucose 0.5g, pyridoxal phosphate 5 mg, distilled water
1000 ml. Add 1% L-lysine or 1% L-ornithine (or 1% L-arginine for arginine dihydrolase
activity testing).
Adjust pH to 6-6.5.

PROCEDURE
Inoculate germs from a young culture, add mineral oil and incubate at 37 °C. Read
results after 24 hours; purple medium for positive and yellow for negative.

EXPLANATION
First the glucose is fermented with acid production, turning the medium yellow. Then
the decarboxylase enzyme hydrolyses the amino acids increasing the pH. The high
pH level turns the medium purple.
NOTES:   
If the test is incubated more than  24 hours then organisms start using the peptone, the resulted products increase the pH and false
positive results may be obtained.
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