• Decarboxylases are a group of enzymes which acts by hydrolyzing an amino acid to form an amine.
• Decarboxylase test is mainly performed to differentiate decarboxylase producing Enterobacteriaceae from other gram-negative rods.
• The decarboxylation of the amino acid yields in an alkaline pH and a change in color of pH indicators bromocresol and cresol red from orange to purple is observed.

Requirements:

1. Media: Brain-heart infusion broth, Blood agar, Decarboxylase broth
2. Test-tubes
3. Incubator
4. Sterile mineral oil

composition of Decarboxylase broth

1. Composition———————————–g/ltr
   • Peptic digest of animal tissue——– (5 g)
   • beef extract ———————————-(5 g)
   • bromocresol purple ———————-(0.1 g)
   • cresol red————————————- (0.005 g)
   • dextrose —————————————(0.5 g)
   • pyridoxal ————————————(0.005 g)
   • amino acid ———————————–(10 g)
2. pH-6.0

Procedure of Decarboxylase test:

1. Glucose-Nonfermenting Organisms:
   • Prepare a suspension (≥McFarland No. 5 turbidity standard) in brain-heart infusion broth from 18-24hr old culture on 5% sheep blood agar.
   • With 4 drops of broth, inoculate each of the three decarboxylase broths (arginine, lysine, and ornithine) and the control broth (no amino acid).
   • To each tube, add a 4-mm layer of sterile mineral oil.
   • Incubate the cultures at 35°-37°C in ambient air.
   • Examine the tubes at 24, 48, 72, and 96 hours.
2. Glucose-Fermenting Organisms:
   • With 1 drop of an 18- to 24-hour brain-heart infusion broth culture, inoculate each tubes.
   • To each tube, add a 4-mm layer of sterile mineral oil.
   • Incubate the cultures for 4 days at 35°-37°C in ambient air.
   • Observe and examine the tubes at 24, 48, 72, and 96 hours.

Results Interpretations:

• Positive test:
  • It is indicated by the alkaline pH and change in coloration from orange to purple.
• Negative test:
  • It is suggested by no change in colour in both test and control tubes and growth is visible in the control tube.

Decarboxylase test-Principle, Procedure and Results

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