Arginine Dihydrolase Test




1. Objective

The objective of the Arginine Dihydrolase Test was to determine whether the test organism produced the enzyme arginine dihydrolase, which hydrolyzed arginine to ornithine, ammonia, and CO₂ through the arginine dihydrolase pathway.

2. Principle

The test was based on the decarboxylation and hydrolysis of arginine.

  • Arginine was first converted to citrulline by the enzyme arginine dihydrolase.

  • Citrulline was then converted to ornithine and ammonia, leading to an alkaline pH.

  • A pH indicator (usually bromcresol purple) in the medium changed color from yellow to purple under alkaline conditions, indicating a positive result.

3. Materials

  • Moeller’s decarboxylase base medium

  • Arginine-containing medium (arginine dihydrolase broth)

  • Sterile mineral oil

  • Inoculating loop

  • Incubator (35–37°C)

  • Test organism (pure culture)

  • Marker pen

  • Control tubes (without arginine)

4. Procedure (Macroscopic)

  • A loopful of pure culture was inoculated into the arginine dihydrolase broth tube.

  • Approximately 1 cm of sterile mineral oil was added on top to create anaerobic conditions.

  • The tube was incubated at 35–37°C for 1–4 days.

  • A control tube without arginine was also inoculated and incubated for comparison.

  • After incubation, the color change in the medium was observed.

5. Result

  • Positive: Medium turned purple (alkaline), indicating production of arginine dihydrolase.

  • Negative: Medium remained yellow, indicating no enzyme activity.

6. Uses

  • The test was used to differentiate Enterobacteriaceae and non-fermenting Gram-negative bacilli.

  • It helped identify bacteria like Enterobacter cloacae, Pseudomonas spp., and Moraxella spp.

  • Commonly included in biochemical identification panels.

7. Consultation

  • Results were interpreted in conjunction with other amino acid decarboxylation tests (like lysine and ornithine).

  • Clinical microbiologists consulted test results for species identification and diagnosis.

  • In diagnostic microbiology, it supported accurate identification of enteric and non-enteric Gram-negative bacteria.

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