• Leucine Amino Peptidase (LAP) test is one of the rapid tests which is employed for the detection of the enzyme leucine amino peptidase.
• The LAP test is generally performed for the preliminary characterization of catalase- negative, gram-positive cocci, particularly non-beta hemolytic cocci.
• Leucine- -naphthalamide impregnated disk acts as a substrate.
• The LAP enzyme is responsible for hydrolysis of substrate that yields -naphthalamine.
• The product on addition to p-methyl (an aminocinnamaldehyde reagent) forms a highly visible red colored Schiff’s base.

Procedure:

1. Place a LAP disk in a sterile petri dish, and leave the disk to warm to room temperature.
2. Lowly dampen the LAP disk either with reagent grade water or with a little sterile distilled water.
3. Use a wooden applicator stick to rub a small amount of several colonies of an 18 to 24 hour pure culture onto a small area of the LAP disk.
4. Incubate at room temperature for 5 minutes.
5. Add 1 drop of cinnamaldehyde reagent after the incubation period, and read within one minute.

Results interpretation:

• Positive result:
  • The positive test is indicated by the development of a red/pink color.
• Negative result:
  • The negative test is suggested by no change or slight yellow color.

Limitations:

• The confirmation of test organism to be gram-positive coccus and catalase negative is mandatory for the LAP test.
• There might be chances of false negative if inadequate columns are taken.

Leucine Amino Peptidase (LAP) test

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