Leucine Amino Peptidase (LAP) Test

Leucine Amino Peptidase (LAP) Test


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Leucine Amino Peptidase (LAP) Test

Objective

To detect the presence of the enzyme leucine aminopeptidase for the preliminary characterization of catalase-negative, gram-positive cocci, especially non-beta-hemolytic cocci.

Principle of Leucine Amino Peptidase (LAP) Test

Leucine aminopeptidase (LAP) test is a rapid test for the detection of enzyme leucine aminopeptidase. In general, Streptococcus pneumoniae and Streptococcus pyogenes, Pediococcus, Lactococcus, and Enterococcus sps are all LAP positive, while other beta-hemolytic Streptococci, Aerococcus sps and Leuconostoc sps are LAP negative. Leucine- β- napthalamide impregnated disk serves as a substrate for the detection of leucine aminopeptidase. The enzyme LAP hydrolyzes the substrate resulting in the production of β- naphthylamine which upon addition of p-dimethyl, aminocinnamaldehyde reagent form a highly visible red-colored Schiff base.

The procedure of LAP Test

  1. Aseptically place a LAP disk in a sterile Petri dish and allow the disk to warm to room temperature.
  2. Moisten the LAP Disk with a small amount of sterile distilled water.
  3. Inoculate with several colonies from an overnight culture plate.
  4. Incubate at room temperature for five minutes
  5. Add one drop of cinnamaldehyde reagent
  6. Observe for the color change in the disk

Result of LAP Test

Leucine Amino Peptidase (LAP) Test

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Positive test: Development of a red color within 1 minute after adding cinnamaldehyde reagent.

Negative test: No color change or development of a slight yellow color.

Limitations of LAP Test

  • The test result depends on the integrity of the substrate-impregnated disk.
  • LAP test is often used in conjunction with PYR and other biochemical tests to help differentiate between catalase-negative, gram-positive cocci.

Quality Control

Positive: Enterococcus faecalis (ATCC29212)

Negative: Aerococcus viridans (ATCC11563)

Leucine Amino Peptidase (LAP) Test

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