LAP Test- Objective, Principle, Procedure, Results, Uses


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.

Procedure of Leucine Amino Peptidase (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 Leucine Amino Peptidase (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.


  • 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)

About Author

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Sagar Aryal

Sagar Aryal is a microbiologist and a scientific blogger. He attended St. Xavier’s College, Maitighar, Kathmandu, Nepal, to complete his Master of Science in Microbiology. He worked as a Lecturer at St. Xavier’s College, Maitighar, Kathmandu, Nepal, from Feb 2015 to June 2019. After teaching microbiology for more than four years, he joined the Central Department of Microbiology, Tribhuvan University, to pursue his Ph.D. in collaboration with Helmholtz-Institute for Pharmaceutical Research Saarland (HIPS), Saarbrucken, Germany. He is interested in research on actinobacteria, myxobacteria, and natural products. He has published more than 15 research articles and book chapters in international journals and well-renowned publishers.

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