Leucine amino peptidase (LAP) test is used for the preliminary characterization of catalase negative, gram-positive cocci. This is a rapid test for detection of enzyme leucine aminopeptidase. Leucine- β- napthalamide impregnated disk serves as a substrate for the detection of leucine aminopeptidas. This enzyme hydrolyse the substrate resulting in production of β- naphthylamine which upon addition of p-dimethyl, aminocinnamaldehyde reagent form a highly visible red colored Schiff base.
LAP test is often used in conjunction with PYR and other biochemical tests to help differentiate between catalase negative, gram-positive cocci. 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.
Procedure/ Method of LAP test:
- Aseptically place a LAP disk in a sterile petri dish, and allow disk to warm to room temperature.
- Moisten the LAP Disk with a small amount of sterile distilled water.
- Inoculate with several colonies from an overnight culture plate.
- Incubate at room temperature for five minutes
- Add one drop of cinnamaldehyde reagent.
Interpretation of Results:
Positive: Development of a red/pink color with 1 minute after addition of cinnamaldehyde reagent
Negative: No change or a slight yellow color
- Confirm that the test organism is a gram positive coccus and is catalase negative before performing the LAP test.
- Ensure that adequate inoculum is used or false negatives may occur.
- Streptococci should be tested prior to 48-hour incubation or sub-cultured prior to testing.
Positive control: Enterococcus faecalis ATCC 29212
Negative Control: Aerococcus viridans ATCC 11563
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