The Leucine Aminopeptidase (LAP) Test: An Introduction and Primary Use
The Leucine Aminopeptidase (LAP) test is a rapid, essential biochemical assay utilized in the clinical microbiology laboratory for the definitive identification and differentiation of certain bacterial species. Specifically, this test targets a group of organisms known as catalase-negative Gram-positive cocci. While morphology and initial Gram stain offer preliminary clues, a precise diagnosis requires enzymatic profiling to distinguish between closely related, yet clinically distinct, genera.
The LAP test’s primary function is to distinguish the LAP-positive group, which includes *Streptococcus* species, *Enterococcus* species, *Lactococcus* species, and *Pediococcus* species, from the LAP-negative or variable group, most notably *Aerococcus* and *Leuconostoc* species. This simple differentiation is critical because the organisms identified often have distinct clinical significance and require different therapeutic approaches, making the LAP test a cornerstone in the initial identification workflow for these pathogens.
Principle of the LAP Test: Molecular Mechanism
The fundamental principle of the LAP test revolves around the detection of the enzyme leucine aminopeptidase. This enzyme, synthesized by certain bacteria, is a type of metallopeptidase that functions to hydrolyze leucine residues from the N-terminus of proteins or peptides. This proteolytic activity is important for bacterial survival, aiding in the metabolism of proteins, and in some cases, contributing to bacterial pathogenesis.
The test utilizes a specialized, absorbent paper disk that is impregnated with the colorless substrate, leucine-ß-naphthylamide (or leucine-p-naphthylamide). This compound serves as the target molecule for the bacterial LAP enzyme. When the test organism possesses the necessary enzyme, it hydrolyzes the leucine-ß-naphthylamide, releasing the free compound ß-naphthylamine, along with a leucine residue. The release of ß-naphthylamine is the key detection event.
Because ß-naphthylamine is colorless, a secondary chemical step is required to visualize the reaction. After a short incubation period, a color-developing reagent, cinnamaldehyde reagent (specifically, p-dimethylaminocinnamaldehyde), is added to the disk. The free ß-naphthylamine reacts with this cinnamaldehyde reagent to form a highly visible, stable Schiff base, which results in a characteristic, bright pink or cherry red color, providing a definitive positive result for the presence of leucine aminopeptidase.
Procedure for Performing the LAP Test
The LAP test is known for its simplicity and rapid turnaround time, making it highly suitable for clinical laboratory settings. The procedure involves several critical steps to ensure accurate results:
The first step involves aseptically placing a LAP disk in a sterile petri dish and allowing it to equilibrate to room temperature. The disk must then be slightly moistened with a small amount of sterile distilled or reagent-grade water. Care must be taken not to oversaturate the disk, as excess moisture can dilute the reaction and lead to an inaccurate result.
Next, a heavy inoculum of the test organism is crucial. Using a sterile wooden applicator stick or inoculating loop, a substantial amount of several colonies from an 18 to 24-hour pure culture is rubbed onto a small area of the moistened LAP disk. An insufficient inoculum is a common cause of false-negative reactions, as there may not be enough bacterial enzyme present to generate a detectable amount of ß-naphthylamine.
The inoculated disk is then incubated at room temperature, typically for a short period of 5 minutes. Following this brief incubation, one drop of the cinnamaldehyde reagent is added directly over the seeded area of the disk. The result is then observed and interpreted rapidly, usually within 1 to 2 minutes of adding the reagent.
Interpretation of LAP Test Results
The interpretation of the LAP test is based on the visible color change occurring after the addition of the cinnamaldehyde reagent. This clear distinction allows for rapid presumptive identification of the organism in question.
A **Positive Result** is indicated by the rapid development of a deep red, reddish-purple, or bright pink/cherry red color on the disk within two minutes of adding the cinnamaldehyde reagent. This color change confirms the presence of the leucine aminopeptidase enzyme. Organisms producing a positive result include *Streptococcus pneumoniae*, *Streptococcus pyogenes*, *Enterococcus* species, *Lactococcus* species, and *Pediococcus* species. Weak positive reactions may appear as a less intense pink color.
A **Negative Result** is indicated by the absence of a red or pink color, or the development of only a slight yellow or orange color, which is attributable to the reagent itself. The lack of color change signifies that the organism does not produce the LAP enzyme or produces it in quantities too low to be detected. Genera that typically yield a negative result include *Leuconostoc* species and some *Aerococcus* species.
Significance and Complementary Role in Identification
While the LAP test is essential, it is rarely used in isolation. Its true diagnostic power is realized when it is integrated into a biochemical testing panel, especially with the catalase test and the Pyrrolidonyl-arylamidase (PYR) test, to systematically narrow down the identification of Gram-positive cocci.
For instance, an organism is first confirmed to be a Gram-positive coccus. If it is catalase-negative, the LAP test helps differentiate it further. The pattern of LAP and PYR results provides a powerful tool for presumptive genus identification, particularly for differentiating clinically important bacteria. For example, *Enterococcus* is typically LAP and PYR positive, while *Streptococcus* species may be positive for one or both, and *Leuconostoc* is typically negative for both. In clinical settings, this streamlined, rapid testing allows for faster administration of appropriate antimicrobial therapy, which is vital for better patient outcomes, especially in severe infections caused by these catalase-negative organisms.
Limitations and Quality Control Considerations
Despite its utility, the LAP test has specific limitations that must be acknowledged. First and foremost, its application is restricted to the identification and differentiation of catalase-negative, Gram-positive cocci. It is not suitable for initial screening of Gram-negative or catalase-positive organisms. Furthermore, as a screening test, it is insufficient on its own for the final, complete identification of a bacterial species and must be supplemented with additional tests, such as serology or molecular methods.
Other crucial technical limitations relate to the procedure. False-negative results can occur if the inoculum is too light, as a high concentration of the enzyme is necessary for the reaction to be observable within the short incubation window. Moreover, the viability and age of the culture are important; *Streptococci*, for instance, should ideally be tested before 48 hours of incubation to ensure enzyme production is active. Quality control is maintained by testing known positive and negative control organisms with each new batch of LAP disks and cinnamaldehyde reagent. The standard positive control is *Enterococcus faecalis* (ATCC 29212), which must produce a red color, and the negative control is *Aerococcus viridans* (ATCC 11563) or another suitable negative organism, which must show no color change, to confirm the reagents’ potency and the test’s reliability.