Contenu:
🏾 Introduction
Phosphatidylcholine, commonly known as lecithin, is a phospholipid widely distributed in animal tissues, egg yolk, and some higher plants. Certain microorganisms produce lecithinase (also called phospholipase C), an enzyme that hydrolyzes lecithin into diglycerides and phosphorylcholine.
Lecithinase activity is a key biochemical characteristic used to identify and differentiate various Gram-positive and Gram-negative bacteria, including pathogenic species such as Clostridium perfringens, Staphylococcus aureus, and Listeria monocytogenes.
🏾 Principle
Egg yolk agar is an enriched and differential medium used to detect lecithinase activity. Lecithin, a major component of egg yolk, is broken down by lecithinase-producing bacteria into insoluble diglycerides and phosphorylcholine. This reaction forms an opaque, white precipitate around bacterial colonies, indicating a positive lecithinase test.
🏾 Medium Composition
Egg yolk agar:
| Component | Quantity |
|---|---|
| Pancreatic digest of casein | 15.0 g |
| Papaic digest of soybean meal | 5.0 g |
| Sodium chloride | 5.0 g |
| Yeast extract | 5.0 g |
| L-Cystine | 0.4 g |
| Hemin | 5.0 mg |
| Vitamin K1 | 10.0 mg |
| Egg yolk emulsion | 100.0 mL |
| Agar | 20.0 g |
pH final 7,0 +/- 0,3 à 25 ° C.
🏾 Procedure for Lecithinase Test
1. Inoculation: Streak a single colony of the test organism in a straight line on egg yolk agar.
2. Incubation Conditions:
- Aerobic bacteria: Incubate at 35–37°C for 24–48 hours.
- Anaerobic bacteria: Incubate in an anaerobic jar at 35–37°C for 24–48 hours.
3. Observation: Examine the plate for an opalescent halo around the colonies.
🏾 Results and Uses
- Positive test: A white, opaque zone extending into the medium around the colony.
- Negative test: No opaque zone formation.
🏾 Applications of Lecithinase Testing
- Pathogenicity studies: Lecithinases contribute to virulence by disrupting host cell membranes.
- Bacterial identification:
- Clostridium perfringens (double zone of hemolysis and lecithinase activity).
- Staphylococcus aureus (lecithinase correlates with coagulase production).
- Pseudomonas aeruginosa and Listeria monocytogenes.
- Differentiation within Bacillus species.
