Lecithinase test

General

Many bacteria have enzymes which can break down lipids, so-called lipases. Lecithinase, which is also called phospholipase C, is such an enzyme that splits the phospholipid lecithin (= e.g. phosphatidylcholine). Phospholipids, which are charged are usually soluble in water, but one of the products which is formed by the splitting, namely a diglyceride, is not charged and it has two long hydrocarbon chains. It is, therefore, unsoluble in water and this is utilized in the lecithinase test, where bacteria are cultivated on egg yolk agar. Egg yolk contains a lot of lecithin.

Method

1. Apply the bacteria in the form of a streak onto the egg yolk agar.
2. Read the plate after 24 h.

• Positive test result: Precipitation around the streak of bacteria.
• Negative test result: No precipitation.

Figure: Bacillus cereus on egg yolk agar. Note the precipitation around the streak of bacteria, which shows that they are positive for lecithinase.

Use

Can among other things be used to differentiate between certain species within the genus Bacillus.