Mannitol egg Yolk Polymyxin agar (MYP agar)
The figure shows MYP agar plates. Plate A is a contol and lacks bacteria. On plate B, suspected colonies of Bacillus cereus have been cultivated. A precipitation can be seen around the streak of bacteria, because lecithin is split into an insoluble diglyceride. For comparison, both Staphylococcus aureus subsp. aureus (to the left) and B. cereus (to the right) have been cultivated on plate C. There is no precipitation around the streak of S. aureus subsp. aureus and, furthermore, manitol is fermented by S. aureus subsp. aureus. - Click on the image to enlarge it.
Image: Ingrid Hansson (SLU), Lise-Lotte Fernström (SLU) & Karl-Erik Johansson (SLU).
Mannitol egg Yolk Polymyxin agar (MYP Agar) is a selective medium for detection and enumeration of Bacillus cereus in food samples. MYP Agar is also called MCS (Mossel Cereus Selective) agar or BCS (Bacillus Cereus Selective) Agar. MYP agar contains mannitol, which B. cereus does not ferment and egg yolk, which in turn contains lecithin that can be cleaved by the phospholipase C of B. cereus. Additionally MYP agar contains polymyxin, which inhibits gram negative bacteria.
MYP agar contains:
The final pH of the medium should be 7.2. Egg yolk and polymyxin are aseptically added when the medium has been sterilized.
If B. cereus is growing on MYP agar a precipitate is formed around the colonies or the streak, and you will not get any yellowing because B. cereus does not ferment mannitol and there will be no decrease in pH around the colonies or the streak.