Description
Phenol Red Egg Yolk Agar with Polymixin B was described in the Australian Standard AS 1766.5 – 1982 for the selective isolation of Bacillus cereus. Many authorities have postulated formulations suitable for isolation of B. cereus. The basis of the AMYL formulation was taken from the work of Mossel et al.
The various plating mediums are based on the principle of providing conditions for the organism to exhibit its haemolytic, lecithinase activity or fermentation properties. This medium depends on egg yolk reaction (lecithinase presence), and lack of mannitol fermentation in the presence of the indicator phenol red. Polymixin B is incorporated to suppress the growth of unwanted micro-organisms. Peptones provide a nutrient medium and sodium chloride maintains osmotic equilibrium. The absence of mannitol fermentation results in an alkaline pH with resultant violet pink background to the colonies. In a review on the various plating media available, Inal found that the perform¬ance of the medium described by Mossel et al., provided very satisfactory results particularly in the presence of mixed inocula. Cocci were not inhibited, but these were usually easily identified due to mannitol fermentation.
Because lecithinase negative strains do occur, confirmation tests are required.
