MYP Agar Plate (100mm plate)

Principle And Interpretation

Bacillus cereus is ubiquitously present in soil, vegetation water and dust. It has been isolated from a large variety of foods, including vegetables, meat, cereals, pasteurized fresh milk and powdered milk (1-3) and processed foods. Under favorable conditions, the organism multiplies and causes gastrointestinal illness (4). It is implicated in two different forms of food poisoning; an emetic illness and a diarrhoeal illness. The emetic illness is mediated by a highly stable toxin that survives high temperature, exposure to trypsin, pepsin and pH extremes. The diarrhoeal illness is mediated by a heat and acid labile enterotoxin. Lecithinase activity is the key reaction in the differential identification of B.cereus, the most commonly encountered and important species in clinical laboratories, from the majority of the other Bacillus species. If unknown isolate produces lecithinase, Bacillus cereus can be presumptively identified by also observing colonial morphology, hemolytic reactivity and motility tests. Mossel et al (5) formulated Mannitol-Egg Yolk-Polymyxin (MYP) Agar, which is recommended by APHA to isolate and enumerate B.cereus from foods (3,4,6,7). When present in large numbers in certain foodstuffs, B.cereus can produce metabolites responsible for the clinical symptoms of food poisoning (5). This medium differentiates B.cereus from other bacteria based on the basis of lecithinase activity, mannitol fermentation and resistance to polymyxin (FD003) (3,8). MYP Agar contains peptone and HM extract, which provide nitrogen source. Mannitol fermentation can be detected by phenol red, which yields yellow colour to the mannitol fermenting colonies due to acid production. Added egg yolk emulsion helps in differentiation of lecithinase producing colonies, which are surrounded by a zone of white precipitate. Addition of PolyB Selective Supplement (FD003) helps to restrict growth of gram-negative bacteria such as Escherichia coli and Pseudomonas aeruginosa. These differentiating media allow differentiation of B.cereus from other Bacillus species by its inability to ferment mannitol and poor sporulation. B.cereus dissimilates egg yolk and gives rise to typical bacilli form colonies.