MYP Agar Base

Principle And Interpretation

MYP Agar Base is used for isolation and identification of Bacillus cereus in accordance with FDA BAM(1). B. cereus is ubiquitously present in soil, on vegetables, and in many raw and processed foods, meat, cereals, pasteurized fresh milk and powdered milk (2-4) and other processed foods. Under favourable 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 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 colony morphology, hemolytic reactivity and motility tests.

MYP Agar Base (M636F) is recommended by FDA BAM to isolate and enumerate B.cereus from foods (5, 4). This medium differentiates B.cereus from other bacteria on the basis of lecithinase activity, mannitol fermentation and resistance to polymyxin (FD003) (5, 6).

Peptone and HM Peptone B provide nitrogen and carbon source, long chain amino acids, vitamins and other essential nutrients. Mannitol acts as the carbon source that upon fermentation yields yellow colour to the colonies. Egg yolk emulsion aids in the differentiation of lecithinase producing colonies, which are surrounded by a zone of white precipitate. Polymixin B Sulphate acts as the inhibitor to restrict the growth of gram negative bacteria. These properties also help in the differentiation of B.cereus from other bacillus species (1).