Modified MYP Agar 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 (1,2,3) and processed foods. Under favourable conditions, the organism multiplies and causes gastrointestinal illness (3). 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. When present in large numbers in certain foodstuffs, B. cereus can produce metabolites responsible for the clinical symptoms of food poisoning (4). This medium differentiates B. cereus from other bacteria based on the basis of lecithinase activity, mannitol fermentation and resistance to polymyxin (FD003). Modified MYP Agar has similar composition to MYP Agar except agar concentration.

Modified MYP Agar Base contains peptone and HM extract, which provide nitrogen and carbon source, long chain amino acids, vitamins and other essential growth nutrients. 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 Polymyxin B Sulphate (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 with reddish zones and white halos. Acid produced by organisms other than B.cereus often diffuse through the medium, making it difficult to distinguish between mannitol fermenters and non-fermenters. So it is advised to transfer the suspected colonies to a fresh medium to visualize the true reaction.

Colonies from Modified MYP Agar Base are subcultured on Nutrient Agar and incubated at 30°C for 24 hours to observe/determine vegetative cells, sporangium and spore morphology and lipid globules within vegetative cell.