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Bacillus Cereus Agar
Bacillus Cereus Agar is a selective medium for the detection, isolation and quantification of Bacillus cereus in foods. In this medium, the low Peptone level and the addition of Sodium Pyruvate improve egg yolk precipitation, thus enhancing bacterial sporulation. The selective agent Polymyxin B suppresses the growth of most bacteria but allows that of B. cereus. Typical B. cereus colonies are crenated, filamentous and peacock-blue, surrounded by a precipitate due to the hydrolytic activity of the enzyme lecitinase on the egg yolk suspension. After 48 hours of incubation, the center of the colony presents a grayish color. The pH indicator Bromothymol Blue detects mannitol utilization by other bacteria of the Bacillus group, which unlike B. cereus form yellow colonies on this medium. Other egg yolk-reacting microorganisms able to grow on this medium such as Staphylococcus aureus, Serratia marcescens or Proteus vulgaris can be distinguished from B. cereus by the color and shape of their colonies, as well as by the presence of a lightening reaction on the egg yolk. There is a choice of 3 sizes - 500g, 2kg, and 10kg.
PREPARATION: Mix 41 grams of the medium in 950 mL of purified water with repeated stirring to dissolve completely. Distribute and autoclave at 121°C for 15 minutes. After cooling to 45-50ºC add 50 mL of sterile egg yolk suspension and 2mL of a filter sterilized solution of Polymyxin B Sulfate (100,000 units).