Bacillus Cereus Selective Agar Base (MYP) ISO

Beef extract and peptone provide nitrogen, vitamins, minerals and amino acids essential for growth. Mannitol is the fermentable carbohydrate providing carbon and energy; Bacillus cereus is mannitol-negative. The mannitol content allows the identification of the accompanying mannitol-positive flora, which are characterized by a yellow color. Phenol red is the pH indicator. Bacteriological agar is the solidifying agent. Bacillus cereus produces lecithinases. The insoluble degradation products from the lecithin present in egg yolk accumulate around the Bacillus cereus colonies, forming a white precipitate.

LC-1343, ISO version

Bacillus Cereus Selective Agar Base (MYP) is a medium recommended by ISO 7932 for the presumptive enumeration of Bacillus cereus with the count plate technique at 30 °C.

Beef extract and enzymatic digest of casein provide nitrogen, vitamins, minerals and amino acids essential for growth. Mannitol is the fermentable carbohydrate providing carbon and energy; Bacillus cereus is mannitol-negative. The mannitol content allows the identification of the accompanying mannitol-positive flora, which are characterized by a yellow color. Phenol red is the pH indicator. Bacteriological agar is the solidifying agent.

Bacillus cereus is resistant to certain concentrations of polymyxin, which inhibits the accompanying flora, and it is effective mainly against Gram-negative organisms. Bacillus cereus produces lecithinases. The insoluble degradation products of the lecithin from egg yolk accumulate around the Bacillus cereus colonies, forming a white precipitate.

The presumptive colonies are large, pink (indicating that mannitol fermentation has not occurred) and generally surrounded by a zone of precipitation (indicating the production of lecithinase).

LC-1343, ISO version

Typical formula in g/L, adjusted and/or supplemented as required to meet performance criteria.

LC-1343, ISO version

Typical formula in g/L, adjusted and/or supplemented as required to meet performance criteria.

Preparation

Suspend 43 grams of the medium in 900 mL of distilled water. Mix well and dissolve by heating with frequent agitation. Boil for one minute until complete dissolution. Sterilize in autoclave at 121 °C for 15 minutes. Cool to 45-50 °C and aseptically add 100 mL of Egg Yolk Emulsion (LC-5152) and, if desired, aseptically add 2 vials of Bacillus Cereus Supplement (LC-6021). Homogenize gently and dispense into appropriate containers.

Instructions for use

Streak plate method:

1. In a Petri dish, add 12-15 mL of molten agar and let it solidify.
2. Inoculate 10 µL of the initial suspension and/or diluted sample.
3. Spread the inoculum with a sterile loop on the agar surface.
4. Incubate the plates in an inverted position at a temperature of 35 ± 2 °C for 24-40 hours.

LC-1343, ISO version

Suspend 46 grams of the medium in 900 mL of distilled water. Mix well and dissolve by heating with frequent agitation. Boil for one minute until complete dissolution. Sterilize in autoclave at 121 °C for 15 minutes. Cool to 44-47 °C and aseptically add 100 mL of Egg Yolk Emulsion (LC-5152) and 2 vials of Bacillus Cereus Supplement (LC-6021). Homogenize gently and dispense into appropriate containers.

For the enumeration of presumptive Bacillus cereus according to ISO 7932:

1. Transfer 0.1 mL of the test sample or of the initial suspension to each of two agar plates. Make decimal dilutions if necessary.
2. When, for certain products, it is desirable to estimate low numbers of B. cereus, the limits of detection may be raised by a factor of 10, by examining 1.0 mL of the test sample if the initial product is liquid, or 1.0 mL of the initial suspension for the other products.
3. Distribute the 1 mL of inoculum either on the surface of a large Petri dish (140 mm) or over the surface of three small dishes (90 mm) using a sterile spreader. In both cases, prepare duplicates by using two large plates or six small plates.
4. Inoculated plates should be incubated for 18 hours to 24 hours at 30 °C. If colonies are not clearly visible, incubate the plates for an additional 24 hours before counting.

LC-1343, ISO version

• Donovan, K.O.: A Selective Medium for Bacillus cereus in Milk, J. appl. Bact., 21; 100:103 (1958)
• Mossel, D.A.A., Koopman, M.J. and Jongerius, E.: Enumeration of Bacillus cereus in Foods. Appl. Microbiol., 15; 650:653 (1967)
• International Standard ISO 7932 Microbiology of food and animal feeding stuffs - Horizontal method for the enumeration of presumptive Bacillus cereus - Colony-count technique at 30 °C.