BACILLUS CEREUS AGAR BASE

Description

Bacillus cereus causes food poisoning due to the consumption of contaminated rice, eye infections and a wide range of other clinical conditions like abscess formation, meningitis, septicemia and wound infection. Bacillus cereus is a known cause of disease mastitis, especially in ewes and heifers among the veterinarians. Holbrook and Anderson developed Bacillus Cereus Agar, which is a highly specific and selective medium for the isolation and enumeration of Bacillus cereus from foods. It supports the growth of even a small number of Bacillus cereus cells and spores in the presence of large number of other food contaminants. The typical colonies of Bacillus cereus are crenate, about 5 mm in diameter and have a distinctive turquoise to peacock blue colour surrounded by a good egg yolk precipitate of the same colour. The bacteria do not ferment mannitol and thus there is no change in colour of the indicator dye around the colonies. Addition of polymyxin-B sulphate at a final concentration of 100 units per ml of medium is sufficient to make the medium selective for the isolation of Bacillus cereus. It suppresses the growth of accompanying bacterial flora. If moulds are suspected in the inoculum, 40 mcg per ml filter-sterilized cycloheximide may be incorporated to suppress the moulds contamination. Some strains of Bacillus cereus have very weak egg yolk reaction. Moreover, on this medium Bacillus cereus is indistinguishable from Bacillus thuringiensis. For the isolation and enumeration of Bacillus cereus in foodstuffs the following method is recommended. Distribute 0.1ml of the homogenized specimen diluted in Peptone Water onto the surface of the medium. Incubate at 37?C under aerobic conditions for 24-48 hours. Possible growth of contaminants is greatly reduced by incubation for 24 hours. Report the results as the number of Bacillus cereus colonies per gram weight of the food sample. Confirmatory tests should be carried out before interpretation.