Isolation and enumeration of Bacillus cereus from foods on a novel chromogenic plating medium

Abstract

Bacillus cereus is an aerobic sporeformer commonly found in raw and processed foods. Foodborne illnesses associated with this pathogen are caused primarily by consumption of cooked foods with inadequate refrigeration. Mannitol-egg yolk-polymyxin (MYP) agar is widely used for isolation and enumeration of the pathogen. In the present study, a new selective medium, which contains a chromogenic substrate (5-bromo-4-chloro-3-indoxyl- myo -inositol-1-phosphate) for the detection of phosphatidylinositol-specific phospholipase C in B. cereus/B. thuringiensis organisms, was evaluated and compared to MYP. Twenty B. cereus strains, four B. thuringiensis strains, 95 other Gram-positive and 14 Gram-negative organisms were examined on BCM®B. cereus/B. thuringiensis (BioSynth) and MYP agar. Only B. cereus and B. thuringiensis formed large (2–7 mm), turquoise, flat colonies±turquoise halos on BCM®after incubation for 24 h at 35°C, while other Bacillus spp. formed white, 1–2 mm colonies or were inhibited. Some Listeria strains formed <1-mm colonies, white or turquoise, on the medium. Of the Gram-positive non- B. cereus/B. thuringiensis organisms tested, growth of >50% was inhibited on BCM®, compared to <1% on MYP. No significant difference (P>0·05) was observed on the two media in the recovery of B. cereus strains from foods inoculated or naturally contaminated with the organisms. However, colony enumeration and isolation were easier on BCM®than on MYP because of higher selectivity and formation of discrete, non-coalescing colonies. Ribotyping (Riboprinter®Microbial Identification System, Qualicon) of five randomly selected food isolates identified three B. cereus and two B. thuringiensis strains. Agar plates were stable for >3 months at 2–5°C.