Development of a new chromogenic medium for the enumeration of Bacillus cereus in various ready-to-eat foods

Abstract

Enumeration of Bacillus cereus in ready-to-eat food with high background flora has been challenging due to the low selectivity and recoverability of the selective media. In this study, a new medium for detection of B. cereus was developed. This medium, B. cereus chromogenic agar (BCCA), contains an egg yolk emulsion for detection of lecithinase activity, a chromogenic substrate specific for detecting alkaline phosphatase activity, and three antibiotics for selectivity. The aim of this study was to compare the performance of BACCA with that of two conventional selective media (MYPA and PEMBA) and two commercialized chromogenic media (CBC agar and Bacara medium). The newly developed media has been evaluated for its inclusivity/exclusivity and recoverability in pure culture and in artificially inoculated ready-to-eat foods, respectively. BCCA exhibited similar or better recoverability and inclusivity/exclusivity for enumeration of B. cereus and B. thuringiensis from pure cultures than two conventional [mannitol yolk polymyxin B agar (MYPA) and polymyxin pyruvate egg yolk mannitol bromothymol blue agar (PEMBA)] and two commercial chromogenic media [chromogenic B. cereus (CBC) agar and Bacara medium]. Furthermore, BCCA yielded similar or enhanced levels of recoverability and superior selectivity of B. cereus and B. thuringiensis from ready-to-eat food products, particularly radish sprouts and a sprout mix, compared with the other media tested. These findings indicate that BCCA comprises a useful option for the isolation and enumeration of B. cereus from commercial foods.