Detection of Emetic Toxin Genes in Bacillus cereus Isolated from Food and their Production of Cereulide in Liquid Culture

Bacillus cereus is a Gram-positive that is ubiquitous in foods and the wider environment. The B. cereus emetic toxin, cereulide, synthesized by nonribosomal peptide synthetase (NRPS), can cause either gastrointestinal disorders and/or emetic form of food poisoning. In this study, 108 B. cereus isolates from foods were tested for a correlation between the presence of emetic toxin genes and cereulide production. Based on 16S rRNA sequence analysis, 107 isolates were identical to B. cereus spp. Further experiments showed that fourteen isolates exhibited 1,271, 2,200 and 188-bp bands, corresponding to amplification of the ces1, ces2 and NRPS gene, respectively. However, only 7 out of 13 strains possessing the NRPS, ces1, and ces2 genes produced cereulide in liquid culture. Strain number 93, isolated from fresh vegetables, produced the highest levels of cereulide (11.17 g/mL); maximum cereulide levels were observed after 4 days of cultivation at 30C in LB broth. The amount of cereulide produced differed between each isolate. Practical ApplicationsThe incident of Bacillus cereus in food could cause food poisoning. The results of our study on the presence of B. cereus in spoiled foods demonstrated that not all the food poisoning B. cereus spp. have the ability to synthesize the emetic toxin, cereulide. Nonribosomal peptide synthetase and cereulide synthetase genes could be used as biological markers for differentiation of cereulide producing and nonproducing B. cereus. Estimation of cereulide level reveals that the amounts of cereulide produced by some strains were very high and could be a threat to human health if consumed in contaminated foods. This study presents proper methods to select cereulide producing food-poisoning B. cereus spp. and gives more information on the condition of cereulide production to prevent future cases of emetic food poisoning.