Purpose: This study assessed cogrowth and cotoxin production of B. cereus and S. aureus in typical ready-to-reheat foods.
Methods: Three strains of B. cereus (two emetic and one diarrheal strain) and two S. aureus enterotoxigenic strains were used in for the inoculations. Bacillus cereus was inoculated either as spores or vegetative cells. Spores were initially treated with 10 minutes at 70, 80, or 90°C to assess the impact of mild heat treatments. Ready-to-reheat lasagne bolognaise packaged in modified atmosphere was purchased at a local supermarket, inoculated, MAP packed (8% O2, balanced by N2) and stored at 12 and 22°C. Microbial and toxin analyses were regularly performed.
Results: At 22°C modified atmosphere did not influence the maximum cell count reached by S. aureus, while for B. cereus a relatively small difference could be seen. When the experiment was performed with vegetative cells instead of spores of B. cereus, a slower initial growth could be seen, but the differences levelled off. The storage day cereulide was detected and the corresponding cereulide concentration were the same. The modified atmosphere at 22°C did not have an influence on the onset of staphylococcal enterotoxin (SE) production or the amount produced. SEs were always produced at a lower cell density than cereulide. At 12°C, a pronounced difference in growth between B. cereus and S. aureus could be seen. At 12°C only SE were produced under all MAP conditions; CER was never found and HBL and NHE production by B. cereus varied in function of MAP.
Significance: Data showed that both B. cereus and S. aureus, as well as their toxins, can be present in high amounts in the same food.