Although these resistant bacteria exist in the original, dry products, they cannot reproduce as a result of the low water content. The more resistant forms are even more likely to survive boiling without any problem. Once the food has cooled down a little bit, this bacterium can reproduce rapidly in the finished dish and even potentially produce toxins. This tends to occur if food is left out unrefrigerated in the kitchen or on a balcony. However, reproduction is only prevented in the fridge if the refrigerator temperature is below 5 °C (this is also an optimal temperature for other food) and the amount of food to be cooled is not too big. Large amounts of rice or pasta take a long time until their inner layers have cooled down sufficiently to prevent reproduction.
Nausea and vomiting can be caused by toxins. These will be formed in sufficient amounts if specific bacillus cereus bacteria (toxin producers) can reproduce extensively, as described above and thus, produce enough toxins. This will lead to nausea and vomiting after a short time, as the end toxins are ingested in this instance. The symptoms will subside within one day.
The bacteria may also cause diarrhoea, if allowed to reproduce rapidly in inadequately refrigerated food. Diarrhoea will occur about 16 hours after ingestion because of the bacteria’s effect directly in the gut.
The share of reported food-borne incidents caused by Bacillus cereus is between 0.3 and 18 % in Europe. A long-term survey carried out in the USA between 1973 and 1987 showed 2 %, studies in Japan and Taiwan (high rice consumption regions) about 30 %.
However, it must be noted that solely cases that are reported are recorded. As nausea and vomiting occur very quickly following a meal and often subside within one and two days without any major consequences, it is difficult to find proper evidence and only a small number of cases are reported and registered.
Diarrhoea cases caused by Bacillus cereus are even harder to spot and, thus, are rarely documented. As a result, it must be assumed that there are far more cases of Bacillus cereus than can be expected based on statistics. Food-borne outbreaks of diseases with a larger number of patients have also been linked to Bacillus cereus in both Europe and the USA. There were 45 food-borne outbreaks reported in the European Union in 2008 that were caused by Bacillus cereus and that affected 1,132 people.