Bacillus Cereus

Bacillus cereus It is a rod-shaped, facultatively aerobic, Gram-positive, spore-forming bacterium. Very widespread in the environment, it is commonly found in the air, dust and soil.

Although there are several strains of Bacillus cereus – some of which are harmless or even beneficial to the human body – the bacterium is known to be a source of food poisoning in humans. They are in particular its toxins to cause damage to the organism, which can manifest itself in different ways:

  1. with nausea and vomiting, symptoms that arise one to six hours after ingestion of contaminated food and can last up to 24 hours → emetic gastroenteritis: preformed emetic toxins are implicated, i.e. already present in the ingested food because they are particularly resistant to heat (such as those produced by Enterococcus faecalis). Only occasionally, emetic gastroenteritis is accompanied by diarrhea. This type of foodborne illness may be difficult to distinguish from that of other short-term foodborne bacterial pathogens, such as Staphylococcus aureus
  1. with abdominal colic and diarrhea, symptoms that appear 8 to 24 hours after ingesting the offending food and can last up to 24 hours → gastroenteritis diarrhoea: Enterotoxins synthesized by the bacterium within the intestine are implicated. Nausea may accompany diarrhea, but vomiting is usually absent

The diagnosis must be supported by the isolation of Bacillus cereus from food, vomit or faeces, and from quantitative cultures on appropriate selective media. As a rule, however, such operations are carried out only for research purposes, since the infection is relatively harmless and usually self-limiting. For this reason, antibiotic therapy is not normally necessary, while correct oral rehydration is the only important precaution to take in the presence of diarrhea.

In some and fortunately rare cases, Bacillus cereus it can however cause septicemia and be fatal.

Among the foods most frequently involved in epidemics Bacillus cereus there are dishes based on:

The infection from Bacillus cereus it is also known as fried rice syndrome, since emetic intoxication has often been documented in subjects who have consumed dishes of fried rice left to rest for hours at room temperature (for example at buffets).

Naturally, the human organism is able to defend itself from infections Bacillus cereus: only when the food contains an excessive number of toxins or bacteria can they take over and cause damage. In particular, in the documented cases the suspect foods contained between 106 e 109 cfu/g (colony forming units per gram).

Curiosity: at the intestinal level, Bacillus cereus competes with other organisms such as Salmonella e Campylobacter for nourishment and adhesion sites. In farm animals such as chickens, rabbits and pigs, some harmless strains of Bacillus cereus they are used as a probiotic food additive to reduce the proliferation of Salmonella in the intestine and cecum. This approach improves both the health of the animal and that of the consumer, reducing the risk of food poisoning Salmonella spp.

Although some strains of Bacillus cereus are psychotrophic and can also grow at refrigeration temperatures (4-6°C), most grow between 15 and 55°C, with optimal growth at 30-37°C. The pH range suitable for the growth of Bacillus cereus is between 5.5 and 8°C.

From what has been explained in the article, we can deduce that:

  • Bacillus cereus it is a ubiquitous bacterium, which increases the possibility of contamination, to the point that the presence of the microorganism in most food raw materials is considered inevitable. Soil is the main source of food contamination with spores Bacillus cereus
  • Refrigeration limits the multiplication of Bacillus cereus lengthening the germination times of spores and generation of toxins. Therefore, incorrect refrigeration of food increases the risk of poisoning.
  • Cooking at 60°C kills the bacteria, but not their emetic toxins, which can remain active at temperatures below 100°C
  • Especially in the catering sector, pre-cooking and subsequent storage of food at temperatures higher than refrigeration temperatures, for many hours before subsequent short cooking, increases the risk of emetic gastroenteritis from Bacillus cereus: the heat-stable emetic toxin formed during the storage phase is not destroyed by subsequent heating
  • B. cereus it is not a particularly acid-tolerant microorganism, so its multiplication is prevented in acidic foods, even at pH values ​​lower than 4.5


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