Bacillus cereus is a rod bacterium, facultative aerobe, Gram positive and sporogenous. Widespread in the environment, it is commonly found in the air, dust and soil.
Infection and Symptoms
Although there are several strains of Bacillus cereus - some of which are harmless or even beneficial for the human organism - the bacterium is known to be a source of food poisoning in humans. They are specifically its toxins to cause damage to the organism, which can manifest itself in different ways:
- 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 involved, that is, 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 infection can be difficult to distinguish from that sustained by other short-term foodborne pathogenic bacteria, such as Staphylococcus aureus
- with abdominal colic and diarrhea, symptoms that arise from 8 to 24 hours from "taking the responsible food" and can last up to 24 hours → diarrheal gastroenteritis: enterotoxins synthesized by the bacterium inside the intestine are involved. Nausea may accompany diarrhea, but vomiting is generally absent
The diagnosis must be supported by the isolation of Bacillus cereus from food, vomit or faeces, and from quantitative cultures on suitable selective media. As a rule, however, these operations are performed only for research purposes, since the infection is relatively harmless and usually self-limiting. For this, antibiotic therapy is not normally necessary, while proper oral rehydration is the " the only important precaution to be taken in the presence of diarrhea.
In some and fortunately rare cases, Bacillus cereus it can however cause septicemic pictures and be fatal.
Infection
Among the foods most frequently involved in epidemics from Bacillus cereus there are dishes based on:
- cereals
- minced meat and meatballs
- vegetables and soups
- milk derivatives
- puddings
Of course, the human body is able to defend itself against infections Bacillus cereus: Only when the food contains an excessive number of toxins or bacteria can these take over and cause damage. In particular, in documented cases the suspected foods contained between 106 and 109 cfu / g (colony forming units per gram).
Curiosity: in the intestine, Bacillus cereus competes with other bodies such as Salmonella And Campylobacter for nourishment and adhesion sites. In farm animals such as chickens, rabbits and pigs, some harmless strains of Bacillus cereus are used as a probiotic food additive to reduce the proliferation of Salmonella in the intestine and cecum. This approach improves both animal and consumer health, reducing the risk of toxic infections from Salmonella spp.
Although some strains of Bacillus cereus are psychotrophic and can also develop 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 it is between 5.5 and 8 ° C.
As explained in the article, we can deduce that:
- Bacillus cereus it is a ubiquitous bacterium, which increases the chances of contamination, to the point that the presence of the microorganism in most food raw materials is to be considered inevitable. Soil is the main source of food contamination with spores of Bacillus cereus
- Refrigeration limits the multiplication of Bacillus cereus lengthening the spore germination times and generation of toxins. Therefore, improper refrigeration of the food increases the risk of toxic infection.
- Cooking at 60 ° C kills bacteria, but not their emetic toxins, which can remain active up to temperatures below 100 ° C
- Especially in the catering sector, the pre-cooking and subsequent storage of food at temperatures above refrigeration, for many hours before a subsequent and short cooking, increases the risk of emetic gastroenteritis from Bacillus cereus: the thermostable emetic toxin formed during the storage phase is not destroyed by the subsequent heating
- B. cereus it is not a particularly acid-tolerant microorganism, so its multiplication is prevented in acidic foods, already at pH values below 4.5
Prevention
To prevent food poisoning from Bacillus cereus we recommend:
- do not store food at room temperature
- store ready-to-eat foods, especially if rich in starch, at a temperature not lower than 60 ° C or not higher than 4 ° C; in the latter case, the food must be quickly cooled and refrigerated within two hours of cooking
- in order to avoid cross-contamination, use well-cleaned pans and dishes for storage, and carefully clean the working surfaces: the spores of Bacillus cereus they have strong adhesive properties, can form biofilms, therefore persist for a long time on such surfaces
