Bacillus cereus is a rod-shaped bacterium, optional aerobic, Gram positive and sporogenic. Very common in the environment, it is commonly found in the air, dust and soil.
Infection and Symptoms
Although there are several Bacillus cereus strains - 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. Its toxins in particular to harm the organism, which can manifest itself in different ways:
- with nausea and vomiting, symptoms that arise from one to six hours after the ingestion of contaminated food and can last up to 24 hours → emetic gastroenteritis : preformed emetic toxins are involved, ie already present in the food ingested because they are particularly resistant to heat ( like those produced by Enterococcus faecalis) . Only occasionally, emetic gastroenteritis is accompanied by diarrhea. This type of food-borne infection can be difficult to distinguish from that sustained by other foodborne pathogenic bacteria in the short term, such as Staphylococcus aureus
- with abdominal colic and diarrhea, symptoms that arise 8 to 24 hours after taking the responsible food and can last up to 24 hours → diarrheal gastroenteritis : enterotoxins are implicated synthesized by the bacterium in the intestine. Nausea can 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 crops on special 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 be taken in the presence of diarrhea.
In some and fortunately rare cases, Bacillus cereus can still cause septicemic pictures and be fatal.
Contagion
Among the foods most frequently involved in Bacillus cereus epidemics 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 Bacillus cereus infections: only when the food contains an excessive number of toxins or bacteria can they take over and produce damage. In particular, in documented cases the suspect foods contained between 106 and 109 cfu / g (colony-forming units per gram).
Curiosity: in the intestine, Bacillus cereus competes with other organisms such as Salmonella and Campylobacter for nutrition and adhesion sites. In farm animals such as chickens, rabbits and pigs, some innocuous 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 infections from Salmonella spp.
Although some Bacillus cereus strains are psychotrophic and may 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 is between 5.5 and 8 ° C.
As explained in the course of the article, we can deduce that:
- Bacillus cereus 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. The soil is the main source of food contamination with Bacillus cereus spores
- Refrigeration limits the multiplication of Bacillus cereus, lengthening the spore germination times and generating toxins. Therefore, incorrect refrigeration of the food increases the risk of 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 the food at temperatures higher than those of refrigeration, for many hours before a subsequent and brief cooking, increases the risk of emetic gastroenteritis from Bacillus cereus : the thermostable emetic toxin formed during the storage phase it is not destroyed by the subsequent heating
- B. cereus 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 foodborne illnesses from Bacillus cereus we recommend:
- do not store food at room temperature
- store ready-to-eat foods, especially those 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 cooled quickly and refrigerated within two hours of cooking
- in order to avoid cross-contamination, use clean pans and plates for storage, and carefully clean the working surfaces: the Bacillus cereus spores have strong adhesive properties, can form biofilms, and therefore persist for a long time on these surfaces
