Bacillus cereus is an aerobic and facultative anaerobic, gram-positive bacteria causing toxin-mediated food poisoning. The optimum temperature for its growth is 30-37°C. However, it can also tolerate a wide range of temperature; some strains can grow up to 55°C while others can grow at temperatures as low as 4 to 5°C. Many strains from dairy products are able to thrive at lower temperatures. The minimum pH requirement for growth is 4. 3, while the maximum pH is around 9. 3. This bacterial population grows best in the presence of oxygen. Nevertheless, since this species is facultative anaerobic it also grows well in anaerobic conditions. Toxin production has been found to be much lower under anaerobic conditions. Bacillus cereus is a spore-forming micro-organism which occurs naturally in most foods. It causes two different and distinct forms of food poisoning: an emetic illness and a diarrhoeal illness.
Bacillus cereus causes food-borne emetic illness by producing a highly stable toxin that survives high temperatures and exposure to trypsin, pepsin and pH extremes. Bacillus cereus levels in foods associated with an emetic form of food poisoning have ranged from 1,000 to 50 billion colony-forming units (CFU)/gram. High numbers of bacterial population have been also detected in faecal samples from affected persons. Diarrhoeal illness is mediated by a heat- and acid-labile enterotoxin produced during growth of vegetative cells in the small intestine. Four different enterotoxins have been characterized. Two are protein complexes such as non-haemolytic enterotoxin (NHE) and haemolysin BL (HBL) and two are neurotoxic proteins such as enterotoxin T (bc-D-ENT) and cytotoxin K. The occurrence of the diarrhoeal-type syndrome has been largely reported in the US and Europe, where the disease has been greatly associated with foods such as meat, vegetable, poultry, desserts and sauces.
Prevalence of the vomiting type syndrome has been reported in the United Kingdom, Canada and the USA, foodborne poisoning is commonly associated with the consumption of cooked fried rice. Serotyping studies conducted by Taylor and Gilbert (1975) showed that strains from incidents of food poisoning in the US and Europe characterised by a longer incubation period and with diarrhoea as the main symptom has provided serotypes 2, 6, 8, 9 and 10, whereas those which caused the rice-associated emetic illness in the United Kingdom, Canada and USA were serotypes 1, 3 or 5. Bacillus cereus food poisoning is mostly associated with rice dishes, and sporadically with other foodstuffs such as meat, vegetable, pasta, dairy products, sauces, soups and sweet pastries. Bacillus cereus food-borne poisoning is also associated with pudding, cream, spices, dry milk, dry potatoes and spaghetti sauces. Bacterial contamination of the food product by the large occurs prior to cooking. Even after cooking the microbial population thrives because vegetative forms can grow over a wide range of temperatures from 25°C to 42°C and are also capable of producing endotoxins.
Endospores have the ability to survive extremely high and low temperatures. When they are allowed to cool fairly gradually, they once again begin to germinate and multiply. Spores are highly resistant to dry heat than moist heat. Emetic toxins are also extremely resistant to heat. They can survive 90 min at 126°C, while diarrhoeal toxins are inactivated at 56°C in 5 min. Clinical Description Bacillus cereus is widely known as the etiological agent of food-borne poisoning, causing both gastrointestinal and non-gastrointestinal diseases. Both diarrhoea and emesis can be clearly distinguished by their clinical features. Diarrhoea is provoked by heat-labile enterotoxins, while emesis is caused by a heat-stable depsipeptide toxin called cellulite. The clinical syndromes associated with the diarrhoeal form of illness are longer incubation about 6 to 24 hours period, very watery diarrhoea (similar to that of C. perfringens), presence of moderate to severe abdominal cramps, nausea and vomiting in some patients, duration of illness ranges from a minimum of 20 hours to a maximum of 36 hours.
Clinical features associated with the emetic form of illness are shorter incubation period of about 1 to 6 hours, presence of vomiting and abdominal cramps; diarrhoea is not severe and is present in only about a third of affected individuals (similar to poisoning by S. aureus), duration of sickness ranges from 8 to 10 hours. In both types of syndromes, fever is exceptional and disease is usually docile. Bacillus cereus also can cause other diseases such as local skin and wound infections, ocular infections, fulminant liver failure. Invasive disease, including bacteraemia, endocarditis, osteomyelitis, pneumonia, brain abscess, and meningitis are also caused by Bacillus cereus. Ocular involvement includes panophthalmitis, endophthalmitis, and keratitis. Apart, it can bring about other infections namely, pneumonia, meningitis, encephalitis, in addition to brain and liver abscesses. It is a rare cause of endocarditis reported in patients with prosthetic heart valves, rheumatic heart disease, intravenous drug use (IVDU), and inpatient with leukaemia.
Food Poisoning by other Bacillus species Toxic food poisoning is not only caused by Bacillus cereus, but several other Bacillus species have also been implicated in food poisoning incidents. The most important species are B. subtilis and B. licheniformis. These organisms are capable of producing an extremely heat-stable toxin which seems very similar to the emetic type produced by B. cereus. These species are widely associated with food products like meat, vegetable, flour, pastry, sausage rolls, meat pies and pastries, curries and various ethnic dishes with rice, and stuffed poultry. Bacillus subtilis is associated mainly with meat, vegetables in pastry, poultry products, and occasionally with bakery items such as bread, crumpets, sandwiches, and ethnic meat or seafood dishes. Food poisoning syndrome associated with B. subtilis causes acute onset nausea, vomiting and stomach cramps, incubation range is 2-5 hours, often with diarrhoea. Poisoning due to B. licheniformis shows the following symptoms; diarrhoea is more common than vomiting, the incubation period is about 8 hours.
Another species which was isolated in high numbers from rice associated with a food poisoning outbreak was B.pumilus. Symptoms were consistent with those similar for food poisoning by other Bacillus species than B. cereus. C. perfringens food poisoning produces a heat-labile toxin which caused predominantly lower gastrointestinal (diarrhoea) disease about 12 hours after ingestion much similar to the diarrheal type syndrome caused by B. cereus. Case Studies In August 2003, five children of a Belgian family became sick after eating pasta salad. The pasta salad prepared on a Friday was taken to the picnic on Saturday. The leftovers had been stored in the refrigerator. The following Monday evening the pasta was served for supper to the children. Since the pasta had an unusual odour three children (aged 14, 10 and 9) did not eat much, they ate very little quantities. After 6 hours the youngest child (aged 7) started vomiting and complained of respiratory distress. She was immediately rushed to the hospital. Upon arrival at the hospital, the other 3 children also felt nauseous and started vomiting. The condition of all the children began to worsen; hence they were transported to the University Hospital in Leuven. However, during transfer, the child (aged 7) had a severe pulmonary haemorrhage, quickly followed by coma, diffuse bleeding, and severe muscle cramps. She died within 20 minutes, at 13 hours after the intake of pasta. An autopsy report revealed the presence of Bacillus cereus in her gut content. The other 3 children were kept under treatment for 8 days and they gradually recovered. This case elucidated the probable severity of the emetic illness, also stressing on the importance of sufficient refrigeration of cooked food. Some B.
cereus strains are known to be psychrotrophic and these strains have the maximum emetic toxin production between 12 and 15°C. In the present case, the temperature of the refrigerator where the pasta salad was stored was 14°C. This probably allowed B. cereus strains to multiply quickly to attain a count of more than 108 CFU/g in 3 days along with very high toxin production. Foodborne poisoning was reported in a military camp in Jurong, Singapore. This food poisoning outbreak involved 19 army personnel who became unwell after breakfast. All these army personnel had been served fried rice by a food stall in a civilian canteen. The illness was characterized by vomiting (89. 5%), abdominal cramps (52. 6%), diarrhoea (47. 4%), headache (47. 4%) and fever (10. 5%) (Tay, Goh & Tan, 1982). The incubation time extended from 13/4 hours to 51/2 hours. Bacillus cereus was detected in the vomitus of the affected soldiers. This makes it clear that Bacillus cereus is a causative agent for the foodborne poisoning resulting in emetic and diarrhoeal illness. This species contaminates food products, especially cooked rice and refrigerated products.
It is therefore essential to follow proper cooking and refrigeration methods. Proper cooking and storage of foods, especially rice cooked for later use, will facilitate to thwart foodborne epidemics. For the most part, B. cereus foodborne outbreak is an outcome of eating cooked foods which are cooled slowly and stored the wrong.
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