What is Bacillus cereus?
Bacillus cereus, a pathogenic, food-borne disease-causing organism, is widely distributed in nature, such as on plants, soils and the GI tracts and mammals of insects and other mammals.
They can also be found in food production plants. Their resistant endospores, which can survive in extreme conditions, allow them to contaminate a wide range of raw materials and foods products. Bacillus cereus, a causative pathogenic organism, causes food poisoning in two ways. Bacillus cereus causes food poisoning by producing complex enterotoxins in the small intestine. Consuming food with heat-stable toxins can cause emetic food poisoning.
It can cause opportunistic infections like bacteremia, septicemia, pneumonia, meningitis, gastritis, liver failure, liver necrosis and brain edema. Other species belonging to Bacillus that are responsible for causing food-borne illness are B. subtilis, B. licheniformis, B. pumilus, B. weihenstephanensis, B. anthracis, B. mycoid, B. pseudomycoides and B. thuringiensis.
Biological characteristics of Bacillus cereus
- Gram-positive bacteria
- Spore-former
- Aerobic-to-facultative
- Motile
- Rod-shaped bacilli
- Grow on temperature range (8 – 55°C)
- Optimum temperature – 25 to 37°C
- pH range (4.9 – 9.3)
- Salt concentration up to 7.5%
- Endospores are resistant to high heat, radiation, desiccation and disinfectants.
Contamination Sources
The heat-resistant spore from B. cereus is capable of contaminating all food processing units, from raw materials to packaging and storage. Biotechnological equipment, machines and machinery are also susceptible to contamination.
The organism can easily be transmitted from soil to vegetables and crops. Foods such as rice, pasta, bread, milk products, meat products and spices, infant food, fish, soups, veggies, and fruits are all frequently contaminated.
B. thuringiensis, when used as a pesticide on plants, can cause enterotoxins that can be harmful to the health of consumers. Numerous outbreaks have been reported from food contaminated by B. cereus.
Bacillus cereus Food Poisoning Epidemiology and outbreaks
Although the Bacillus cereus outbreak was first identified in Norwegian hospitals, no details about specific populations have been provided. Watery diarrhea was the only manifestation of the outbreak, which affected elderly and people with low stomach acid.
In 1971, 1000 people in the United Kingdom were affected by nausea and vomiting after eating contaminated rice from Chinese restaurants. Bacillus cereus food poisoning, which is the third most frequent bacterial outbreak in Hungary, has caused 117 cases between 1960 and 1968.
Between 1982 and 1986, Japan saw 73 cases of B. cereus-related outbreaks with 1,323 cases. Out of 26,173 cases in Taiwan with 20 deaths, 18% were caused by B. cereus food poisoning.
Every year, food poisoning from B. cereus is reported in every country, including the United Kingdom, Japan, England, Wales, Northern Europe, North America, and Scotland. There have been fewer cases of food poisoning in Canada, England, Wales, Japan, and the USA than in other countries. This could be due to differences in cooking and eating habits.
Bacillus cereus foodborne toxins
Bacillus cereus produces protein toxin, including diarrheal toxin as well as emetictoxin.
Diarrheal toxin
When foods containing vegetative cells are eaten, a diarrheal toxin forms and begins to grow in the small intestinal tract. Pronase sensitive proteases include trypsin, pronase and chymotrypsin.
The infectious dose that can cause diarrheal illness is approximately 104 to 109 CFU/gm. Incubation begins between 8 and 16 hours after ingestion, and lasts for only 24 to 48 hours. The symptoms include mild diarrhea, abdominal cramps, and watery stool.
Children and patients with immunocompromised may experience bloody diarrhea or necrotic enteritis, which can lead to liver failure and brain swelling. Three chromosomally encoded enterotoxins are associated with diarrheal illnesses: Hemolysin BL, Nonhemolytic enterotoxin and Cytotoxin k (CytK).
Hbl, a primary virulence element in B. cereus diarrheal foods poisoning, forms a transmembrane por in the small intestinale by osmotic hydrolysis. Nhe shares homology with Hbl, but has a three-part component as well as a pore-forming toxins. CytK is a prototype toxin which causes bloody diarrhea or necrotic enteritis. It is a b barrel pore-forming toxic toxin.
Emetic toxin
A small, cyclic heat-stable protein peptide is responsible for the more severe and acute emetic reaction. After ingestion of preformed contaminated food, the incubation period begins between 2 and 5 hours. The illness can be shown by 105 to108 cells per gram.
These symptoms may be similar to S. aureus food toxicity, which can cause nausea, vomiting, and abdominal cramps that can last up to 24 hours. Although the mechanism of the emetic poisoning is not yet known, it is believed that the 1.2kDa cereulide toxins form ion channels in the membrane and create holes.
Emetic toxins are heat stable and can withstand cooking processes such as frying, roasting and boiling. B. cereus toxicities can also be caused by other environmental factors like temperature, pH, atmospheric content, nutrient sources, and food consistency.
Detection methods of Bacillus cereus
1. Culture methods
B. cereus culture can be done using laboratory media like blood agar and nutrient agar. But polymyxin B resistant B. cereus requires selective media such as polymyxin B-pyruvate-egg yolk-mannitol-bromothymol blue agar (PEMBA) and mannitol-egg yolk-polymyxin B agar (MYP).
After 24 hours of incubation, pink colonies can be seen with clear zones of precipitation. When there are fewer organisms in the sample, the MPN (minimum probable number) method is used. To confirm the presence, confirmatory, and complete tests are performed.
2. ELISA
The commercially available ELISA method is used to detect toxins. However, it is not accurate in determining the toxin-producing activities of B. cereus. It can detect either hemolysin B or nontoxic proteins.
3. Reverse passive latex agglutination (RPLA) enterotoxin assay
To activate them biologically, the sample is boiled. This yields a positive result. The test detects hemolysin B, but the toxins are not detected in high glucose levels.
4. PCR
PCR assays can detect the presence of toxin by amplifying the B. cereus genome sequence. Toxin genes block information about virulence strains, so additional complementary tests are required.
Treatment of Bacillus cereus Food Poisoning
- Bacillus cereus food poisoning can be self-limiting and resolves in 24 to 48 hours.
- Fluid therapy and bed rest are essential. If the illness is very severe, antibiotics can be used to treat it.
Prevention and control measures of Bacillus cereus Food Poisoning
- B. cereus spores are resistant to heat treatment, even though proper cooking can kill vegetative cells, but they remain viable.
- It is important to eliminate contamination before spores can germinate.
- It is important to cool food quickly before storage, and then properly reheat it before consumption.
- Bacillus spp. can eat foods with low pH (4.3). They cannot survive in low-acid food products.
- Food processing plants can trace the presence and distribution of spores from farm through packaging.
- Good hygiene and food handling habits should be observed.
- To eliminate food poisoning, educate food handlers about food safety management.
