Food infection by Bacillus cereus

Bacillus cereus or B.cereus is a type of bacteria that produces toxins. These toxins can cause two types of illness: one type characterized by diarrhea (a slower onset) and the other, called emetic toxin (a rapid onset), by nausea and vomiting. It resembles Staphylococcus aureus (staph) food poisoning in its symptoms and incubation period. These bacteria are present in foods and can multiply quickly at room temperature. Some strains of this bacteria are harmful to humans and cause foodborne illness, while other strains can be beneficial as probiotics for animals.

Other strains morphology

Characteristics

•         Gram positive large (width > 1 um) rod,

•         spore former -- central spore or paracentral

•         grows aerobically and anaerobically

•         beta hemolytic

•         usually motile

•         may be present in stools of healthy individuals

•         grown out of food samples after heat shock --> treat sample at 70^o C for 10 minutes; or after ethanol shock --> mix 1:1 with absolute ethanol for 1 hour

•         Widely disseminated in nature

Habitat

Bacillus cereus has no specific geographic distribution. It is widely distributed around the world, but has no known animal reservoir. B.cereus is a spore-forming soil microbe. The microbe is able to grow either in the presence or in the absence of oxygen with PH 4.3-9.3. Its spores are sufficiently heat-resistant to survive pasteurization treatment of milk and standard cooking temperatures reached in domestic kitchens. It cannot survive the high-temperature treatment used to process canned foods.

 Brief History

The first reported description of Bacillus cereus food poisoning was published in 1950. B.cereus has been recognized as an agent of food poisoning since 1955. There are only a few outbreaks a year reported by CDC. The earliest documented US outbreak occurred in 1969. Two years later, the United Kingdom confirmed its first outbreak of Bacillus cereus food poisoning. Between 1972 and 1986, 52 outbreaks of food-borne disease associated with B. cereus were reported to the CDC included such foods as beef, turkey, and Mexican foods (in 2003, there were two), but this is thought to represent only 2% of the total cases which have occurred during these periods. It is not a reportable disease, and usually goes undiagnosed because of symptomatic similarities to S.aureus intoxication (B. cereus vomiting-type) or C.perfringens food poisoning (B. cereus diarrheal type).

Sources    

A variety of foods, particularly rice and leftovers, as well as sauces, soups, and other prepared foods that have sat out too long at room temperature. Moreover, as a common soil inhabitant, Bacillus cereus is most likely to be found on harvested crops – grains, fruits and vegetables. Because the spores are heat-resistant and survive normal cooking conditions, Bacillus cereus food poisoning is most commonly associated with consuming cooked, starchy foods – such as rice dishes – that have been held at room temperature for several hours after cooking. Most common sources are rice, spices and dairy products.

Bacillus cereus food poisoning

Causes of B.cereus poisoning

This illness occurs via ingestion of contaminated food. A good example of this is when food is not cooked at the correct temperature which results in it being undercooked. This situation is compounded when the undercooked food is then stored in the fridge at the wrong temperature.

This results in warm, undercooked food which is a ripe breeding ground for germs and bacteria. These bacteria thrive in these conditions and produce spores –a series of seed like structures. These spores produce toxins (called ‘enterotoxins’) which are heat resistant and cause two types of food poisoning i.e. emetic and diarrheal.

There is no evidence that human carriage of the organism or other means of contamination play a role in transmission. It is not known whether the ingested organisms multiply and make toxin in vivo or whether a preformed toxin is present in food.

Diagnosis of Human Illness:

Confirmation of B. cereus as the etiologic agent in a foodborne outbreak requires either:

Ø  Isolation of strains of the same serotype from the suspect food and feces or vomitus of the patient.

Ø  Isolation of large numbers of a B.cereus serotype known to cause foodborne illness from the suspect food or from the feces or vomitus of the patient.

Ø  Isolation of B. cereus from suspect foods and determining their enterotoxigenicity by serological (diarrheal toxin) or biological (diarrheal and emetic) tests.

Target Populations:

All people are believed to be susceptible to B. cereus food poisoning. However, some individuals, especially young children, are particularly susceptible and may be more severely affected.      

Preventing bacillus cereus poisoning

• Keep prepared foods under refrigeration (5°C) until you are ready to serve.
• Keep hot foods at a temperature greater than 60°C.
• Try to avoid preparing foods too far in advance.
• For heated products that are going to be eaten at a later time ensure that the product is correctly cooled after cooking.
• Wash hands with hot soapy water and dry thoroughly before preparing food.
• Keep utensils and kitchen clean.
• Avoid placing raw food on top of or next to cooked food
•  Reading instructions carefully about cooking times
• Using separate chopping boards, utensils etc for raw and cooked foods.

Treatment

Persons with B. cereus food poisoning require only supportive treatment.

Ø  Oral rehydration.

Ø  Occasionally, intravenous fluid and electrolyte replacement for patients with severe dehydration.

Ø  Antibiotics are not indicated.