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Bacteriology at UW- Madison |
Bacillus cereus Spore
Stain
Bacillus cereus has been recognized as an agent of food poisoning since 1955. Between 1972 and 1986, 52 outbreaks of food-borne disease associated with B. cereus were reported to the CDC, but this is thought to represent only 2% of the total cases which have occurred in that time.
B. cereus causes two types of food-borne intoxications (as opposed to infections). One type is characterized by nausea and vomiting and abdominal cramps and has an incubation period of 1 to 6 hours. It resembles Staphylococcus aureus food poisoning in its symptoms and incubation period. This is the "short-incubation" or emetic form of the disease. The second type is manifested primarily by abdominal cramps and diarrhea with an incubation period of 8 to 16 hours. Diarrhea may be a small volume or profuse and watery. This type is referred to as the "long-incubation" or diarrheal form of the disease, and it resembles more food poisoning caused by Clostridium perfringens. In either type, the illness usually lasts less than 24 hours after onset. In a few patients symptoms may last longer.
The short-incubation form is caused by a preformed heat-stable enterotoxin of molecular weight less than 5,000 daltons. The mechanism and site of action of this toxin are unknown. The long-incubation form of illness is mediated by a heat-labile enterotoxin (molecular weight of approximately 50,000 daltons) which activates intestinal adenylate cyclase and causes intestinal fluid secretion.
B. cereus food poisoning occurs year-round and is without any particular geographic distribution. The short-incubation form is most often associated with fried rice that has been cooked and then held at warm temperatures for several hours. The disease is often associated with Chinese food restaurants. In one reported outbreak, macaroni and cheese made from powdered milk turned out to be the source of the bacterium.
Long-incubation B. cereus food poisoning is frequently
associated
with meat or vegetable-containing foods after cooking. The bacterium
has
been isolated from 50% of dried beans and cereals and from 25% of dried
foods such as spices, seasoning mixes and potatoes. One outbreak of the
long-incubation form was traced to a "meals-on-wheels" program in which
food was held above room temperature for a prolonged period.
Summary and Analysis of a Report of Bacillus cereus Food Poisoning Associated with Fried Rice at Two Child Day Care Centers -- Virginia, 1993 (from CDC)
Summary
On July 21, 1993, a regional public health facility received reports
of acute gastrointestinal illness that occurred among children and
staff
at two jointly owned child day care centers following a catered lunch.
The catered lunch was served on July 21 to 82 children aged less than or equal to 6 years, and to nine staff; dietary histories were obtained for 80 persons. 67 ate the catered lunch. A case was defined as vomiting by a person who was present at either day care center on July 21. Fourteen (21%) persons who ate the lunch became ill, compared with none of 13 who did not. Symptoms included nausea (71%), abdominal cramps or pain (36%), and diarrhea (14%). Twelve of the 14 cases occurred among children aged 2.5-5 years, and two occurred among staff. The median incubation period was 2 hours (range: 1.5-3.5 hours). Symptoms resolved a median of 4 hours after onset (range: 1.5-22 hours).
Chicken fried rice prepared at a local restaurant was the only food significantly associated with illness; illness occurred in 14 (29%) of 48 persons who ate chicken fried rice, compared with none of 16 who did not.
The rice had been cooked the night of July 20 and cooled at room temperature before refrigeration. On the morning of the lunch, the rice was pan-fried in oil with pieces of cooked chicken, delivered to the day care centers at approximately 10:30 a.m., held without refrigeration, and served at noon without reheating.
Following the outbreak, health officials recommended to day care staff and restaurant food handlers that the practice of cooling rice or any food at room temperature be discontinued, food be maintained at proper temperatures (i.e., below 41 F {5 C} or above 140 F {60 C}), and a thermometer be used to verify food temperatures.
Analysis
The emetic ("short incubation") form of the disease,which occurred
in this outbreak, is mediated by a highly stable toxin that survives
high
temperatures and exposure to trypsin, pepsin, and pH extremes; the
diarrheal
syndrome is mediated by a heat- and acid-labile enterotoxin that is
sensitive
to proteolytic enzymes.
The diagnosis of B. cereus food poisoning can be confirmed by the isolation of greater than or equal to 105 B. cereus organisms per gram from epidemiologically implicated food. Underreporting of such outbreaks is likely because illness associated with B. cereus is usually self-limiting and not severe. In addition, findings of a recent survey about culture practices for outbreaks of apparent foodborne illness indicate that 20% of state public health laboratories do not make B. cereus testing routinely available.
Fried rice is a leading cause of B. cereus emetic-type food poisoning in the United States. B. cereus is frequently present in uncooked rice, and heat-resistant spores may survive cooking. If cooked rice is subsequently held at room temperature, vegetative forms multiply, and heat-stable toxin is produced that can survive brief heating, such as stir frying. In the outbreak described in this report, vegetative forms of the organism probably multiplied at the restaurant and the day care centers while the rice was held at room temperature.
The day care staff and restaurant food handlers in this report were unaware that cooked rice was a potentially hazardous food. This report underscores the ongoing need to educate food handlers about basic practices for safe food handling.
Bacillus cereus Colonies
on Blood Agar