Three second-year medical students eat at a local Chinese restaurant that is perhaps better known for its low prices than the quality of its cuisine. Their lunch consists of hot and sour soup, spring rolls, fried rice, and three other Chinese entrees. The servings are plentiful, so the left-over fried rice and two of the entrees are taken back to their apartment and stored in their refrigerator. The students decide to reheat the food and have it for lunch two days after bringing it home. Two hours later - ironically right in the middle of their medical microbiology class - all three students are overcome with an urge to vomit and quickly excuse themselves from class. A bout of severe nausea, vomiting, and abdominal cramps follows, but only one of them experiences diarrhea (which is relatively mild). Being highly inquisitive and dedicated medical students who always like to practice their diagnostic skills, they check each other's vital signs, which turn out to be normal. The stool produced by the one student to experience mild diarrhea is watery, with no visible signs of blood or mucus. The symptoms all three victims subside within 10 hours, without treatment and without noticeable sequelae. None of the students have traveled outside of the city in which their university is located in the past six months. The reheated Chinese food is the only meal at which they all ate the same things since their lunch at the Chinese restaurant itself.

Question 3.1: How did this disease incident come about?

The fact that all three students were afflicted at the same time suggests a common source of their disease. Common-source gastrointestinal afflictions are usually traced to contaminated water or food. The students have not traveled to an area where the water is likely to be contaminated, so contaminated food is the more likely cause. The lack of any evidence for an active microbial infection suggests some type of toxin-related food poisoning. The Chinese food is the most likely source of the problem because it is the only food eaten by all three students during the past few days. Theoretically they could have been infected during the original meal at the restaurant, but the stored and reheated food is naturally more suspicious.

Question 3.2: What is the most likely causative agent?

Assuming that the reheated Chinese food caused this problem, we are dealing with a very short incubation period (only about 2 hrs). The only forms of microbial food poisoning likely to manifest in such a short period of time are those caused by Staphylococcus aureus or Bacillus cereus. (It should be realized, however, that incubation times are not always definitive for the diagnosis of bacterial food poisoning; incubation times can vary depending on the number of organisms or amount of toxin consumed.) The symptoms of these two diseases are similar, so either one is a reasonable possibility. However, the meal that apparently caused the problem included reheated fried rice and the organism most commonly associated with that food is Bacillus cereus. In fact, most cases of acute B. cereus food poisoning with a short incubation period (< 6 hrs) are associated with reheated fried rice.

Question 3.3: How does this causative agent produce disease?

B. cereus is a spore-forming bacterium that is commonly present in many natural environments. If rice is contaminated with B. cereus spores, the flash frying used in the preparation of certain rice dishes (e.g., fried rice) does not raise the temperature enough to destroy spores (which are exceedingly heat-resistant). In fact, the cooking may actually activate spores, which may then germinate and grow in fried rice when it is stored in the refrigerator. As the organism grows in the stored food, it produces a heat-stabile enterotoxin that is not inactivated by reheating.

Contamination of rice with B. cereus is also attributed to a practice used in many Asian restaurants, in which large portions of boiled rice are allowed to drain without refrigeration to avoid clumping. The boiling activates B. cereus spores naturally present in the rice, after which they germinate, grow, and produce the heat-stable enterotoxin in the unrefrigerated rice as it drains. Flash frying then fails to inactivate the toxin as the final dishes are prepared. In this case, the victims are sickened by the meal they eat at the restaurant, rather than by leftover food.

Several exotoxins produced by B. cereus strains may contribute to their pathogenicity. One of these (a presumptive toxin at this point) produces vomiting when fed to rhesus monkeys, but it does not produce fluid accumulation in the rabbit intestine, and it does not stimulate the adenylate cyclase-AMP system (as does the cholera toxin).

Question 3.4: Does this causative agent produce any other forms of GI disease?

B. cereus actually produces two forms of food poisoning. The emetic form featured in this case has a relatively short incubation period (1-6 hrs) and primarily causes nausea, vomiting, and abdominal cramps. Diarrhea is also possible, but appears in only 1/3 of the victims. The emetic form involves a Staphylococcus aureus-like enterotoxin, most likely the presumptive vomiting toxin described above. This disease is almost always associated with reheated fried rice, especially after a period of refrigeration.

The diarrheal form of B. cereus food poisoning has a somewhat longer incubation period (8-16 hrs) and primarily produces watery diarrhea and abdominal cramps. Some victims experience vomiting, but most do not. The diarrheal form involves an enterotoxin similar to the E. coli LT. The toxin stimulates the adenylate cyclase-AMP system in intestinal epithelial cells, thereby causing fluid loss. This disease is most often associated with fried rice, but has also been attributed to meat, vegetables, dried beans, and cereals.

Question 3.5: How should this disease be treated?

Treatment is generally not necessary for cases of B. cereus food poisoning. The symptoms resolve on their own in a relatively short period of time, as they did in this case. Replacement of fluids may be required if a severe case causes dehydration.