Case Study

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A 15-year-old high school student and several of her friends ate lunch at a local Chinese restaurant. They all were served the daily luncheon special, which consisted of sweet and sour pork with vegetables and fried rice. All the girls developed nausea, vomiting, abdominal pain, and diarrhea within 6 hours of eating lunch. They were all sent to the local emergency department for work-up of their suspected food poisoning. The children are all otherwise healthy and none are on any medication and have no known allergies.

Which of the following is the most likely cause of these symptoms?

A) Bacillus cereus
B) Staphylococcus aureus
C) Clostridium botulinum
D) Clostridium perfringens
E) EHEC (Enterohemorrhagic Escherichia coli)

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The Answer:


The correct answer is Choice A.

The most likely cause of the patient's symptoms is infection by Bacillus cereus, (Choice A) a Gram-positive rod that causes foodborne illness. Improperly cooked food allows bacterial endospores to survive while subsequent improper refrigeration allows for germination of the bacterial endospores and production of enterotoxins.

Bacillus cereus causes two separate forms of illness: an emetic form and a diarrheal form. Improperly cooked and stored rice is one of the most commonly implicated foods associated with the emetic form. The emetic form is caused by a toxin named cereulide, which leads to nausea and vomiting a short time (2 to 6 hours) after ingestion. The diarrheal form is caused by three toxins: Hemolysin BL, Nonhemolytic Enterotoxin, and Cytotoxin K. Many different types of food are associated with the diarrheal form and symptoms of watery non-bloody diarrhea and abdominal cramps occur 8 to 20 hours after ingestion.

Staphylococcus aureus (Choice B) is a Gram-Positive cocci which is found in clusters. Staphylococcus aureus is coagulase positive and causes a variety of infections including impetigo, cellulitis, osteomyelitis, endocarditis, and myositis. Staphylococcus aureus food poisoning occurs due to the ingestion of preformed toxins (heat and acid-stable staphylococcal enterotoxins). Illness can occur as soon as 4 hours after eating contaminated food. Staphylococcal enterotoxins act directly on neural receptors in the upper gastrointestinal tract which leads to stimulation of the vomiting center of the brain. Typical foods associated with Staphylococcus aureus food poisoning include ham, cream filled pastries, and potato salad.

Clostridium botulinum (Choice C) is a Gram-Positive anaerobic spore forming rod which causes botulism. Botulism is caused by the ingestion of a pre-formed neurotoxin. Botulism toxin prevents the release of the neurotransmitter acetylcholine, which interferes with neurotransmission at peripheral cholinergic synapses. Clinical disease occurs within 12 to 36 hours after ingestion and is characterized by a flaccid paralysis. The cranial nerves are affected first, particularly those involving the eyes causing diplopia and blurred vision. Difficulty swallowing is another early sign. The paralysis descends with subsequent involvement of respiratory muscles. The toxin does not produce systemic signs of fever or sepsis and patients typically succumb to paralysis and respiratory failure.

Clostridium perfrigens (Choice D) is a Gram-Positive anaerobic spore forming rod. Clostridium perfrigens food poisoning often occurs when poultry, meat, or fish is precooked and then reheated before serving. Spores of Clostridium perfrigens resist the first heating and then germinate in the food. Upon ingestion, the organisms sporulate in the intestine and form a heat-labile enterotoxin which inhibits glucose transport, damages the intestinal epithelium, and causes protein loss into the intestinal lumen. Clostridium perfrigens food poisoning is characterized by watery diarrhea, severe cramping, and abdominal pain which begins 8 to 24 hours after the suspected meal. The illness typically lasts 24 hours or less.

Enterohemorrhagic Escherichia coli (Choice E) is a Gram-Negative rod. EHEC occurs by ingestion of contaminated beef which produces SLT I and II (Shiga-like toxin (verotoxin)). The toxin causes rearrangement of actin, damage to microvilli, and systemic toxemia, however, it is not invasive. The diarrhea is copious, watery, and bloody with minimal neutrophils. This diarrhea is associated with serious complications, including hemorrhagic colitis, hemolytic uremic syndrome (HUS), and thrombotic thrombocytopenic purpura (TTP). The particular serogroup associated with Enterohemorrhagic Escherichia coli is O157:H7.

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