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Clinical manifestation of Bacillus cereus
A. Food poisoning
Two forms of food poisoning: vomiting disease (emetic form) and diarrheal disease (diarrheal form).
- Emetic form
The emetic form of disease results from the consumption of contaminated rice. Most bacteria are killed during the initial cooking of the rice, but the heat-resistant spores survive. If the cooked rice is not refrigerated, the spores germinate, and the bacteria can multiply rapidly. The heat-stable enterotoxin that is released is not destroyed when the rice is reheated. The emetic form of disease is an intoxication, caused by ingestion of the enterotoxin and not the bacteria. Thus the incubation period after eating the contaminated rice is short (1 to 6 hours), and the duration of illness is also short (less than 24 hours). Symptoms consist of vomiting, nausea, and abdominal cramps. Fever and diarrhea are generally absent.
- Diarrheal form
The diarrheal form of B. cereus food poisoning is a true infection, resulting from ingestion of the bacteria in contaminated meat, vegetables, or sauces. There is a longer incubation period, during which the organism multiplies in the patient’s intestinal tract, and the release of the heat-labile enterotoxin follows. This enterotoxin is responsible for the profuse watery diarrhea, nausea, and abdominal cramps.
B. Eye infections
- B. cereus ocular infections usually occur after traumatic, penetrating injuries of the eye with a soil contaminated object.
- These include severe keratitis and endophthalmitis.
C. Other infections
- B. cereus has also been associated with localized infections, such as wound infections, and with systemic infections, including endocarditis, catheter-associated bacteremia, central nervous system infections, osteomyelitis, and pneumonia.
- The presence of a medical device or intravenous drug use predisposes to these infections.
Pathogenicity and virulence factors of Bacillus cereus
- B. cereus is most commonly associated with food poisoning, but the organism can also cause post-traumatic ophthalmitis, which requires rapid, aggressive management locally.
- The organism is widespread in the environment and is found in most raw foods, especially cereals such as rice.
- Gastroenteritis caused by B. cereus is mediated by one of two enterotoxins.
- The heat-stable, proteolysis-resistant enterotoxin causes the emetic form of the disease. The mechanism of action of the heat-stable enterotoxin is unknown.
- The emetic form is manifested by nausea, vomiting, abdominal cramps, and occasionally diarrhea and is self-limiting, with recovery occurring within 24 hours.
- The heat-labile enterotoxin causes the diarrheal form of the disease; each stimulates the adenylate cyclase–cyclic adenosine monophosphate system in intestinal epithelial cells, leading to profuse watery diarrhea.
- The diarrheal form has an incubation period of 1–24 hours and is manifested by profuse diarrhea with abdominal pain and cramps; fever and vomiting are uncommon.
B. cereus is the most commonly encountered species of Bacillus in opportunistic infections including posttraumatic eye infections, endocarditis, and bacteremia. The pathogenesis of B. cereus ocular infections is also incompletely defined.
- At least three toxins have been implicated; they are necrotic toxin (a heat-labile enterotoxin), cereolysin and phospholipase C (a potent lecithinase).
- It is likely that the rapid destruction of the eye that is characteristic of B. cereus infections results from the interaction of these toxins and other unidentified factors. Bacillus species can colonize skin transiently and can be recovered as insignificant contaminants in blood cultures.
- In the presence of an intravascular foreign body, however, these organisms can be responsible for persistent bacteremia and signs of sepsis (i.e., fever, chills, hypotension, shock).
- Infections of other sites are rare and usually involve intravenous drug abusers or immunocompromised patients.