Bacillus cereus is a toxin-producing facultatively anaerobic gram-positive bacteria. The bacteria is commonly found in the environment, is often found in soil and vegetation, and can be present in foods. It can quickly multiply at room temperature. There are two main types of intestinal illnesses caused by B. cereus. One is diarrheal, and one leads more to nausea/vomiting. B. cereus has also been implicated in infections of the eye, respiratory tract, and in wounds. The pathogenicity of B. cereus, whether intestinal or nonintestinal, is intimately associated with the production of tissue-destructive exoenzymes. Among these secreted toxins are four hemolysins, three distinct phospholipases, an emesis-inducing toxin, and proteases.
B. cereus illness is related to many foods - beef, turkey, rice, beans, vegetables. Specifically, the diarrheal illness is often related to meats, milk, vegetables, and fish. The emetic-type illness is most often associated with rice products, but it has also been associated with other types of starchy products such as potato, pasta, and cheese products. Some food-mixtures (sauces, puddings, soups, casseroles, pastries, and salads, have been associated with food-borne illness in general.
Bacillus cereus is caused by the ingestion of food contaminated with either the enterotoxigenic B. cereus or with the emetic toxin. In non-gastrointestinal illness, reports of respiratory infections similar to respiratory anthrax have been attributed to B. cereus strains harboring B. anthracis toxin genes.
The Centers for Disease Control (CDC) website states that there were 619 confirmed outbreaks of Bacillus-related poisoning from 1998 through 2015, involving 7385 illnesses. In this timeframe, there were 75 illnesses and three deaths due to confirmed Bacillus-related illness. The website states there were 19,119 outbreaks overall, and 373,531 illnesses. It refers to 14,681 hospitalizations and 337 deaths. These statistics refer to all Bacillus-related illness, and not just B. cereus related illness.
The Bad Bug Book further breaks this down and states there are an estimated 63,400 episodes of B. cereus illness annually in the United States. In 2005, 2006, and 2007 there were 13 confirmed outbreaks and 37.6 suspected outbreaks during this period, involving over 1000 people.
All people are considered to be susceptible to B. cereus infection. Mortality is rare. The emetic enterotoxin has been found to be associated with a few cases of liver failure and death in otherwise healthy people. There is a newly found cytotoxin that has been associated with a B. cereus strain that caused one confirmed outbreak and three deaths. The infective dose or the number of organisms most commonly associated with human illness is 10^5 to 10^8, but pathogenicity arises from the preformed toxin, not from the bacteria themselves.
There are two types of food-borne B. cereus illness. In the first, contaminated food (many types of food, often left at room temperature) makes its way to the small intestine where the toxin, in this case, a large-molecular-weight protein, is released. This can lead to diarrhea, cramps, and sometimes nausea. Usually, vomiting is not present in this form of illness. Incubation for the first type is 6 to 15 hours. In the second type, affected food, most often starchy food, and classically, rice, contains a different type of toxin (cereulide, an ionophoric low-molecular-weight dodecadepsipeptide that is pH-stable and heat and protease-resistant). This toxin causes emetic-type B. cereus illness. The incubation period for this type is 30 minutes to 6 hours.
Endophthalmitis is a vision-threatening eye infection resulting from microbial infection of the inner eye. The hallmark of the ophthalmic lesion is a corneal ring abscess, which may be accompanied by rapid progression of pain, proptosis, chemosis, retinal hemorrhage, and perivasculitis. Systemic manifestations include fever, leukocytosis, and general malaise.
The non-hemolytic enterotoxin (NHE) from Bacillus cereus is an activator of the NLRP3 inflammasome and pyroptosis. 
The pathogenicity of B. cereus, whether inside or outside of the gastrointestinal (GI) tract, is associated with exoenzyme production. Among the secreted toxins are four hemolysins, three distinct phospholipases, an emesis-inducing toxin, and three pore-forming enterotoxins: hemolysin BL (HBL), nonhemolytic enterotoxin (NHE), and cytotoxin K. In the gastrointestinal tract (small intestine), vegetative cells, ingested as viable cells or spores, produce and secrete a protein enterotoxin and induce a diarrheal syndrome, whereas emetic toxin, a plasmid-encoded cyclic peptide (cereulide), is produced in food products and ingested as a formed toxin. In rabbit ligated ileal-loop assays, culture filtrates of enterotoxigenic strains induce fluid accumulation and hemolytic, cytotoxic, dermonecrotic, and vascular permeability activities in rabbit skin.
The enterotoxin is composed of a binding component (B) and two hemolytic components, designated HBL. Also, diarrheagenic in the gastrointestinal tract is a nonhemolytic three-component enterotoxin, designated NHE. The emetic toxin, which induces a vomiting syndrome, is synthesized in the contaminated food product, usually rice, pasta, or dairy products, in which B. cereus is growing and may represent a product of growth.
History and Physical
For diarrheal-type illnesses, symptoms include watery diarrhea and abdominal pain and cramping. Some nausea can be associated but rarely vomiting. Symptom onset is usually within 6 to 15 hours of eating food that was left at room temperature (less than 140 degrees and more than 40 degrees) for more than 2 hours.
For emetic-type illness, symptoms include nausea and vomiting, similar to the presentation associated with Staphylococcus aureus food-borne illness. Symptom onset is generally within 30 minutes to 6 hours of consuming rice or starchy foods that were left at room temperature, even after they have been reheated.
Symptoms for both types usually resolve within 24 hours after onset. In complicated cases, severe manifestations can include severe systemic and pyogenic infections, septic meningitis, gangrene, cellulitis, panophthalmitis, lung abscess, infant death, and endocarditis. Risk factors for the complicated and severe disease include immunosuppression, intravenous drug abuse, and the neonatal period, as well as having an indwelling catheter or having a traumatic or surgical wound.
There is some difficulty differentiating B. cereus infections from other similar infections. The diarrheal type of B. cereus infection is very similar to Clostridium perfringens food poisoning. The symptoms of the emetic type parallel S. aureus.
B. cereus can be confirmed as the source of a foodborne outbreak by (1) isolation of strains of the same serotype from a food source and the feces or vomitus of a patient, (2) isolation of large numbers of B. cereus serotype known to cause foodborne illness from the food/feces/vomitus of a patient, or (3) isolation of B. cereus from suspect foods and confirmation of the enterotoxigenic through serology (for the diarrheal toxin) or biological tests (diarrheal and emetic).
In other infections like lung abscess and endophthalmitis, evaluation is made with whatever fluid is available. For example, the diagnosis of endophthalmitis can be made by the gram-staining of vitreous fluid.
Differential diagnosis of B.cereus infections include:
Viral infections (e.g., rotavirus)
Bacterial infections (e.g., Campylobacter, Shigella, Salmonella, E. coli, Y. enterocolitica, V. cholera, C. difficile)
Patients using systemic steroids, hospitalized, or neutropenia has a bad prognosis when combined with bacteremia. Patients with hematologic malignancies and neutropenia, along with B. cereus infections, have a worse prognosis, which improves with the resolution of neutropenia.
Complications of B. cereus infections are uncommon and only occur in people with compromised immune systems. The complications include gangrene, cellulitis, and aseptic meningitis.
Deterrence and Patient Education
Patients should be educated about the importance of handwashing for the prevention of these bacterial infections. Also, they should be provided with the necessary literature regarding the appropriate receiving, handling, processing, and storage of food and food products in order to prevent the infection.
Enhancing Healthcare Team Outcomes
Bacillus cereus food poisoning is a relatively common presentation to the emergency department. Hence, nurses and nurse practitioners should be familiar with its presentation and management. The key feature of bacillus cereus poisoning is that the symptoms usually occur rapidly after the ingestion of the contaminated food. However, the symptoms can be confused with C.difficile colitis. The diarrhea is watery and quite pronounced. Fortunately, the majority of patients only require oral hydration. Mortality is very rare with this infection.
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B. Cereus rods
Contributed by Frederick C. Michel, ASM
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