Bacillus cereus

Overview[edit | edit source]

Bacillus cereus is an endemic, soil-dwelling, Gram-positive, rod shaped, beta hemolytic bacteria that causes foodborne illness.^[1] It is the cause of "Fried Rice Syndrome". B. cereus bacteria are facultative aerobes, and like other members of the genus Bacillus can produce protective endospores.

Pathogenesis[edit | edit source]

• B. cereus is responsible for a minority of foodborne illnesses (2–5%). It is known to create heavy nausea, vomiting, and abdominal periods. ^[2] Generally speaking, Bacillus foodborne illnesses occur due to survival of the bacterial spores when food is improperly cooked.^[3] This problem is compounded when food is then improperly refrigerated, allowing the spores to germinate.^[4] Bacterial growth results in production of enterotoxin, and ingestion leads to two types of illness, diarrheal and emetic syndrome.^[5]

• The diarrheal type is associated with a wide-range of foods, has an 8–16 hour incubation time and is associated with diarrhea and gastrointestinal pain. Also know as the long-incubation form of B. cereus food poisoning, it can be difficult to differentiate from poisoning caused by Clostridium perfringens.^[6]
• In the emetic form, cooked rice that is improperly refrigerated is the most common cause, leading to nausea and vomiting 1–5 hours after consumption. This form can be difficult to distinguish from other short-term bacterial foodborne pathogens (e.g. Staphylococcus aureus).^[6]

• It was previously thought that the timing of the toxin production might be responsible for the two different types, but in fact the emetic syndrome is caused by a toxin called cereulide that is found only in emetic strains and is not part of the 'standard toolbox' of B. cereus. Cereulide a dodecadepsipeptide produced by non-ribosomal peptide synthesis (NRPS), which is somewhat unusual in itself. It was shown independently by two research groups to be encoded on a plasmid, which is called pCERE01 ^[7] or pBCE4810 ^[8]. Interestingly, this plasmid shares a common backbone with the virulence plasmid pXO1, which encodes the anthrax toxin genes in B. anthracis, but with a different pathogenicity island. Periodontal isolates of B. cereus also possess distinct pXO1-like plasmids.==Ecology==

B. cereus competes with other microorganisms such as Salmonella and Campylobacter in the gut, so its presence reduces the numbers of those microorganisms. In food animals such as chickens,^[9] rabbits^[10] and pigs,^[11] some harmless strains of B. cereus are used as a probiotic feed additive to reduce Salmonella in the intestines and cecum. This improves the animals' growth as well as food safety for humans who eat their meat.

Some strains of B. cereus produce cereins, bacteriocins active against different B. cereus strains or other Gram-positive bacteria.^[12]

Diagnosis[edit | edit source]

In case of foodborne illness, the diagnosis of B. cereus can be confirmed by the isolation of more than 10⁵ B. cereus organisms per gram from epidemiologically implicated food, but such testing is often not done because the illness is relatively harmless and usually self-limiting.^[13]

Prognosis[edit | edit source]

Most emetic patients recover within six to 24 hours, but in some cases, the toxin can be fatal.^{[14][15][16][17][18]} In 2014, 23 neonates receiving total parenteral nutrition contaminated with B. cereus developed septicaemia, with three of the infants later dying as a result of infection. ^[19][20]

Reproduction[edit | edit source]

At 30 °C (86 °F), a population of B. cereus can double in as little as 20 minutes or as long as 3 hours, depending on the food product.^[21]

Gallery[edit | edit source]

• 
  A closer view of PHIL 12377, this photograph depicts the colonial morphology displayed by Gram-positive Bacillus cereus bacteria, which was grown on a medium of sheep’s blood agar (SBA), for a 24 hour time period, at a temperature of 37°C. From Public Health Image Library (PHIL). ^[22]
• 
  This photograph depicts the colonial morphology displayed by Gram-positive Bacillus cereus bacteria, which was grown on a medium of sheep’s blood agar (SBA), for a 24 hour time period, at a temperature of 37°C. From Public Health Image Library (PHIL). ^[22]
• 
  Bacillus cereus showing hemolysis on sheep blood agar. From Public Health Image Library (PHIL). ^[22]
• 
  Bacillus cereus showing hemolysis on sheep blood agar. From Public Health Image Library (PHIL). ^[22]
• 
  Blood agar and bicarbonate agar plate cultures of Bacillus cereus. From Public Health Image Library (PHIL). ^[22]
• 
  Sheep blood agar plate culture of Bacillus anthracis and Bacillus cereus. From Public Health Image Library (PHIL). ^[22]
• 
  Bacillus cereus. Gram stain. From Public Health Image Library (PHIL). ^[22]
• 
  Bacillus cereus. Leifson flagella stain. From Public Health Image Library (PHIL). ^[22]

Treatment[edit | edit source]

Antimicrobial Regimen[edit | edit source]

• 1. Food poisoning^[23]

• Preferred regimen: Food poisoning is usually self-limited and requires no antibiotic therapy.

• 2. Bacteremia

• Preferred regimen: Vancomycin 15 mg/kg IV q12h
• Alternative regimen: Clindamycin 600 mg IV q8h
• Note (1): Bacillus cereus is commonly resistant to beta-lactams.
• Note (2): Pseudobacteremia is transient and usually results from contaminated blood cultures, gloves, or syringes.

• 3. Meningitis or brain abscess

• Preferred regimen: Vancomycin 15 mg/kg IV q12h
• Alternative regimen: Clindamycin 600 mg IV q8h
• Note: Blood culture isolates are mostly contaminates until proven otherwise, especially in intravenous drug user population.

• 4. Endophthalmitis

• Preferred regimen: Clindamycin 450 μg intravitreal AND Gentamicin 400 μg intravitreal OR Dexamethasone intravitreal AND Vancomycin 15 mg/kg IV q12h
• Alternative regimen: Clindamycin 600 mg IV q8h
• Note: Ophthalmological consultation, culture ocular fluids, early vitrectomy, and intravitreal antibiotics are necessary.

• 5. Endocarditis

• Preferred regimen: Vancomycin 15 mg/kg IV q12h
• Note: Most blood cultures in intravenous drug users are contaminates or represent transient bacteremia.

• 6. Soft tissue infection

• Preferred regimen: Vancomycin 15 mg/kg IV q12h
• Alternative regimen: Clindamycin 600 mg IV q8h

• 7. Pneumonia

• Preferred regimen: Vancomycin 15 mg/kg IV q12h
• Alternative regimen: Clindamycin 600 mg IV q8h

References[edit | edit source]