Fact sheet on presumptive Bacillus cereus

General
The term „presumptive Bacillus cereus“ describes a group of closely related Bacillus cereus strains that can cause disease in humans and animals.
Bacillus cereus (B. cereus) is a bacterium that forms spores as resistant survival forms. The spores are particularly heat-stable, can survive heating steps and may even be induced to germinate by heat shock. Due to the resistance of the spores to environmental influences such as heat and desiccation, these organisms are widespread in the environment. Complete avoidance of B. cereus in foods is rarely possible (e.g. in hermetically sealed canned goods).
In addition, these bacteria are capable of producing various toxins, some of which can also be heat-stable. The toxins can lead to intoxications that cause vomiting or diarrhoea. The type of illness depends on the specific toxin. Not all members of this group have the potential to cause disease; this depends on their ability to produce toxins.

Characteristics

• Rod-shaped bacteria
• Gram-positive
• Oxidase-negative
• Catalase-positive
• Spore-forming
• Toxin-producer (also produces heat- and acid-resistant toxins, among others)

Origin / Occurrence

• B. cereus and its spores are widespread in our environment (e.g. soil, dust, water, animals and humans)
• Particularly found in plant-based foods but also in animal-derived products (especially dairy products)

Significance

• Potential foodborne pathogen. Frequently causes foodborne illnesses, including outbreaks in catering or communal feeding settings.
• The acid- and very heat-stable emetic (vomiting) toxin (cereulide) of B. cereus can survive common heating steps in food production and is formed in the food → oral intake of the toxin after growth of the organism and toxin formation in the food
• The diarrhoeal enterotoxins (Hbl, Nhe, CytK) are heat-labile and are produced by B. cereus only in the intestine → oral intake especially of spores, but also of vegetative cells
• Bacillus thuringiensis (Bt), a member of the presumptive B. cereus group, is used as a biological insecticide, especially in organic crop production. If the time between application and harvest is too short, increased counts can occur on various vegetables, e.g. tomatoes. Literature reports that B. thuringiensis or its toxins have in rare cases caused foodborne gastroenteritis in humans.

Clinical picture

• Emetic toxin (cereulide or emetic toxin): incubation period: 0.5 to 6 hours, duration of illness: usually 6 to 24 hours, symptoms: nausea and severe vomiting (emetic form), less commonly abdominal cramps and diarrhoea
• Diarrhoeal enterotoxins (Hbl, Nhe, CytK): incubation period: 8–16 hours, duration of illness: usually 12 to 24 hours, symptoms: diarrhoea
• Minimal intoxication dose:
  • For the emetic toxin (cereulide): generally at least 10⁴ CFU/g (in rare cases from 10³ CFU/g)
  • For diarrhoeal enterotoxins (Hbl, Nhe, CytK): generally at least 10⁵ CFU/g (for CytK also lower cell counts)
  • The minimal intoxication dose also depends on the properties of the B. cereus strains. There are B. cereus strains that produce large amounts of toxin (high-producers), others produce only small amounts of toxin (low-producers)

Examples of affected foods

• cooked rice, cooked pasta
• vegetables
• sauces, soups
• desserts, e.g. pudding, semolina porridge
• spices, herbs, sprouts, salads, dried mushrooms
• cereal products
• dairy products

→ Foods of plant origin, especially starchy products, are far more often the cause of gastrointestinal illnesses than foods of animal origin

Growth conditions and toxin production

• Temperature:
  • Optimum: +30 °C to +40 °C
  • Minimum: cold-tolerant strains can grow even under refrigeration at +4 °C
  • In general, no growth above +58 °C (exception for thermotolerant B. cereus strains)
• pH: growth at pH 5.5 to 8.0 (exceptions even down to pH 4.4)
• a_w-value: growth at a water activity of at least 0.92
• Salt tolerance: 0.5 to 9 %
• Oxygen requirement: facultative anaerobe, prefers aerobic conditions
• Toxin production: dependent on the B. cereus strain, generally under optimal growth conditions

Inactivation by heating

• At +72 °C for at least 2 minutes exposure time vegetative bacteria are killed (Note: monitor core temperature) → spores are very heat-resistant and even survive cooking processes
• Heat-resistant spores are only killed by pressure sterilisation / autoclaving at temperatures such as +125 °C for 15 to 20 minutes
• Inactivation of the heat-stable cereulide toxin only from +121 °C for at least 120 minutes

1. Check processes, production protocols and documentation
2. Killing of vegetative bacteria from a core temperature of +72 °C for at least 2 minutes and heat-resistant spores from +125 °C for 15 to 20 minutes. Inactivation of heat-stable cereulide toxins from +121 °C for at least 120 minutes (case-by-case assessment)
3. Verify correctness of processes and temperatures
   1. In general, holding temperatures above +60 °C are required
   2. Rapid cooling: after 3 hours a core temperature of at least +7 °C must be reached (special caution is required for large quantities)
4. Verify effectiveness by follow-up testing

1. Request traceability of the cold chain during transport or from the supplier
2. Check incoming and storage temperatures
3. Monitor refrigeration temperatures and cold chains during processing, transport and sale
4. If overage (or excessive shelf life) is suspected, carry out appropriate storage tests
5. Verify effectiveness by follow-up testing

1. Check products on receipt for e.g. incoming/transport temperature, shelf life, packaging, deviations or abnormalities
2. This also applies to goods before use in production. If in doubt, do not use the food and consult supervisors
3. Check microbiological specifications. If necessary, arrange laboratory testing of suspicious products or request results from the supplier
4. Verify effectiveness by follow-up testing

1. Maintain distance from fields with animal husbandry; keep wild animals away from fields, e.g. with fences or nets
2. Water hygiene: contaminated water must not be used for irrigation. Irrigation and drainage systems must be appropriate. Regular monitoring of microbiological water quality
3. No microbiologically contaminated fertilisers should be used in crop production
4. Personnel and work hygiene during harvest or downstream steps
5. Remove soil and contaminants by washing
6. Verify effectiveness by follow-up testing

1. Check the performance of cleaning and disinfection:
   1. Hygiene plan
   2. Execution of cleaning and disinfection e.g. disinfectant dosing, contact time, use of suitable and clean materials and cleaning cloths
2. Avoid moisture residues and biofilms. Ensure good drying of equipment, utensils and surfaces
3. Train staff regarding potential errors and corrective measures
4. Repeat cleaning and disinfection
5. Verify effectiveness by visual on-site checks, environmental monitoring and follow-up testing