Bacillus cereus is a spore-forming bacterium that occurs naturally in many kinds of foods and can cause illness in humans. It forms spores that are resistant to heating and dehydration and can therefore survive cooking and dry storage as well as a heat-stable toxin.
Forms recalcitrant spores and a heat-stable toxin
It can cause serious illness in two different ways: after contaminated food is eaten the bacteria produce toxins in the small intestine leading to diarrhoea, cramps, and sometimes nausea (but usually not vomiting). B. cereus can also make a different toxin in (usually) rice and other starchy foods which causes nausea and vomiting in 30mins to 6 hours. Usually clears up in about a day. Both kinds of illness are generally self-limiting but can cause serious complications, although rarely in otherwise healthy people.
Refrigeration at 4°C or lower is vital – B. cereus can form spores which can germinate at higher temperatures and the more bacteria, the more toxin, and the greater the chance that you’ll get sick. When foods containing B. cereus spores are at the right (or wrong!) temperature the spores may germinate, the bacteria may grow and produce toxins that make people sick. Really sick. This is frequently linked with reheated rice but also other starchy foods of plant origin such as pasta, potatoes, pastries and noodles.
B. cereus can cause vomiting or diarrhoea and, in some cases, both. This depends on the kinds of toxin it produces.
When B. cereus grows and produces emetic toxin in food, it can cause vomiting, even if the food is cooked again and no live bacteria are eaten. This is because the toxin is not easily destroyed by heating. When food containing live B. cereus is eaten the bacteria may grow and produce another toxin – diarrhoeal toxin – in the gut. This results in – you’ve guessed it – diarrhoea.
Illness from B. cereus can be prevented by making certain that hot foods are kept hot and cold foods are stored cold. It is important to remember that re-heating food that has been ‘temperature abused’ will not make it safe.
Growth and Control
Range 4-55°C, emetic strains have a minimum of 10°C
Maximum toxin production at 20-25°C, toxin production range 10-40°C
Oxygen required for production of emetic toxin
Minimum Water Activity
With NaCl >0.93 and <0.95 aw
With glycerol 0.93 aw
Spores more resistant to dry than moist heat, and are also more resistant in oily foods. Cooking at or below 100°C may allow spore survival.
Emetic toxins remain active after 150 min at 100°C (pH values 8.7 to 10.6)
Generally vegetative cells decline rapidly in stomach acid, however some may survive depending on food and level of stomach acidity. Spores are resistant to gastric acidity (between pH 1 and pH 5.2)
Emetic toxin stable between pH 2 and pH 9
Spores survive long periods in dry foods e.g. population unchanged after 48 weeks in cereal (aw 0.27-0.28)
Vegetative cells destroyed by frying, grilling, roasting and pressure-cooking
Spores (depends on strain and food):
D100°C = 1.2-7.5 minutes in rice
D120°C (mean of 465 datapoints) = 2.5 seconds
D120°C (mean of 19 datapoints in oily foods, e.g. pumpkin pie, soybean oil) = 3.4 min
Emetic toxins inactivated 90 min at 100°C at pH 8.6
Diarrhoeal toxin inactivated 5 min at 56°C
Vegetative cells inactivated in yoghurt (pH 4.5) and fruit juice (pH 3.7, 5-6 log10 reduction within a few hours depending on temperature)
Diarrhoeal toxins unstable outside range pH 4-11 (Jenson and Moir, 2003)
Vegetative cells inhibited at aw < 0.91
Vegetative cell growth inhibited by sorbic acid, benzoate, sorbate, ethylenediaminetetraacetic acid (EDTA) and polyphosphates
Spore germination and outgrowth inhibited by nisin (NB: Nisin is not sporicidal)
Disinfectants / Sanitisers
Most food industry sanitisers destroy vegetative B. cereus cells on surfaces – but it’s the spores that are a bastard to kill.
Phenolics, QACs, alcohols, bisguanides, organic acids, esters and mercurials have little sporicidal effect. QACs are sporistatic but you really need an oxidising agent such as AntiBak– see here for suggestions.
Glutaraldehyde, formaldehyde, chlorine, iodine, acids, alkalis, hydrogen peroxide, peroxy acids, ethylene oxide, ß-propionolactone and ozone are all sporicidal at high concentrations with long contact times
Chlorine disinfectants are often recommended against spores; most bleaches contain about 5% sodium hypochlorite and are effective against vegetative B. cereus cells but not spores
B. cereus-associated foodborne illness occurs as two distinct intoxication syndromes: emetic and diarrhoeal. Recovery is rapid for both syndromes, usually within 12-24 hours. There are usually no long-term effects, but severe consequences, including fatalities, can occasionally occur.
Incubation: 0.5-6 hours.
Symptoms: Nausea, vomiting, malaise, occasionally followed by diarrhoea.
Dose: Large numbers in the range of 105 to 108/g viable cells are required before toxin (cereulide) becomes detectable in the food. Emetic toxin concentration in foods implicated in an outbreak in Japan ranged from 0.01 to 1.28 μg/g. An intoxication dose of 8 μg kg-1 body weight has been suggested (Paananen et al., 2002).
Incubation: 8-16 hours
Symptoms: Abdominal pain, watery diarrhoea, occasional nausea.
Dose: 105-107 (total cells). Foods with such high populations of B. cereus may not be acceptable to the consumer.
At Risk Groups: All people are susceptible to intoxication, but intensity of symptoms varies between individuals.
Treatment: Treatment is usually not given. Fluids are administered when diarrhoea and vomiting are severe.
B. cereus is a spore former. It is widely distributed in nature and contaminates virtually every agricultural commodity. It has been isolated from soil, dust, cereal crops, vegetation, animal hair, fresh water and sediments, although it is not generally isolated from fish (ICMSF, 1996).
Human: Can be transiently carried in the intestine of healthy humans (14-43%) (Jenson and Moir, 2003). However, no person-to-person transmission has been reported.
Food: Transmission is predominantly foodborne. Most raw foods will contain B. cereus spores, as do many dried herbs, spices and dehydrated foods. Emetic illness is frequently linked with raw starchy foods of plant origin (such as rice, pasta, potatoes, pastries and noodles). In 95% of emetic cases, fried or cooked rice is implicated (Jenson and Moir, 2003). Diarrhoeal illness is often associated with meat products, soups, vegetables, sauces and milk/milk products. Dairy products may spoil through the growth of spores that survive pasteurisation.
Plague and Pestilence
Most B. cereus food poisoning incidents are from cereal-based or protein-based foods, slowly cooled and stored between 10 and 50°C. This allows surviving spores to germinate and reach numbers high enough to cause illness. Rice is frequently a culprit. Global oubreaks include:
Pancakes; (5 cases) Commercial eatery. Temperature abuse and poor storage of pancake batter.
Savoury rice, potato, mashed pumpkin (suspected); (27 cases). Food vehicles not identified.
Japan, contaminated milk in school lunch: 1877 cases (emetic).
Norway, fish soup: 20 cases (diarrhoeal). Inadequate cooling.
Denmark, meat with rice: >200 cases (diarrhoeal).
Spain, cooked noodles: 13 cases (diarrhoeal). Inadequate cooling of cooked noodles.
Norway, vanilla sauce: >200 cases (diarrhoeal). Prolonged storage at ambient temperature.