Bacillus cereus Spores and Foodborne Illness

Bacillus cereus is a bacterium found throughout the environment, notable for forming highly resistant structures called spores. This allows the bacterium to survive in a dormant state, withstanding conditions that kill other microbes. Its prevalence in soil and on vegetation makes it a frequent presence in many raw food ingredients. The primary concern arises from its capacity to cause foodborne illness when foods are handled or stored improperly, allowing the bacteria to grow and produce toxins.

The Nature of Bacillus cereus Spores

Bacterial spores are dormant structures that some bacteria produce to survive unfavorable environmental conditions. For Bacillus cereus, these spores allow the organism to endure stress like nutrient deprivation. The process of forming a spore, known as sporulation, involves encapsulating its genetic material within a durable, multi-layered coat. This protective shell makes the spore resistant to many harsh treatments.

The resilience of Bacillus cereus spores is a factor in food safety. They can withstand desiccation, radiation, chemical disinfectants, and heat. While normal cooking temperatures kill the active form of the bacteria, the spores can survive. This means that even after cooking, viable spores may remain in the food.

When conditions become favorable, such as when cooked food cools slowly at room temperature, the spores can germinate. During germination, the spore transitions back into an active, growing vegetative cell. Once active, these cells can multiply and produce the toxins responsible for causing illness.

Common Sources and Contamination Pathways

The spores of Bacillus cereus are widespread in soil, on decaying organic matter, and carried by dust, which allows them to contaminate raw agricultural products. Grains and starchy foods are common vehicles for these spores. Rice, pasta, potatoes, and various spices are often associated with B. cereus.

Contamination can occur from the farm to the processing facility. Since the spores are in soil, they can be present on harvested vegetables and crops. If processing equipment is not properly cleaned, these spores can persist and cross-contaminate other foods.

Spores can remain dormant through processing steps like drying or pasteurization. The primary risk arises when these foods are cooked and then improperly stored. For instance, a large pot of rice or a sauce left to cool at room temperature for several hours creates an ideal environment for surviving spores to germinate and multiply.

Illnesses Caused by Bacillus cereus

Bacillus cereus causes two distinct types of food poisoning, each from a different toxin with different symptoms. The first is the emetic, or vomiting, syndrome. This illness is caused by a heat-stable toxin called cereulide, which is produced in the food before consumption.

The symptoms, primarily nausea and vomiting, appear rapidly, within 30 minutes to six hours. This syndrome is most often linked to starchy dishes like fried rice or pasta that have been cooked and then held at room temperature.

The second form is the diarrheal syndrome. This is caused by enterotoxins produced within the small intestine after a person ingests the bacteria or its spores. Because the toxin is produced inside the body, symptom onset is slower, occurring between 6 and 15 hours after eating the food. The primary symptoms are watery diarrhea and abdominal cramps, and this illness is associated with meats, stews, sauces, and vegetables.

Both syndromes are self-limiting, with symptoms resolving within 24 hours. The illness can be more severe in individuals with compromised immune systems or other health conditions. While rarely fatal, the prevalence of the bacteria makes it a common source of foodborne gastroenteritis worldwide.

Preventing Bacillus cereus Food Poisoning

Preventing Bacillus cereus food poisoning requires proper food handling, particularly temperature control after cooking. The goal is to prevent surviving spores from germinating and multiplying. This is achieved through the rapid and proper cooling of cooked foods.

Cooked foods should be cooled to below 41°F (5°C) as quickly as possible, ideally within two hours. When storing food, it should be kept refrigerated at or below 40°F (4°C) or held hot above 140°F (60°C). These temperatures inhibit the growth of the bacteria.

Reheat leftovers to an internal temperature of at least 165°F (74°C) to kill any vegetative cells that grew during storage. However, this will not destroy the heat-stable emetic toxin, cereulide. If this toxin has already formed in improperly stored food, reheating will not make it safe. The best practice is to avoid leaving cooked starchy foods at room temperature for more than two hours.