Clostridium botulinum

Agricultural Disaster Preparedness and Recovery, Food Safety May 29, 2013 Print Friendly and PDF

Clostridium botulinum

Keith R. Schneider, Mickey E. Parish, and Renée M. Goodrich

A photomicrograph of Clostridium botulinum type A viewed using a Gram stain technique. Image courtesy of CDC/Dr. George Lombard.

Botulism is caused by the nerve toxins produced by the bacterium Clostridium botulinum, and can cause a serious paralytic condition that can lead to death. Clostridium botulinum is ubiquitous in nature, but most prevalent in soil and water. Although the bacteria and their spores are harmless, their toxins cause illness and death.

There are three types of botulism, each named accordingly to the mode of acquisition of the toxin. Food-borne botulism results from the ingestion of the botulinum toxin. Wound botulism is a result of organisms growing and producing their toxin in a wound. Infant botulism is the most common, caused by C. botulinum producing toxins in the intestine. The Centers for Disease Control and Prevention (CDC) suggests a fourth type of botulism, of which no origin can be determined and results from the colonization of the intestines of adults and children. These cases of botulism have been caused by botulinum neurotoxins produced by non C. botulinum species.

Clostridium botulinum Foodborne Illness

Based on differences in antigenicity among toxins, there are seven strains of C. botulinum. Each strain is characterized by its ability to produce a protein neurotoxin, enterotoxin or haemotoxins. Types A, B, E, and F cause botulism in humans, while types C-alpha, C-beta and D cause botulism in animals and birds. Type G was identified in 1970 but has not been determined as a cause of botulism in humans or animals.

The organisms are characterized as slightly curved, Gram-positive, motile, anaerobic rods that produce heat-resistant spores. The spores become activated in a low acid (pH higher than 4.6), anaerobic environment with temperatures between 40°F to 120°F, high moisture content and lacking competing bacterial flora. According to the Centers for Disease Control and Prevention, there were 169 reported cases of botulism in 2001: thirty-three food-borne, 112 infant, and twenty-three wound botulism.

Due to increasing knowledge and education in proper storage and handling of foods, the incidence of C. botulinum in the United States is rare. However, the bacterium and its spores are ubiquitous and often unavoidable in nature.

Food-borne botulism is caused by the ingestion of toxins produced by the bacterium in foods that have not been properly handled or canned. The toxin can be inactivated by heat, but can be found in canned vegetables, meat, and seafood products that were not sufficiently heat-treated before canning.

Infant botulism is the most common form of botulism. It is caused by the ingestion of C. botulinum spores which produce a neurotoxin in the intestine. According to a report by the CDC that reviews the epidemiology of botulism in the United States, infants hospitalized with botulism have a pattern of higher birth weights, "and their mothers tend to be white, older, and better educated."

Wound botulism is the rarest form and occurs when C. botulinum infects a wound and produces toxins that are carried through the rest of the body via the bloodstream.

The fourth class of botulism consists of cases in which a cause cannot be determined. One theorized source is surgical procedures on the gastrointestinal tract that may alter the normal bacterial population of the intestines, resulting in a favorable environment for botulism bacterium to grow. Cases of botulism associated with clostridia other than C. botulinum, include adult and infant botulism associated with C. baratii, caused by Type F neurotoxin.

Those at Risk

Although food-borne botulism is rare and one of the least common of the foodborne diseases, C. botulinum can infect all persons because infection may result from ingestion of contaminated foodstuffs. It takes only a small amount of toxin to cause illness. However, the immunocompromised, the very young and old may suffer from more serious side affects. Foodborne botulism is not spread from person to person; the toxin must be ingested. Home-canned products, especially low-acid food products, are responsible for most cases of foodborne botulism. Improperly-handled commercial food products have also contributed to outbreaks.

Foods Commonly Associated with Clostridium botulinum

Clostridium botulinum is present in water and soil, so potentially any food which comes into contact with them presents a potential hazard. However, improperly processed home-canned low-acid vegetables and contaminated meats are the usual cause of food-borne botulism. Foods associated with botulism include canned asparagus, green beans, garlic in oil, corn, soups, ripe olives, tuna fish, sausage, luncheon meats, fermented meats, salad dressings, and smoked fish.

Spores have also been found on the surfaces of vegetables and fruits. During the canning process, foods undergo a hot fill process and oxygen is removed, leaving the food in an anaerobic environment. Certain foods such as meat are able to bind oxygen to create an anaerobic environment. Home canning processes for low-acid foods can present an extremely high risk if processing times and temperatures are inadequate and cans or jars are not be handled properly. The National Center for Home Food Preservation has home food preservation recipes that have been tested to ensure that they are adequate to prevent the growth of C. botulinum.

Even cooking with oils and grease can create an anaerobic environment. In addition, fresh produce can be a source of food-borne botulism. Outbreaks stemming from the growth of botulinum toxin from unrefrigerated cabbage have been documented.

Strains of C. botulinum can grow between 38°F and 115°F. This means that strains can grow at normal refrigeration and higher temperatures, making it difficult to prevent bacterial growth and toxin production. If the proper temperature to kill the spores is not reached, the resulting environment can be perfect for growth and the production of neurotoxins.

Infant botulism has been linked to the ingestion of C. botulinum spores in honey, corn syrup, and other foods. The C. botulinum spores germinate, colonize, and produce toxins in the intestinal tract of infants (intestinal toxemia botulism). Infants less than one year old lack the proteins in their intestines needed to destroy either the vegetative or spore form of C. botulinum, so these products are not recommended for infants.

Additional Resources

Centers for Disease Control and Prevention

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This work is supported by the USDA National Institute of Food and Agriculture, New Technologies for Ag Extension project.