Clostridium botulinum
From a food safety view Clostridium botulinum is one of the most important of the pathogenic foodborne bacteria because of the severity of the symptoms caused by powerful neurotoxins. In the past it has been associated with inadequately canned protein foods. Today it is relatively rare and most likely associated with home preservation of foods. Although, there is increasing concern over convenience foods and foods containing fewer preservatives, minimal processing and a greater reliance on chilling for preservation. Some of these newer foods may fall within the growth range of C. botulinum.
The organism is globally distributed and is primarily a soil organism however, certain strains can be found in sediments associated with aquatic environments.
It is an anaerobe which means it requires reduced levels of oxygen for growth and it is a spore former.
Foodborne botulism is an intoxication almost always associated with ingestion of preformed botulinum neurotoxin.
Infant botulism results after ingestion of spores of C. botulinum which subsequently germinate, multiply and produce neurotoxin in the infant's intestinal tract. Honey has been linked to infant botulism but surveys of commercial foods including honey have not identified any with a high incidence of spores.
Storage of low acid cooked foods at ambient temperatures for extended periods can create conditions suitable for the growth of this organism. Outbreaks have occurred from baked potato stored at ambient temperature for several days and spiced onions sautéed in margarine that were kept warm and used throughout the day. Garlic butter was also implicated in an outbreak in North America in 1985. The butter was made with an aqueous mixture of garlic in soybean oil. The pH of the garlic in oil was above 4.6 and the product had been stored at ambient temperature.
The growth of other organisms like moulds has been shown to reduce the acid level in some tomato products to a level where outgrowth of surviving spores of C. botulinum occurred. Home preservation of tomatoes and tomato products is common. The product should not be consumed if mould growth is evident.
Foods particularly non acidified foods should not be stored for extended periods at ambient or warm temperatures.
It is important to follow the recommendations of the food manufacturer with respect to storage temperature and shelf life.
Description
Clostridium botulinum are anaerobic, Gram-positive spore-forming rods, with the spores being very heat resistant. The trend towards vacuum packaging of chilled foods provides for a potential risk if temperature abuse is allowed to occur. They can be isolated from the soil and marine environment. Some strain (non-proteolytic) can grow slowly at temperatures down to 3.3°C. They usually will not produce toxins at pH values less than 4.6 and water activity values of less than 0.94. The toxin is one of the most potent toxins known and 10-6g is sufficient to kill an adult human. The toxin is easily destroyed by boiling for 10 minutes. Nitrate and nitrite are preservative which are effective in inhibiting the growth of C. botulinum.
Nature of acute disease
Foodborne disease caused by C. botulinum is referred to as botulism. It is caused by the ingestion of a neurotoxin produced by the microorganism in the food. The toxin can be destroyed by normal cooking procedures. Infant botulism can also occur which is thought to occur from the ingestion of C. botulinum spores with honey being the major food implicated. In the latter illness spores in the gastrointestinal tract will germinate and multiply, producing the toxin. There is now an antidote to the toxin which has reduced the mortality rate somewhat.
Characteristics of Illness
Include weakness, fatigue and dizziness, followed by blurred vision and progressive difficulty in speaking and swallowing. Weakening of the respiratory muscles is also observed and death may occur due to respiratory failure.
Symptoms usually occur 18 to 36 hours after ingestion of the food containing the toxin, although cases have varied from a few hours to several days.
If not treated botulism can cause death. Once treated recovery can take weeks to years depending upon the severity of the poisoning.
A very small amount (a few nanograms) of toxin can cause illness.
Diagnosis of human illness
Botulism is difficult to diagnose by clinical symptoms alone as it is often confused with other illnesses such as Guillain-Barré syndrome. The most direct and effective way is to demonstrate the presence of toxin in the serum or faeces of the patient or in the food that the patient consumed. Currently, the most and widely used method for detecting toxin is the mouse bioassay. This usually takes 3 days after isolation of the toxin.
Complications
Fatality rates for botulism are around 5-15%. Treatment requires the administering of antisera and the use of respiratory support systems.
Susceptible individuals
All people are believed to be susceptible to botulism.
Associated foods
It is generally thought that any low acid foods (pH above 4.6) can support the growth of C. botulinum and its subsequent production of toxin. Despite this an outbreak of botulism occurred in canned tomatoes, which had a low pH. It was thought that the production of toxin occurred due to the growth of mould increasing the pH of the product. Some foods that have been associated with cases of botulism include home preserves: meat, fish, rockmelon (cantaloupe), vegetables; soft cheeses, potato salad and garlic stored in oil. Infant botulism has mainly been associated with the consumption of honey.
Food analysis
Cultural methods can be used to detect the presence of the microorganism in foods. These tests can take up to 10 days. Mouse bioassay can then be used to test for the presence of the toxin. See Hocking et al (1997) for more details.
Outbreaks
The incidence of the disease is low, but the mortality rate is high if not treated immediately. There have been no reported cases of botulism in Australia since 1991 when a couple became ill from consuming home-preserved unacidified asparagus. In Australia between 1942 and 1984 there were 5 outbreaks of botulism, affecting 53 people with 9 fatalities. Foods implicated include canned vegetables (mushrooms and asparagus) and canned tuna.
Overseas outbreaks include:
- Japan - 36 people became ill and 11 people died after eating fried stuffed lotus roots. The lotus roots were vacuum packaged and sold unrefrigerated. These conditions allowed for the growth and toxin production of C. botulinum.
- Canada - At least 37 people were affected after eating at a restaurant that used an unacidified temperature abuse garlic in oil which had been stored at room temperature.
- USA - Cases of botulism have occurred due to the consumption of potato salad. Three outbreaks have occurred, resulting in 50 people ill and one fatality. It is thought that the C. botulinum was present in the baked potatoes used for the salad. These had been stored at ambient temperatures for several days before use.
- USA - sauteed onions were implicated in an outbreak in a restaurant in America. 28 people were affected, with one fatality.
- UK - contaminated hazelnut yogurt was responsible for an outbreak resulting in 27 people ill and one fatality. The source of the C. botulinum was the hazelnut puree used in the yogurt.
Prevention
When processing shelf-stable products ensure that the heating processing is designed to destroy spores of C. botulinum. Any process used should be designed by a person with experience in the design of safe heat processes. Alternatively, the pH of the product can be adjusted to below 4.5 and a pasteurisation process is then sufficient. When making home preserves bottle only acidic fruit such as apples, pears, stone fruit, pineapple, berries etc should be used. Melons, tomatoes and many tropical fruits must have some acid added to them before they are bottled. Vegetables must be bottled in at least half vinegar and half water, and when making flavoured oils, use only dried herbs and vegetables or soak fresh herbs and vegetables in vinegar before adding to the oil. In manufactured meats, ensure correct levels of nitrites are used to inhibit the growth of C. botulinum. Do not stored perishable foods within the Temperature Danger Zone (5°C to 60°C). Always evenly reheat food to above 75°C. When reheating a in microwave, periodically stir the food to ensure even heating.
Further Information
US Food and Drug Administration Bad Bug Book
National Center for Food Preservation (USA)
Literature
- Hocking, A.D. et al. (1997). Foodborne Microorganisms of Public Health Significance. 5th ed. North Sydney. AIFST NSW Branch Food Microbiology Group.
- Doyle, M.P. (1989). Foodborne Bacterial Pathogens. Marcel Dekker; New York.