Clostridium Botulinum and Botulism

Clostridium botulinum is a Gram positive, obligate anaerobic (only grows in absence of oxygen), spore-forming bacterium. The organism forms spores that are resistant to many common food process controls. Botulinal neurotoxins produced by vegetative cells (growing form) of this anaerobic bacterium are among the most potent biological neurotoxins known.

Clostridium botulinum is commonly found in soils and marine sediments throughout the world. Since it is found in the soil, it may contaminate vegetables cultivated in or on the soil and will colonise the gastro-intestinal tract of fish, birds and mammals.

Not all C. botulinum cause illness in humans. Strains produce one of seven known types of neurotoxins (A to G). Only those producing types A, B, E and F (rarely) cause botulism in humans (WHO, 2002). Strains are also separated into two groups based on physiological differences: Group I (can produce A, B or F toxin) are proteolytic and cause food spoilage; Group II (can produce B, E or F toxin) are non-proteolytic and may be present in foods without obvious spoilage.

The Clostridium botulinum bacterium produces spores that are heat resistant and cannot be killed simply by boiling. Canned food sterilisation processes are specifically designed so as to eliminate the possibility of any of its spores surviving. The toxin is, however, fairly heat sensitive and heating at 80°C for 30 minutes or 100°C for 10 minutes will destroy the active toxin. Toxins are slightly more heat stable at lower pH values.

If canned foods like meat, fish or vegetables are under-processed or become contaminated with Clostridium botulinum after processing, then fatal food poisoning will result. The sealed can provides an ideal, anaerobic (oxygen free) environment for the bacterium to grow. The factors which limit the growth of Clostridium botulinum include:

  1. Low pH: Clostridia will not produce toxin in acid or acidified foods (i.e. below pH 4,5).
  2. Low water activity: A minimum of 0.94 is needed to support bacterial growth and toxin production. This water activity corresponds to a 10% salt (NaCl) solution, which is why salting is sometimes used as a method of preservation. Drying is another method or reducing the water activity of the food.
  3. Temperature: As general rule refrigeration reduces the ability of Clostridium botulinum to grow, as most strains of grow optimally at 35 – 40°C. It is however important to note that some strains can grow at temperatures as low as 3°C.
  4. Food Preservatives: Many preservatives (nitrite, sorbic acid, phenolic antioxidants, polyphosphates, etc.), at specific concentrations, inhibit Clostridium botulinum growth.
  5. Other micro-organisms compete for nutrients and space with Clostridium botulinum and reduce its ability to thrive.

What is botulism? Botulism is a paralytic illness caused by a neuro-(nerve) toxin that is produced by the bacterium Clostridium botulinum. The botulinum toxin is one of the most potent toxins known. Food contaminated with only a few nano grams (lO -9 g) can be toxic. There are five different types of botulism:

  1. Food-home Botulism: An intoxication most commonly found in home preserved foods, usually due to under-processing.
  2. Wound Botulism: Caused when the bacterium manages to grow inside a deep wound (usually a puncture wound, gunshot wound or associated with drug abuse).
  3. Infant Botulism: Caused when the intestine of the infant (less than one year of age) is colonised with the bacterium, which then produces toxin. Honey is known to be a source of Clostridium botulinum spores and is therefore not recommended for babies younger than one year old.
  4. Adult Infectious Botulism: This is similar to Infant Botulism, but is usually found in patients with intestinal diseases or after bowel surgery.
  5. Inadvertent Botulism: following treatment with botulinum toxin injection (Botox), used to treat various muscular problems and cosmetic treatments.

Botulism is characterised by symmetric, descending paralysis. Typical symptoms include double or blurred vision, drooping eyelids, headaches, dizziness, difficulty swallowing, dry mouth and muscle weakness. Symptoms generally occur within 12-36 hours after the food is eaten, but can occur as early as 6 hours or as late as 10 days (depending on the type of toxin). The toxin works by binding to receptors in the neuromuscular junctions of nerves and blocking the release of acetylcholine from these nerves that stimulates muscle fibres. In severe cases, extensive respiratory muscle paralysis leads to respiratory failure and death. In the past, about 50% of patients affected, died. Nowadays there is an antitoxin available and the recovery rate is much greater, although many patients still require a ventilator and take several months to recover. The antitoxin must be given as soon as possible after intoxication is suspected since once the toxin attaches to the nerve receptor site, the antitoxin is ineffectual.


Are canned foods safe?

Commercially canned foods are one of the safest and nutritionally most sound foods available to the consumer. All canning processes are strictly controlled to ensure that pathogenic organisms do not survive the process.

When are canned foods not safe?

If a can is damaged or dented or if it appears distorted or swollen, this could be an indicator that bacteria are at work inside the can. The contents should not be consumed under any circumstances. Clostridium botulinum does not necessarily produce gas when it grows and not all strains are proteolytic / putrefactive, so the presence of the toxin will not necessarily be obvious. Any suspect product should therefore not be consumed.

Is botulism only found in canned foods?

Food-borne botulism has often been found most commonly in home-canned and bottled vegetables, but there have been a few cases from commercially canned vegetables, meat and fish. It is not limited to canned products and can occur in any low acid food (pH > 4,5). Outbreaks have also been reported in products like vacuum packed fish, garlic and herbs in oil, baked potatoes in foil.

If botulism is suspected what should we do?

Botulism is classified as a medical emergency and medical help should be sought immediately. All relevant information regarding the suspect food must be given to the doctors. It is best to refrigerate or freeze the samples, although freezing the samples will reduce the viability of any live bacteria, it will preserve the toxin for identification.


  1. Botulism, U.S.A. Centres for Disease Control, gov
  2. Clostridium botulinum. U.S. Food and Drug Administration Centre for Food Safety and Applied Nutrition. Bad Bug Book.
  3. Clostridium botulinum. World Health Organisation. International Programme on Chemical Safety and Poisons Information. Monograph 858. Bacteria.
  4. ICMSF, International Commission on Microbiological Specifications for Foods (1996) Clostridium botulinum. In: Roberts TA, Baird-Parker AC, Tompkin RB (Eds.) Micro-organisms in Foods 5 Microbiological Specifications of Food Pathogens, pp 66-111. Blackie Academic and Professional, London.
  5. Ministry for Primary Industries New Zealand Hazard Data Sheets

Updated for F.A.C.S. by ABe (2016).