Clostridium botulinum - Botulism (Clinical forms): Overview.


Characteristics of Clostridium botulinum and botulinum toxin/s 


Clostridium botulinum is an strict anaerobic grampositive bacillus, genus Clostridium, family Clostridiaceae, spore-forming, that produces a neurotoxic toxin. This bacterium is usually found in soil and untreated fresh water and sediments (oceans, lakes), with a worldwide distribution. In some circumstances this organism can contaminate food and grow in them to produce their toxin/s. Botulism, a serious form of food poisoning, results from ingestion of food containing the toxin. Although this disease is rare, their mortality rate is high. When the toxin type was determined among 1036 cases detected in US between 1899 and 1990, 384 were for A toxin, 106 due to B toxin, 105 to E toxin and 3 to F toxin. Sometimes may be cases due to two toxins, for example A and B.

All forms of human or animal botulism, are caused by absorption of botulinum toxin formed during the multiplication of the bacterium Clostridium botulinum. The toxin has a toxicity (neurotoxicity) very high, so that exerts its action at extremely low levels, is thermolabile, while spores of the bacteria are heat resistant and survive in foods heated to over 100ºC, such as canned under thermal treatment. Besides, some strains of C. botulinum, C. butyricum, C. baratii and C. argentinense can produce botulinum neurotoxins.

There are seven toxin types (A - G) differentiable by neutralization tests, useful for clinical and epidemiology. Types A, B, E and F are the main causes of human botulism, while types C and D are found in cases of animal botulism, the most affected being wild birds and poultry, cattle, horses and some species of fishes. Types A and B are the most common in men, and are mainly related to contamination of home-prepared canned vegetables, but in Europe these types have also been found in relation to meat products. The E-type (fish) found in aquatic environments and correlates with E botulism cases concerning contaminated fish or shellfish, and is increasing. The F type is exceptional. Type C is subdivided into C1 (neurotoxin) and C2 (not neurotoxin affects vascular permeability and enterotoxigenic). The G type is produced by C. argentiniense (isolated from ground in Argentina, serum of deceased patients, although it is unclear involvement).

Toxins are synthesized during the growth of the bacterium as an inactive protein (150 kDa), which is released from the bacteria during lysis. To activate the toxin formed should degrade into two polypeptide chains (50 and 100 kDa).            

C. botulinum, can differentiate into groups according to their culture characteristics, biochemical and physiological. All cultures type A and some of the type B and F are proteolytic. Cultures of C. botulinum toxins produced by C and D are not proteolytic, when cultured in a medium with coagulated egg white or flesh. All types E and some of the type B and F are non-proteolytic, but have characteristics of carbohydrate metabolism that differ from non-proteolitics groups types C and D. The strains of type G not have It has been studied in sufficient detail for effective and satisfactory characterization.

The optimum temperature for growth and toxin production is about 35ºC for the proteolytic strains; for non-proteolytic strains is 26-28ºC. Non-proteolytic types B, E and F strains can produce toxin at refrigeration temperatures (3-4ºC). Toxins of non-proteolytic strains do not show maximum toxicity until activated toxin with trypsin. Toxins from proteolytic strains are generally produced in its activated form.

Clinical forms of botulism

There are four clinical forms of human botulism:

  • Food poisoning (foodborne botulism), due to the ingestion of food contaminated with Clostridium botulinum in which the toxin is formed.
  • Wound botulism, produced by infection of wound with Clostridium botulinum and developing toxin in vivo after growing the bacteria in the wound.
  • Infant botulism, produced by production of toxin in the digestive tract of young children colonized by Clostridium botulinum (the most common cause is due to the ingestion of contaminated honey or corn syrup), followed by absorption of the toxin. These patients may have antibodies to the toxin.
  • Adult intestinal botulism, caused by digestive colonization by Clostridium botulinum in the elderly, followed by absorption of the toxin. These patients may have antibodies to the toxin.

In any of the clinical forms of human botulism, and equal in the animal botulism, toxin penetrates the blood from the gastrointestinal tract when ingested preformed with a food, or when produced by the bacterium that colonizes the digestive tract (young children or adults), or in exceptional cases children from an infected wound with the bacteria. There are some foods that are more likely than others to contain botulinum toxin. Foods with a pH less than 4.5 are more difficult to be cause of botulism since at this pH C. botulinum is unable to multiply and produce toxin (this is the case of fruit juices, vinegar marinated food, etc.). Conversely, foods with pH equal or higher than 4.5 can cause botulism, since in them the multiplication and toxin production is possible (this is the case of meat, fish, vegetables, prepared dishes, etc.), on all those foods oxygen-free exposure, as with canned or vacuum-packed food, and having a pH greater than 4.6. Examples are dangerous food: cured ham, smoked, canned fish or vegetable (subjected to heat treatment insufficient to kill spores), etc. The cans contaminated with C. botulinum are usually curved, though this does not occur with type E.

The toxin absorbed irreversibly binds to the neuromuscular junctions of motor neurons, preventing the release of acetylcholine and causing flaccid paralysis or muscle weakness.

Clinically is characterized by acute flaccid paralysis, which usually begins with bilateral involvement of the cranial nerves, affecting the muscles of the face, head and pharynx, and then descends symmetrically to affect the chest muscles and limbs. Death, when it occurs, it is due to respiratory failure by paralysis of the tongue and pharyngeal muscles occluding the upper airway, or by paralysis of the diaphragm and the intercostal muscles. For this reason, patients should receive botulinum antitoxin and respiratory intensive care needed.