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The below mentioned article provides a study note on Clostridium Botulinum which causes botulism due to the ingestion of its “preformed” toxin. This form of food poisoning is characterised by oculomotor paralysis, pharyngeal paralysis, aphonia. Eight antigenic types of toxin (A-H) are known. Of these, types A, B and E are mostly responsible for the illness.
Morphology and Staining of Clostridium Botulinum:
Spore forming Gram-positive bacilli, with rounded ends, 4-6 x 0.9-1.2µl, occur singly or in pairs. Spores are oval, sub-terminal and bulging. This bacillus is motile.
Cultural Characteristics of Clostridium Botulinum:
They are strict anaerobes and grow on ordinary media anaerobically at 37°C. RCM yields abundant growth. Surface colonies are large, greyish, irregular, semitransparent with a central nucleus, haemolytic on horse blood agar. Meat is digested and usually blackened by toxigenic types A, B and F. All types produce lecithinase.
Resistance of Clostridium Botulinum:
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Though the spores of CI. botulinum are destroyed by autoclaving at 121°C for 15 minutes, they are resistant to 100°C for 4 to 5 hours. Spores of type A of CI. botulinum are more resistant to heat and may survive up to 30 minutes at 120°C, whereas types B, C, D and E are resistant to heat. The resistance of the spores of CI. botulinum to radiation is of great significance in food processing factory.
Antigenic Structure of Clostridium Botulinum:
On the basis of their antigenically distinct toxins, eight serotypes – A, B, C, D, E, F, G and H—of CI. botulinum have been distinguished.
Toxins liberated by different strains have identical pharmacological action, but they are neutralized by their specific antitoxin.
Haemagglutinin, haemolysin and haemolytic lecithinase are also produce in little quantity by some strains.
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Toxin:
CI. botulinum produces a powerful exotoxin which can be destroyed at a temperature of 80°C for 30-40 minutes. The toxin of type A has been purified as a pure crystalline protein which is lethal to a mouse in a dose of 0.00000033 mg.
One mg of toxin can kill more than 200 million mice. The lethal dose of toxin for human being is not yet well known and it may be less than one microgram since it is the most toxic substance so far known. Only 1 kg of botulinum toxin can kill one billion people. The toxigenicity of CI. botulinum is most probably under control of temperate phage.
Botulism may be of three types:
(a) Food borne botulism;
(b) Infant botulism;
(c) Wound botulism.
(a) Food Borne Botulism:
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It is a rare disease of intoxication caused by ingestion of preformed toxin in food contaminated with CI. botulinum. Sometimes bacteria may also produce toxin in the intestine. Human disease is usually caused by type A, B and E, rarely by types C, G and F. The toxin is absorbed from the intestine into blood.
Symptoms appear 12 to 36 hours after consumption of contaminated food. Symptoms are vomiting, thirst, constipation, difficulty in speaking and breathing. Diarrhoea is not symptom. Mostly prognosis is poor with 25-70% fatality and death ensues due to respiratory failure.
Source of Botulism:
Home preserved vegetables, meat, fish (raw or tinned fish) may cause food borne botulism.
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Mode of Action of Toxin:
After absorption of toxin into blood (toxaemia), the toxin acts by blocking the release of acetylcholine at synapses and neuro-muscular junction which leads to neurological signs and symptoms (oculomotor and pharyngeal paralysis, vomiting and constipation).
(b) Infant Botulism:
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Though this condition is rare, usually 6 months old infants are affected with the start of mixed feeding. This condition is not due to ingestion of preformed toxin but due to consumption of food contaminated with CI. botulinum. Honey has been reported as source of infection.
The spores after germination produce botulinum toxin, which causes toxaemia. The disease is manifested by constipation, weakness, lethargy and cranial palsies. It may be very mild or fatal depending on the absorption of toxin into blood. Infected infants are managed by supportive care and assisted feeding.
(c) Wound Botulism:
It is extremely rare. During wound infection toxin is produced at the site of wound infection and absorbed into blood. This toxaemia may be responsible for neurological manifestations similar to those of blood-borne botulism.
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Pathogenesis and Clinical Features:
Botulism is not an infection, but is an intoxication resulting from the ingestion of food in which CI. botulinum has grown and produced toxin (preformed toxin). The toxin acts by blocking release of acetylcholine at neuromuscular junctions. Flaccid paralysis results.
Symptoms begin 18-24 hours after ingestion of toxic food with visual disturbances (incoordination of eye muscles, double vision), inability to swallow and speech difficulty); signs of bulbar paralysis; death is due to respiratory paralysis and cardiac arrest. Gastrointestinal symptoms are not always prominent, there is no fever, the patient remains fully conscious until shortly before death; the mortality rate is high.
Laboratory Diagnosis of Clostridium Botulinum:
Toxin can be demonstrated in the patient’s serum and in contaminated food, can be neutralized by specific antitoxin in mice and also can be detected by passive haemagglutination and radioimmunoassay.
Presence of toxin in suspected residual food, patient’s serum and intestinal content can be demonstrated by injecting the filtrate—obtained by macerating specimen of food, stool, vomitus in sterile isotonic saline—into mice. The test animal (unprotected) develops dyspnoea, flaccid paralysis and dies within 24 hours.
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Demonstration of Organisms:
Gram-positive spore forming bacilli can be demonstrated in smears made from suspected food, and intestinal content. Specimens should be heated first at 65°C to 80°C for 10 minutes (to destroy non-sporing bacilli) and then inoculated on blood agar (incubated anerobically) and Rebertson’s Cooked Meat (RCM) medium. Toxigenicity test in mice should be performed to detect toxin culture filtrate.
In absence of toxin, mere demonstration of bacilli in food and stool is of no significance.
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Treatment and Prophylaxis:
Potent antitoxin to 3 types should be given intramuscularly. Tin containing contaminated food may “swell” due to gas production by CI. botulinum. Food from such can should not be consumed.
Since botulism is due to consumption of contaminated canned or preserved food, home processing of meat and vegetables (peas, beans, carrots) for preservation is not advisable. Since a low pH inhibits the growth of CI. botulinum, acid fruits may be preserved in bottle at home.
Once the toxin has entered into the nervous tissue and gets fixed in the synapses, the symptoms develop. This toxin can no longer be inactivated by antitoxin. Only supportive care may bring down the mortality rate from 85% to 20%. When an outbreak occurs, antitoxin can be given intramuscularly to those who consumed the suspected food.
