Clostridium botulinum

It is the causative agent of botulism.

Morphology

It is a non capsulated, Gram positive bacilli, motile by peritrichous flagella and bears oval, bulging, subterminal spores.

Classification

Cl. botulinum has been classified on the antigenicity of Botulinum toxin into seven types A to G.

Human Pathology

Pathogenicity is due to Botulinum toxin. This neurotoxin is a protease that cleaves the proteins involved in the release of acetylcholine at the neural synapses and neuromuscular junction. Hence, acetylcholine cannot be released causing a flaccid paralysis.

Three types of botulism are seen: Food-borne, Wound and Infant Botulism. Food-borne is due to preformed toxin while other two forms are due to spore contamination in food (honey) or wound which then produce toxin.

Food-borne Botulism: Food can get contaminated with Cl. botulinum spores which germinate under anaerobic conditions to produce the potent neurotoxin. It is commonly associated with improperly canned meat and meat products and alkaline foods such as beans, mushrooms, peppers etc.

The first symptoms of botulism are nausea, vomiting, weakness, and vertigo (dizziness). These are followed by neurological symptoms: visual impairments (blurred or double vision), loss of normal throat and mouth functions (difficulty speaking and swallowing; dry mouth, throat, and tongue; and sore throat), general fatigue, lack of muscle coordination, and difficulty in breathing. Gastrointestinal symptoms may include abdominal pain, diarrhea, or constipation. Death is usually caused by respiratory failure and airway obstructions. Symptoms appear in 12-36 hours typically. Presents as a descending paralysis.

Infant Botulism: Associated with ingestion of honey contaminated with Cl. botulinum spores in infants typically less than 2 months of age. Clinical features are similar to food borne botulism.

Wound Botulism: Wounds can get contaminated with spores, which then germinate locally to produce the botulinum toxin which gets absorbed into the body. Symptoms are similar to food borne except for gastrointestinal symptoms. Seen in drug abusers.

Prevention of botulism can be done by proper canning of food, discard swollen, gassy or spoiled canned foods, adding nitrates and nitrites to meat and meat products, refrigerate all food within 2 hours of cooking and boiling foods for at least 10 minutes and avoiding feeding honey to infants.

Laboratory diagnosis of Botulism

Contaminated food and fecal matter can be used as specimens for diagnosis. Gram stain may show typical Gram positive bacilli.

Culture is not always done. It is done under anaerobic conditions on either blood agar, nutrient agar, Robertson’s cooked medium (RCM) where it causes blackening of meat particles and gas production and lactose egg yolk milk agar medium where it produces opalescence and pearly layer.

Diagnosis is mainly done clinically and by the demonstration of toxin in clinical samples by inoculating intraperitoneally into mice or guinea pigs. The test animal dies if toxin is present in the sample while control animals given antitoxin will survive.