Clostridium botulinum

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Clostridium botulinum, the causative agent of botulism, is a gram-positive, spore-forming, anaerobic rod. The spores of C. botulinum are found worldwide in soil, pond, and lake sediments. Additionally, this bacterium releases an extraordinarily potent neurotoxin; the lethal dose for humans is less than 1 mg, making it the most potent toxin known to man.

The C. botulinum neurotoxin is heat-labile, rapidly destroyed at 100oC. The spores, however, resist boiling for long periods and must be destroyed by moist heat at 121oC. The spores can germinate, grow, and release the potent exotoxin in foodstuffs under anaerobic conditions; home-canned vegetables and home-preserved fish are the most common source of botulism.

Symptoms of botulism occur within 18 to 96 hours following ingestion of contaminated food. The toxin, resistant to enzymes in the gastrointestinal tract, is absorbed through the small intestine. Once absorbed, the toxin acts on the neuromuscular junctions causing muscle paralysis. Vision problems and difficulty swallowing and breathing are among the first symptoms to develop. Without administration of the appropriate antitoxin, respiratory paralysis and death may soon follow. The toxin-mediated disease is lethal in 10% to 20% of cases.

Because of botulinum toxin's extraordinary toxicity, it has received attention as a possible agent in bioterrorism and biological warfare. According to the Centers for Disease Control and Prevention (http://www.bt.cdc.gov/Agent/Botulism/FactSheet.pdf), several countries have developed botulinum toxin as an aerosol weapon. Although human studies on inhalational botulism have not been attempted, it is speculated that the effects will be similar to food-borne botulism and feared that the mortality rates may be much higher (60%).

Figure 1: Gram stain of C. botulinum. Spores are stained pink; bacilli stain purple. Photo credit: George Lombard, Centers for Disease Control and Prevention.

Figure 2: Gentian violet stain of C. botulinum.

Figure 3: C. botulinum spores stained with malachite green stain. The endospores of C. botulinum when stained using the malachite green staining method will appear as green spheres, while the bacilli themselves will turn purple in color. Photo credit: Larry Stauffer, Oregon State Public Health Laboratory.

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