What Is Botulism? - Food Poisoning Caused By the Bacterium Clostridium botulinum

What Is Botulism? - Food Poisoning Caused By the Bacterium Clostridium botulinum
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  • 1:26 Clostridium botulinum
  • 2:36 Botulism Exotoxin
  • 3:56 Botulism Food Poisoning
  • 6:16 Infant Botulism
  • 7:28 Symptoms and Treatment
  • 8:41 Lesson Summary
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Lesson Transcript
Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

The most toxic natural substance on Earth is not the venom of a snake or spider. Instead, it is a toxin made by a bacterium called Clostridium botulinum. We will examine Botulism, a type of food poisoning caused by this toxin.

Lake Erie Fish Kills

In the 1990s, large numbers of dead fish began periodically washing up on the shores of Lake Erie. Following the fish were scores of dead seabirds. The local scientists quickly converged on the lake, taking all kinds of samples, in an attempt to discover the reason for all the deaths. Interestingly, the cause of death nearly across the board seemed to be drowning. How is it possible that aquatic birds and fish could drown? These are organisms specifically evolved to live in water. And why so many all at once? Each answer led to more questions.

Eventually, the data points started to come together. The water at the bottom of the lake had an abnormally low oxygen content, called anoxia, for an abnormally long time. The dead and dying fish were all paralyzed. Unable to move, they couldn't swim out of the anoxic waters or away from predators. The smaller paralyzed fish were easy prey for larger fish and birds. The fish and birds that ate these fish also showed signs of paralysis. Unable to keep their heads above the surface, the birds drowned as well. All this evidence pointed to an outbreak of one very dangerous bacterium: Clostridium botulinum.

Clostridium Botulinum

Clostridium botulinum is an obligate anaerobic, gram positive, endospore-forming, rod-shaped bacteria that can cause a very deadly form of food poisoning. Let's break that down a little bit. C. botulinum is an obligate anaerobe, meaning it cannot survive in the presence of oxygen, and can only grow by fermentation in anoxic environments. The bacterium is gram positive, meaning it has a thick peptidoglycan cell wall. It also forms very resistant endospores that are able to survive unfavorable environmental conditions. These endospores are everywhere. If you brush your hand across the desk in front of you and examined the dust you stirred up, it would contain C. botulinum endospores. These endospores will only become active, growing bacteria if they land in an anoxic, protein-rich environment.

With this information, the Lake Erie outbreak starts to make more sense. The bacteria were able to grow in the abnormally anoxic waters. They were passed up through the food chain, first infecting fish then fish-eating birds. But, why were the infected animals paralyzed?

Botulism Exotoxin

C. botulinum produces a very potent exotoxin. An exotoxin is a toxin that is produced by, in this case, a bacterial cell and released into the environment around the cell. The botulism exotoxin is only released by the vegetative, meaning actively growing, bacteria and is the most potent and deadly natural substance known. One gram of aerosolized botulism toxin is enough to kill about a million people. Toxic doses are measured in billionths of a gram.

So, how does this toxin work? Well, the Lake Erie outbreak provides some clues, with the major clue being the paralysis experienced by fish and birds. Botulism toxin is a potent neurotoxin that interferes with the neurotransmitters that pass signals from the nerves to the muscles. The toxin blocks the release of acetylcholine from the nerves. Normally, acetylcholine is released from the nerve cells and binds to the muscles, initiating contractions. When the botulism neurotoxin blocks acetylcholine release, this results in flaccid paralysis, which means the muscles are limp and unable to contract. Aquatic animals that can't move their muscles will drown, just like the fish and birds in Lake Erie.

Botulism Food Poisoning

But, this is a lesson about food poisoning, so with this background in place, let's shift our focus to humans. Since C. botulinum endospores are everywhere, it is inevitable that they will contaminate food. You might think that if the endospores are everywhere and that so little of the toxin is required to be lethal, why aren't large numbers of people dying from botulism every day? The answer again lies in the organism itself. The endospores require very specific, anaerobic and protein-rich conditions to germinate into growing cells and produce toxins. Foods that are processed with high heat or acids will kill C. botulinum.

Exposure to oxygen prevents the endospores from germinating and producing toxins. The toxin itself is simply a protein, so heating foods to 80-90 degrees Celsius for 10 minutes is enough to break down any toxin proteins already present in the food, rendering it harmless. It is important to note here, that illness is caused by the toxin, not the bacteria itself. C. botulinum that are unable to produce the exotoxin are harmless, and, even if the cells aren't actively growing, residual toxin can result in illness.

Problems arise when the safeguards for food preservation and preparation are not followed and people consume the toxin. It has become much less common today, but many people used to can their own vegetables. Failure to properly heat sterilize vegetables can allow C. botulinum to survive. Some canned veggies, like beans, are often used cold and uncooked in salads.

Eating them without heating them to the required 80-90 degrees Celsius can result in botulism toxin consumption. Of course, commercially canned foods can also be contaminated with endospores, but regulations ensure more consistent handling and processing, thus reducing infections from this source. Still, any dented or damaged cans should be discarded, and you should never taste test these foods.

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