MadSci Network: Microbiology

Re: How do botulism spores form?

Date: Sun Jan 16 21:34:16 2000
Posted By: George Stearns, Grad student, Food Microbiology, University of Nebraska-Lincoln
Area of science: Microbiology
ID: 947049922.Mi

How do botulism spores form?

We worry all the time about food poisoning from home made canned foods. I understand most heating kills living cells but not their "spores." Well how do the bacteria know to form spores? I can't imagine they know in advance that we're going to be cooking our jams!

The answer to the first question is that the bacteria make spores in response to the conditions in which they live. Under adverse conditions, such as when the temperature becomes too hot, or if there is not enough water or nutrients available to grow, the bacteria produce spores. The process of making a spore takes about eight hours from start to finish. The spores are produced when the bacteria changes all of its processes required to grow and divide to those process required to make a spore. Those changes are made in response to the bacteria's environment.

The simplest answer to the second part of the questions is that the bacteria do not know that they are going to be cooked but they make spores long before they end up in the canning jar. In fact, the spores are usually on the food prior to harvest. I have included a much more thorough answer below.

Botulism is caused by an organism known as Clostridium botulinum whose most notable attributes are the formation of spores and the production of a deadly toxin that affects the nervous system (i.e., neurotoxin). In sifting through the scientific literature I did not find a great deal of material about how this bacteria actually forms spores. The probable reason for this is that many research institutions require special permission to work with organisms that are highly lethal. Clostridium botulinum is also very sensitive to oxygen. The growing cells must be kept away from oxygen. Maintaining these conditions requires a great deal of expensive equipment.

Thus, to study the process of sporulation scientists tend to use what is termed a "model organism," or one that carries out the same, or a similar process, but is easier to work with. For this purpose a common bacteria that is not dangerous, but forms spores has been studied extensively, in an effort to understand how spores form in bacteria. The particular bacterium that has been chosen is called Bacillus subtilis and the answer that I am providing is based on the work carried out in B. subtilis.

B. subtilis produces spores in response to its environment. The bacterium is constantly measuring the external environment for the pH (the term pH is used to describe how acidic something is), the temperature, and water or nutrient availability. All of these parameters cause the bacteria to alter its growth pattern so that it may survive any adverse conditions. Some of the alterations are short term in nature, while others, like forming spores, represent a longer protective mechanism, allowing some cells to survive as spores. When the environment is sensed as being unfavorable the organism makes some drastic changes in order to produce the spores. Some of the parts that make up the bacterial cell are no longer produced while the production of new parts begins. The actual process is quite complex and I believe is beyond the intent of this question.

The spore is a dormant form of the bacterium that is much more resistant to chemicals, heat, air, and drying. Once the spore is formed, it remains in that state until favorable conditions are encountered. If the conditions become favorable the spore will "germinate," thereby producing a growing cell that can replicate into more bacterial cells. In the case of C. botulinum, spores will not germinate if the pH is too low.

This is the reason that many canning procedures call for the addition of citric acid or some other acid. Most fruits have enough acid in them to lower the pH to a level where the spores will not germinate. In addition, the sugar used in making jams holds onto the water so that the bacteria cannot use it for growth. A certain amount of "available" water must be present for the bacteria to grow. Other canned produce, such as green beans, do not have enough acid to prevent germination of the spore, nor do they hold onto the water like sugar does. Those are the usual sources of botulism poisoning. It is my recommendation that canning instructions be followed religiously for these reasons.

The reason that heat treatment is used in canning jams is to deal with the spores of molds - the main reason for spoiled jams. The spores produced by molds have different characteristics than those produced by C. botulinum and molds can grow in conditions where C. botulinum cannot. Molds require less water and can grow at much lower pH than C. botulinum.

Genetics of Bacterial Diversity, D. Hopwood, ed., 1990.

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