FOOD MICROBIOLOGY: SPORES
Diagram
Two kinds of bacteria, two kinds of survival
Spore-forming bacteria
Make tough resting spores that survive boiling water for minutes, and drying, and chemicals. The spores wake up and grow when conditions turn friendly again.
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Non-spore-forming bacteria
Have no resting form. Cooking kills most vegetative cells, and drying and chemicals kill them too.
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The risk
Cooking kills the ordinary cells and leaves the spores, so a cooked food cooled slowly can grow spore-formers that the cooking never touched.
Figure 1: Spore-formers survive heat and drying that kill ordinary cells. Drawn in our own style from the FTN Food Microbiology notes, Units 1 and 4, and from Jay, Loessner and Golden, Modern Food Microbiology, chapter 24.
Introduction
Spore-forming and non-spore-forming bacteria are two groups sorted by whether they can make a spore, a tough resting form that survives heat, drying and chemicals. Both can spoil food or cause illness, but spore-formers survive processing that kills ordinary cells, and their spores wake up and grow when conditions turn friendly again. That is why cooked rice, cooked meat and canned food carry a spore risk that raw food does not: the cooking kills the competition and leaves the spores.
The FTN Food Microbiology notes, Unit 1, define the spore and name Bacillus and Clostridium from soil, and Unit 4 describes the three spore-formers that matter most in food. Jay, Loessner and Golden give Gram-positive spore-forming bacteria their own chapter on food poisoning. This article follows those sources.
Quick comparison
Table 1: Spore-forming bacteria compared with non-spore-forming bacteria
| Aspect | Spore-forming bacteria | Non-spore-forming bacteria |
|---|---|---|
| Resting form | Make tough resting spores | No resting form: only vegetative cells |
| Survives boiling water | Yes, spores survive boiling water for minutes | No, cooking kills most vegetative cells |
| Survives drying | Yes, spores survive heat and drying | No, drying kills or stops ordinary cells |
| Survives chemicals | Spores resist chemicals that kill vegetative cells | Killed by sanitisers and disinfectants at the normal dose |
| Examples | Bacillus and Clostridium species, from soil | Salmonella, Staphylococcus aureus, E. coli and Listeria |
| Foods | Cooked rice and starchy foods, cooked meat and gravy, canned goods and vacuum packs | Raw meat and poultry, eggs, milk, salads and ready-to-eat foods |
| Illness | Botulism, Bacillus cereus vomiting and diarrhoea, Clostridium perfringens cramps and diarrhoea | Salmonellosis, staphylococcal food poisoning, E. coli infection |
| How illness happens | Spores survive cooking, wake up as food cools, grow, and make toxin, or release toxin in the gut | Cells swallowed with food grow in the gut, or toxin made in the food is eaten |
| Control | Cook in small batches, cool fast, reheat fully, use proper canning heat, and keep acid foods below pH 4.6 | Cook through, chill fast, keep raw food away from ready-to-eat food, and wash hands |
| Detection | Isolation uses selective media and spore testing, as for Bacillus cereus from food | Isolation uses enrichment broth, selective plates and confirmation tests |
Scientific explanation
Definition
A spore is a tough resting form made by some bacteria and molds that survives heat, drying and chemicals: the food example in the FTN notes is Clostridium spores, which survive boiling and later grow in badly cooled cooked food. Spore-forming bacteria are the bacteria that make such spores, mainly Bacillus and Clostridium species from soil. Non-spore-forming bacteria have no resting form: they live only as vegetative cells, and cooking, drying and sanitisers kill them.
Working principle
Spore-forming bacteria from soil, such as Bacillus and Clostridium species, survive heat and drying, so they can persist through processing that kills weaker cells. Clostridium botulinum is an anaerobe whose spores survive heating and grow in oxygen-free foods such as canned goods, vacuum packs and home preserves, making botulinum toxin, one of the strongest poisons known. Clostridium perfringens grows in meat and gravy cooked in bulk and cooled slowly: its spores survive cooking, wake up as food cools, and grow fast in the warm middle of a large pot. Bacillus cereus spores survive cooking in rice, pasta and other starchy foods: left warm, they grow and make two kinds of toxin, one causing vomiting within a few hours, the other causing diarrhoea within 8 to 16 hours. Non-spore-forming bacteria have no such second chance: cooking kills most vegetative cells, so their risk is in raw food, in food handled after cooking, and in food held warm, not in food that was cooked and cooled.
Apparatus
The books used here name no laboratory apparatus for telling the two apart, so no apparatus list is given.
Measurement procedure
Isolation of Bacillus cereus from food uses selective media and spore testing, matching the organism to cooked rice and starchy foods. The books used here name no single standard procedure for telling spore-formers from non-spore-formers as a pair: the distinction is made by heating the sample, which kills vegetative cells and leaves spores, and by the foods and illnesses each group is tied to, as given above.
Factors affecting readings
Temperature and cooling rate decide the spore risk: spores survive cooking, wake up as food cools, and grow fast in the warm middle of a large pot, so cooking in small batches and cooling fast are the controls. Acidity decides the botulinum risk: acid foods below pH 4.6 stop Clostridium botulinum, so proper canning heat, acid, chilled storage and never using swollen or damaged cans are the controls. Oxygen decides it too: Clostridium botulinum is an anaerobe and grows in oxygen-free foods such as canned goods and vacuum packs. For non-spore-formers, temperature and handling decide the risk, as for Salmonella and Staphylococcus aureus.
Formulas and calculations
The books used here give no formula for this comparison, so no formula or worked calculation is given.
Industrial applications
- Spore-forming bacteria: the reason cooked rice is cooled fast, stored cold and reheated fully, with a short storage life.
- Spore-forming bacteria: the reason meat and gravy are cooked in small batches and cooled fast, not left in a large pot.
- Spore-forming bacteria: the reason canning needs proper heat, acid foods are kept below pH 4.6, and swollen or damaged cans are never used.
- Non-spore-forming bacteria: the reason raw meat is cooked through and kept away from ready-to-eat food.
- Non-spore-forming bacteria: the reason salads and sandwiches are protected from handlers and from raw food.
- Non-spore-forming bacteria: the reason hand washing and clean water are food safety controls.
Advantages and limitations
- Spore-forming bacteria, advantage to the bacteria: spores survive boiling water for minutes, and drying and chemicals, so they persist through processing.
- Spore-forming bacteria, limitation to the maker: they can be controlled by fast cooling, full reheating, proper canning heat and acidity below pH 4.6.
- Non-spore-forming bacteria, advantage to the maker: cooking kills most vegetative cells, so thorough cooking removes the risk.
- Non-spore-forming bacteria, limitation as a risk: they grow fast in food held warm, and some, such as Staphylococcus aureus, make toxin in the food before it is eaten.
FAQ
What is the main difference between spore-forming and non-spore-forming bacteria?
Spore-formers make tough resting spores that survive boiling water for minutes, and drying and chemicals, while non-spore-formers have no resting form and are killed by cooking.
Why is cooked rice a classic spore-former food?
Bacillus cereus spores survive cooking in rice, and left warm they grow and make toxin, so cooked rice held at room temperature is the classic vehicle.
Why must meat and gravy be cooled fast?
Clostridium perfringens spores survive cooking, wake up as food cools, and grow fast in the warm middle of a large pot.
What stops Clostridium botulinum in canned food?
Proper canning heat, acid foods below pH 4.6, chilled storage, and never using swollen or damaged cans.
Do spores survive drying?
Yes: spores survive heat and drying, which is why spore-formers from soil persist through processing that kills weaker cells.
References and further reading
- FTN subject: Food Microbiology, Unit 1: Microorganisms in Food, Sources and Factors; Unit 4: Foodborne Illness, Food Poisoning and Disease.
- James M. Jay, Martin J. Loessner and David A. Golden, Modern Food Microbiology, 7th ed., Springer, 2006 (Google Books page read)
