FOOD MICROBIOLOGY: FOODBORNE PATHOGENS
Diagram
Two foodborne germs, two doses
Salmonella
Lives in the gut of animals and people. Reaches food through raw meat, poultry, eggs and produce. Usually needs many cells to cause illness.
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Shigella
Spreads from person to person and through food handled by an infected person. Causes dysentery with very few cells, sometimes fewer than 100.
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The choice
Both cause diarrhoea and both are Gram-negative rods, but their source, dose and disease differ, so their prevention differs too.
Figure 1: Salmonella comes mainly from animals and raw food; Shigella comes mainly from infected people handling food. Drawn in our own style from the FTN Food Microbiology notes, Unit 4, and from Jay, Loessner and Golden, Modern Food Microbiology, chapter 26.
Introduction
Salmonella and Shigella are the two classic causes of foodborne diarrhoea, and both are Gram-negative rods, but they reach food by different routes, need very different numbers of cells to cause illness, and cause different diseases. Salmonella lives in the gut of animals and people and usually needs many cells, while Shigella spreads from person to person and can cause dysentery with very few cells, sometimes fewer than 100. Jay, Loessner and Golden give the two one chapter, on foodborne gastroenteritis caused by Salmonella and Shigella, and the FTN Food Microbiology notes, Unit 4, describe each one in turn.
Both are controlled by cooking, clean water and keeping raw food away from ready-to-eat food, but the details differ, and this article follows the FTN notes and the Jay chapter.
Quick comparison
Table 1: Salmonella compared with Shigella
| Aspect | Salmonella | Shigella |
|---|---|---|
| Lives in | The gut of animals and people | People, and it spreads from person to person |
| How it reaches food | Faecal contamination of food and water, through raw meat and poultry, eggs, unpasteurized milk and raw produce | Food handled by an infected person, and food handled after cooking |
| Common vehicles | Raw meat and poultry, eggs, unpasteurized milk, raw produce, and foods held warm | Salads, sandwiches and other foods handled after cooking |
| Infectious dose | Usually high, but can be low in fatty foods, which protect cells from stomach acid | Very low, sometimes fewer than 100 cells |
| Disease | Salmonellosis: fever, cramps and diarrhoea lasting several days | Dysentery: bloody diarrhoea, cramps and fever |
| Onset | Symptoms start 12 to 72 hours after eating | The books used here give no onset time for Shigella, so none is stated |
| Growth in food | Grows in foods held warm | Spreads through handling rather than growing in the food |
| Prevention | Cooking through, keeping raw meat away from ready-to-eat food, and hand washing after handling raw meat | Hand washing, clean water, and keeping ill food handlers away from food |
| Detection | Isolation uses enrichment broth, selective plates and confirmation tests, from suspect food to a named organism | The books used here name no separate isolation procedure for Shigella, so none is given |
| Temperature group | A mesophile, so warm food left out is its best chance to grow | The books used here give no temperature group for Shigella, so none is stated |
Scientific explanation
Definition
Salmonella is a Gram-negative rod that lives in the gut of animals and people and spreads through faecal contamination of food and water: it reaches food through raw meat and poultry, eggs, unpasteurized milk and raw produce, and it grows in foods held warm. Shigella is a Gram-negative rod that causes dysentery with a very low infectious dose, sometimes fewer than 100 cells, and it spreads from person to person and through food handled by an infected person.
Working principle
Salmonella reaches food from animals and from faecal contamination, and it grows in food held warm. The infectious dose is usually high but can be low in fatty foods, which protect cells from stomach acid. Symptoms start 12 to 72 hours after eating, with fever, cramps and diarrhoea lasting several days. Shigella works differently: it does not need to grow in the food, because so few cells cause illness. It spreads from an infected person to food, often food handled after cooking, such as salads and sandwiches, and it causes bloody diarrhoea, cramps and fever. That is why the prevention differs: Salmonella is stopped by cooking through and by keeping raw meat away from ready-to-eat food, while Shigella is stopped by hand washing, clean water and keeping ill food handlers away from food.
Apparatus
The laboratory tools named in the FTN notes for Salmonella are enrichment broth, selective plates and confirmation tests. The books used here name no separate apparatus list for Shigella, so none is given.
Measurement procedure
Isolation of Salmonella from food uses enrichment broth, selective plates and confirmation tests, showing the full path from suspect food to a named organism. The books used here name no separate isolation procedure for Shigella, so none is given here: the distinction between the two is made from their source, dose and disease, as given above.
Factors affecting readings
For Salmonella, temperature and the food itself decide the risk: it grows in foods held warm, and fatty foods lower the dose needed, because fat protects the cells from stomach acid. For Shigella, the handler decides the risk: an infected person handling food after cooking is the route, so hand washing and keeping ill handlers away from food are the controls. Clean water matters for both, because faecal contamination of water carries Salmonella, and unclean water carries Shigella.
Formulas and calculations
The books used here give no formula for this comparison, so no formula or worked calculation is given.
Industrial applications
- Salmonella: the reason raw meat and poultry are cooked through, and raw meat is kept away from ready-to-eat food.
- Salmonella: the reason eggs and unpasteurized milk are handled with care, and foods held warm are watched.
- Salmonella: laboratory isolation from suspect food, by enrichment broth, selective plates and confirmation tests.
- Shigella: the reason hand washing after the toilet is a food safety rule, not only a hygiene rule.
- Shigella: the reason ill food handlers are kept away from food, especially food handled after cooking.
- Shigella: the reason salads and sandwiches, which are handled after cooking, are watched as vehicles.
Advantages and limitations
- Salmonella, advantage to the analyst: it is found by a standard path of enrichment broth, selective plates and confirmation tests.
- Salmonella, limitation as a risk: it usually needs many cells, but fatty foods protect it from stomach acid, so the dose can be low.
- Shigella, advantage to the analyst: its route is clear, through an infected handler, so the control is clear too.
- Shigella, limitation as a risk: very few cells cause illness, sometimes fewer than 100, so a small lapse in hand washing is enough.
FAQ
What is the main difference between Salmonella and Shigella?
Salmonella comes mainly from animals and raw food and usually needs many cells, while Shigella comes mainly from infected people handling food and needs very few cells, sometimes fewer than 100.
Why can fatty food make Salmonella more dangerous?
Because fat protects the cells from stomach acid, so the infectious dose can be low in fatty foods.
What illness does Shigella cause?
Dysentery, with bloody diarrhoea, cramps and fever.
How soon do Salmonella symptoms start?
12 to 72 hours after eating, with fever, cramps and diarrhoea lasting several days.
How is Shigella prevented?
By hand washing, clean water, and keeping ill food handlers away from food, especially food handled after cooking.
References and further reading
- FTN subject: Food Microbiology, 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)
