Food Poisoning Bacteria: E. coli and Listeria

This lesson continues the study of specific food poisoning bacteria, covering Escherichia coli (E. coli) and Listeria monocytogenes, as required by the AQA GCSE Food Preparation and Nutrition specification (8585), section 3.4. Both bacteria can cause serious illness, and you need to know their sources, symptoms, onset times, and prevention methods.

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Escherichia coli (E. coli)

Escherichia coli (usually written as E. coli) is a large group of bacteria. Most strains of E. coli are harmless and live naturally in the intestines of humans and animals. However, some strains are pathogenic and can cause severe food poisoning. The most dangerous strain is E. coli O157:H7 (often shortened to E. coli O157), which produces a powerful toxin called Shiga toxin (also known as verocytotoxin).

Sources of E. coli O157

Source	Detail
Raw and undercooked beef	Especially minced beef (burgers, meatballs) — the bacteria live in the intestines of cattle and can contaminate meat during slaughter
Unpasteurised milk and dairy	Can contain E. coli from infected cattle
Contaminated water	Especially from agricultural areas where animal waste enters water supplies
Unwashed vegetables and salad	Contaminated by animal manure used as fertiliser or irrigation with contaminated water
Person-to-person	Through poor hand hygiene; very low infective dose means it spreads easily
Contact with farm animals	Especially cattle, sheep and goats

Key Feature: Low Infective Dose

A crucial characteristic of E. coli O157 is its very low infective dose — as few as 10 bacteria can cause illness. This is much lower than other food poisoning bacteria, which typically require thousands or millions of bacteria. This means:

• Even a small amount of contamination can cause illness
• Cross-contamination is particularly dangerous
• Person-to-person spread is more likely
• Thorough cooking is essential

Why Minced Beef Is Particularly Risky

Whole cuts of beef (steaks) carry bacteria only on the outside surface, which is killed when the outer surface is cooked (even if the inside remains rare). However, mincing mixes the outer surface bacteria throughout the meat, so bacteria can be present in the centre of a burger. This is why:

• Steaks can be served rare (as long as the outside is seared)
• Burgers and mince must be cooked all the way through to 75°C at the core
• There should be no pink meat in the centre of a burger

flowchart LR
    A["Whole Steak"] --> B["Bacteria on<br/>OUTER surface only"]
    B --> C["Searing kills<br/>surface bacteria"]
    C --> D["Can be served rare"]

    E["Minced Beef"] --> F["Bacteria MIXED<br/>throughout"]
    F --> G["Must cook to 75°C<br/>at the CORE"]
    G --> H["No pink meat<br/>in centre"]

    style A fill:#27ae60,color:#fff
    style D fill:#27ae60,color:#fff
    style E fill:#e74c3c,color:#fff
    style H fill:#e74c3c,color:#fff

Symptoms of E. coli O157 Food Poisoning

Feature	Detail
Onset time	1–4 days (typically 3–4 days)
Duration	Usually 5–10 days, sometimes longer
Main symptoms	Severe abdominal cramps, bloody diarrhoea, vomiting, fever
Serious complications	Haemolytic uraemic syndrome (HUS) — kidney failure, particularly in children under 5 and elderly people; can be fatal

Exam Tip: The serious complication of E. coli O157 is HUS (haemolytic uraemic syndrome), which causes kidney failure. This is a key distinguishing feature in exam questions.

Prevention of E. coli

Prevention Method	Explanation
Cook minced beef thoroughly	Core temperature must reach at least 75°C — no pink meat in burgers
Use pasteurised milk	Avoid unpasteurised (raw) milk and dairy products
Wash all vegetables and salad	Especially those to be eaten raw; wash under running water
Prevent cross-contamination	Separate raw meat from ready-to-eat foods; use different chopping boards
Thorough handwashing	After handling raw meat, using the toilet, and after contact with farm animals
Treat water	Ensure drinking water is treated; avoid untreated water from agricultural areas

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Listeria (Listeria monocytogenes)

Listeria monocytogenes causes an infection called listeriosis. While it is less common than Campylobacter or Salmonella, it is one of the most serious food poisoning bacteria because it has a high mortality rate (approximately 20–30% of clinical cases can be fatal). It is particularly dangerous for pregnant women and their unborn babies.

Key Feature: Growth at Fridge Temperatures

A critical characteristic of Listeria is that it can grow at low temperatures, including fridge temperatures (0–5°C). Most other food poisoning bacteria grow very slowly or not at all at fridge temperatures, but Listeria continues to multiply, although more slowly than at warmer temperatures.

This means that:

• Refrigeration does not provide complete protection against Listeria
• Foods stored for long periods in the fridge can accumulate dangerous levels of Listeria
• Use-by dates are particularly important for high-risk foods

Sources of Listeria

Source	Detail
Soft and mould-ripened cheeses	Brie, Camembert, blue cheese (unless made from pasteurised milk and cooked until hot)
Pâté	All types, including vegetable pâté
Ready-to-eat chilled foods	Pre-packed sandwiches, prepared salads, deli meats
Smoked fish	Smoked salmon, smoked mackerel
Unpasteurised milk	Can contain Listeria
Pre-prepared salads	Including washed salad bags
Cooked sliced meats	Deli counter meats that may be stored for extended periods
Soil	Listeria lives naturally in soil and can contaminate vegetables

Symptoms of Listeriosis

Feature	Detail
Onset time	1–70 days (highly variable; often 2–3 weeks)
Duration	Variable; can be prolonged
Mild form	Flu-like symptoms: fever, muscle aches, nausea, diarrhoea
Severe form	Meningitis (inflammation of brain membranes), septicaemia (blood poisoning)
In pregnancy	Miscarriage, stillbirth, premature birth, or severe infection in the newborn baby
Mortality rate	Approximately 20–30% in clinical cases

Listeria and Pregnancy

Listeria is particularly dangerous during pregnancy because:

• Pregnant women are about 10 times more likely to get listeriosis than the general population
• Even a mild infection in the mother can cause miscarriage, stillbirth or premature birth
• The baby may be born with a severe infection (neonatal listeriosis)
• This is why pregnant women are advised to avoid soft mould-ripened cheeses, pâté, and other high-risk foods for Listeria

Exam Tip: If an exam question mentions a pregnant woman, think of Listeria. The advice to pregnant women to avoid soft cheese, pâté and certain other foods is specifically because of the risk of listeriosis.

Prevention of Listeria

Prevention Method	Explanation
Respect use-by dates	Listeria grows in the fridge, so do not eat food past its use-by date
Avoid high-risk foods in pregnancy	Soft mould-ripened cheeses, pâté, smoked fish, pre-prepared salads
Keep fridge at 0–5°C	Slows Listeria growth (but does not stop it)
Eat ready-to-eat chilled foods promptly	Do not store for extended periods
Use pasteurised dairy products	Pasteurisation kills Listeria
Wash vegetables thoroughly	Especially those to be eaten raw
Follow storage instructions	Consume foods within the time stated once opened
Reheat food thoroughly	To at least 75°C at the core

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Comparing E. coli O157 and Listeria

Feature	E. coli O157	Listeria
Primary sources	Raw/undercooked beef, unpasteurised milk	Soft cheese, pâté, chilled ready-to-eat foods
Onset time	1–4 days	1–70 days (often 2–3 weeks)
Key symptoms	Bloody diarrhoea, severe cramps	Flu-like symptoms; can cause meningitis
Serious complication	HUS (kidney failure)	Meningitis; miscarriage/stillbirth in pregnancy
Most at risk	Children under 5, elderly	Pregnant women, unborn babies, elderly, immunocompromised
Unique feature	Very low infective dose (as few as 10 bacteria)	Can grow at fridge temperatures (0–5°C)
Key prevention	Cook burgers to 75°C core; no pink meat	Respect use-by dates; avoid certain foods in pregnancy

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