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This lesson introduces the three main types of microorganisms relevant to food safety, as required by the AQA GCSE Food Preparation and Nutrition specification (8585), section 3.4. You need to understand the different types of microorganisms, the conditions they require to grow and multiply, and why they are important in food preparation and nutrition. This knowledge underpins the entire food safety topic and is frequently examined.
Microorganisms (also called microbes) are tiny living organisms that are too small to see with the naked eye. They are found everywhere — in the air, in water, on surfaces, on skin, and in food. While many microorganisms are harmless or even beneficial, some can cause food spoilage and food poisoning.
The three main types of microorganisms relevant to food safety are:
Each type has different characteristics, but they all require specific conditions to grow and multiply.
Bacteria are single-celled organisms that are the most common cause of food poisoning. They reproduce by binary fission — a single bacterium splits into two identical cells. Under ideal conditions, bacteria can divide every 10 to 20 minutes, meaning that a single bacterium can multiply into millions within just a few hours.
| Feature | Detail |
|---|---|
| Size | Microscopic — typically 1–5 micrometres |
| Reproduction | Binary fission (splitting in two) |
| Speed | Can double every 10–20 minutes in ideal conditions |
| Types | Pathogenic (cause illness), spoilage (cause food to deteriorate), beneficial (used in food production) |
| Examples | Salmonella, E. coli, Campylobacter, Listeria, Staphylococcus aureus |
It is important to distinguish between these two types:
Exam Tip: A common exam question asks students to explain why pathogenic bacteria are dangerous. The key point is that they do not change the appearance, taste or smell of food, so contaminated food may look perfectly normal.
Yeasts are single-celled fungi that are found naturally on the surface of fruits, in the air, and in soil. They reproduce by budding — a small bud forms on the parent cell and eventually breaks away to form a new yeast cell.
| Feature | Detail |
|---|---|
| Size | Microscopic — larger than bacteria but still invisible to the naked eye |
| Reproduction | Budding |
| Action | Fermentation — yeasts feed on sugars and produce carbon dioxide and alcohol |
| In food spoilage | Cause fermentation in fruit juices, jams, and sugary foods; can produce off-flavours and fizzing |
| Beneficial uses | Bread making (CO₂ makes dough rise), beer and wine production (alcohol), Marmite |
The process of fermentation is a key concept. When yeasts feed on sugar, they produce carbon dioxide gas and ethanol (alcohol):
Sugar → Carbon dioxide + Alcohol (ethanol)
This reaction is used deliberately in bread making (the CO₂ causes the dough to rise, and the alcohol evaporates during baking) and in brewing (the alcohol is the desired product). However, unwanted yeast activity can cause food spoilage — for example, fruit juices may start to ferment and taste fizzy or alcoholic.
Exam Tip: Remember the dual nature of yeasts — they can be both beneficial (bread, brewing) and a cause of food spoilage (unwanted fermentation).
Moulds are multicellular fungi that grow in thread-like structures called hyphae. A mass of hyphae is called a mycelium. Moulds reproduce by releasing spores into the air. When these spores land on a suitable surface, they germinate and grow into new mould colonies.
| Feature | Detail |
|---|---|
| Size | Visible to the naked eye as fuzzy growths (individual spores are microscopic) |
| Structure | Thread-like hyphae forming a mycelium |
| Reproduction | Spores released into the air |
| Appearance | Often green, white, grey, blue or black fuzzy patches on food |
| In food spoilage | Grow on bread, cheese, fruit, and other foods; produce musty off-flavours |
| Beneficial uses | Blue cheese (Stilton, Roquefort), Camembert and Brie, Quorn (mycoprotein), penicillin |
Some moulds produce mycotoxins — poisonous substances that can cause illness. You should not simply cut mould off food and eat the rest, because:
All microorganisms require specific conditions to grow and multiply. These conditions can be remembered using the mnemonic FATTOM:
mindmap
root((FATTOM))
Food
Nutrients for growth
Especially protein-rich foods
Acidity
Most bacteria prefer neutral pH 6.6-7.5
Acidic conditions slow growth
Temperature
Danger zone 5°C to 63°C
Optimum around 37°C
Time
Bacteria can double every 10-20 min
Risk increases the longer food is in danger zone
Oxygen
Aerobic bacteria need oxygen
Anaerobic bacteria grow without oxygen
Moisture
Water is essential for growth
Drying food removes moisture
Microorganisms need nutrients to grow. They prefer foods that are high in protein and moisture, such as:
These are known as high-risk foods and will be covered in detail in the next lesson.
Most bacteria thrive in conditions that are neutral to slightly acidic (pH 6.6–7.5). Highly acidic conditions (low pH) slow bacterial growth or kill bacteria, which is why vinegar (acetic acid) and lemon juice (citric acid) are used in food preservation (e.g., pickling).
| pH Level | Effect on Bacteria |
|---|---|
| Below 4.5 | Most bacteria cannot grow — acidic preservation |
| 6.6–7.5 | Optimum range for most bacteria |
| Above 9.0 | Most bacteria cannot grow — too alkaline |
Temperature is one of the most critical factors in food safety:
| Temperature | Significance |
|---|---|
| -18°C | Freezer temperature — bacteria are dormant (not killed) |
| 0–5°C | Fridge temperature — bacterial growth is very slow |
| 5–63°C | DANGER ZONE — bacteria multiply rapidly |
| 37°C | Optimum temperature for most pathogenic bacteria (human body temperature) |
| 63°C | Hot holding temperature — bacteria cannot multiply |
| 75°C | Core cooking temperature — most bacteria are killed |
| 100°C | Boiling point of water |
Exam Tip: The danger zone (5°C to 63°C) is one of the most frequently tested facts in the food safety topic. You must know these specific temperatures.
Given suitable conditions, bacteria can multiply extremely quickly. The longer food spends in the danger zone, the more bacteria will be present. This is why:
This is why vacuum packing does not eliminate all bacterial risk — anaerobic bacteria can still grow in the absence of oxygen.
Bacteria need water to grow. Foods with high moisture content (known as high water activity) support bacterial growth more readily. This is why:
When bacteria are introduced to food in ideal conditions, their population follows a characteristic pattern known as the bacterial growth curve:
graph LR
A["Lag Phase<br/>Bacteria adapt<br/>to environment"] --> B["Log Phase<br/>Rapid exponential<br/>growth"]
B --> C["Stationary Phase<br/>Growth rate equals<br/>death rate"]
C --> D["Death/Decline Phase<br/>Nutrients depleted<br/>waste products toxic"]
style A fill:#3498db,color:#fff
style B fill:#e74c3c,color:#fff
style C fill:#f39c12,color:#fff
style D fill:#95a5a6,color:#fff
| Phase | Description |
|---|---|
| Lag phase | Bacteria are adapting to their new environment. There is little increase in numbers. This is the safest time to eat food. |
| Log (exponential) phase | Bacteria multiply rapidly by binary fission. Numbers double at regular intervals. This is the most dangerous phase. |
| Stationary phase | The rate of bacterial multiplication equals the rate of bacterial death. Nutrients are running low and waste products are accumulating. |
| Death (decline) phase | Bacteria begin to die as nutrients are exhausted and toxic waste products build up. Numbers decrease. |
Exam Tip: You may be asked to sketch or interpret a bacterial growth curve. Remember that the log phase is where the greatest danger lies — bacteria are multiplying fastest. In food safety, we aim to keep bacteria in the lag phase by controlling conditions.
| Feature | Bacteria | Yeasts | Moulds |
|---|---|---|---|
| Type | Single-celled organisms | Single-celled fungi | Multicellular fungi |
| Reproduction | Binary fission | Budding | Spores |
| Speed | Very fast (10–20 min) | Slower than bacteria | Slowest of the three |
| Visibility | Invisible | Invisible (unless colonies form) | Visible fuzzy growths |
| Food poisoning | Major cause | Rarely | Rarely (mycotoxins possible) |
| Food spoilage | Yes (slime, off-smells) | Yes (fermentation) | Yes (fuzzy growth) |
| Beneficial uses | Yoghurt, cheese | Bread, beer, wine | Blue cheese, Quorn |
Exam Tip: For a 6-mark question on microorganisms, structure your answer by naming the type of microorganism, explaining how it reproduces, describing the conditions it needs to grow, and giving an example of how it causes food poisoning or spoilage. Always use correct terminology such as "binary fission," "danger zone" and "pathogenic."
A Year 7 end-of-term barbecue at a secondary school features hot dogs, burgers, salads and ice cream. One parent volunteer prepares burgers on a wooden chopping board and sets it aside to answer a question from a teacher. The board is left on the picnic table in direct sunlight (28°C) for 90 minutes before being used — without washing — to slice tomatoes for a salad bowl. This worked example walks through exactly why that single action posed a serious food-safety threat, using the microorganism biology from this lesson.
Step-by-step analysis:
1. **Initial contamination**: raw burger meat typically carries between **100 and 10,000 cells per gram** of spoilage and occasional pathogenic bacteria — commonly *E. coli*, *Salmonella* and *Campylobacter*. When juices from the meat soaked into the board, a conservatively estimated **100 cells per square centimetre** transferred to the wooden surface.
2. **FATTOM conditions met**: the board now held **food** (protein residues), at near-neutral **acidity**, warm **temperature** (28°C — centre of the danger zone), given **time** (90 minutes), with **oxygen** and **moisture** from the blood and juices.
3. **Binary fission in action**: at 28°C many pathogenic bacteria divide every **20 minutes**. Over 90 minutes that is **4.5 generations**, meaning the population multiplies by 2⁴·⁵ ≈ **23 times**. Our starting 100 cells/cm² has become roughly **2,300 cells/cm²**.
4. **Transfer to salad**: sliced tomatoes pick up juices and bacteria directly from the contaminated board. Because tomatoes are a **ready-to-eat** food that will not be cooked further, whatever bacteria transferred will arrive on the plate alive.
5. **Consumption and illness**: the infective dose for *E. coli* O157 is extraordinarily low — **as few as 10 cells** can cause severe illness, especially in children (YOPI group). Guests eating the salad are therefore exposed to a dose that could easily exceed the infective threshold by orders of magnitude.
Diagnostic reasoning:
1. **Why did this happen despite the food being "fresh"?** Because the bacteria concerned are **pathogenic, not spoilage** — they do not change the appearance, smell or taste of the salad. The tomato looks and smells fine.
2. **Why did 90 minutes matter so much?** Because bacterial multiplication is **exponential**, not linear. Twice as long in the danger zone means **vastly more** bacteria, not just twice as many.
3. **What should the volunteer have done?** Used separate colour-coded boards for raw meat (red) and salad (green), or washed the board with hot soapy water and a sanitiser before switching tasks, or kept the board in the shade and in a sealed container.
This example brings together bacterial reproduction (binary fission), FATTOM conditions, the danger zone and the distinction between pathogenic and spoilage organisms — all core Section 4 content.
Misconception: "If food has been cooked properly, it cannot make anyone ill."
This is only partly true. Thorough cooking (core 75°C) kills most living pathogenic bacteria, but some bacteria such as Staphylococcus aureus and Bacillus cereus produce heat-resistant toxins while the food was warm. These toxins survive cooking. In addition, food can easily be re-contaminated after cooking via unwashed hands, dirty utensils or chopping boards. Cooking is only one critical control point; post-cook hygiene and temperature control are equally important.
Exam-style question (6 marks): Explain what microorganisms need to grow and multiply in food, and give two ways of preventing bacterial growth.
Grade 3-4 response:
Bacteria need warmth, food and water to grow. You can stop them by keeping food in the fridge or cooking it. Bacteria can make people ill with diarrhoea and vomiting.
(AQA: "Limited knowledge, some relevant points but no specific detail.")
Grade 5-6 response:
Bacteria need food, warmth, moisture, time and the right pH to grow — this can be remembered as FATTOM (Food, Acidity, Temperature, Time, Oxygen, Moisture). The temperature range where they grow fastest is the danger zone of 5-63°C. Ways of preventing growth include: (1) storing high-risk foods in the fridge at 0-5°C so they are too cold for bacteria to multiply; (2) cooking food to a core temperature of 75°C, which kills most bacteria; (3) drying or salting food to remove moisture; (4) using acidic pH in pickling or fermenting.
(AQA: "Clear knowledge with specific terminology and temperatures.")
Grade 7-9 response:
Microorganisms require six conditions summarised by the mnemonic FATTOM: Food (organic nutrients, especially protein and carbohydrate), Acidity (neutral pH 6.5-7.5 for most pathogens), Temperature (the danger zone 5-63°C, with optimum growth around 37°C), Time (exponential growth via binary fission — doubling every 10-20 minutes gives over 2 million from one cell in 7 hours), Oxygen (aerobic, anaerobic or facultative depending on species), and Moisture (water activity, a_w, above 0.85). Preventive controls manipulate one or more of these variables. Chilling to 0-5°C slows metabolism and binary fission dramatically but does not eliminate Listeria which is psychrotrophic. Freezing to -18°C induces dormancy (not death). Thorough cooking to a core of 75°C for 30 seconds (or equivalent time-temperature combinations) denatures enzymes and destroys most vegetative cells, though spore-formers (Bacillus cereus, Clostridium) and heat-stable toxins (S. aureus) are unaffected. Water-activity reduction through drying, salting or sugar-curing lowers a_w below the threshold for bacterial growth, which is why jerky, salami and jam have long shelf lives. pH reduction through fermentation, pickling or acidification (to pH < 4.5) inhibits most pathogens. Evaluation: the most reliable approach combines multiple controls (known as hurdle technology) rather than relying on a single factor, because any one control can fail — for example, soft cheese chilled at 5°C can still harbour Listeria, so use-by dates and YOPI advice supplement temperature control.
(AQA: "Detailed, thorough knowledge with sophisticated terminology, accurate scientific detail and evaluative judgement.")
This content is aligned with the AQA GCSE Food Preparation and Nutrition (8585) specification, Section 4: Food safety. For the most accurate and up-to-date information, please refer to the official AQA specification document.