Microorganisms: Bacteria, Yeasts and Moulds

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.

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What Are Microorganisms?

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:

1. Bacteria
2. Yeasts
3. Moulds

Each type has different characteristics, but they all require specific conditions to grow and multiply.

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Bacteria

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.

Key Facts About Bacteria

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

Pathogenic Bacteria vs Spoilage Bacteria

It is important to distinguish between these two types:

• Pathogenic bacteria cause food poisoning. They do not usually change the appearance, taste or smell of food, which makes them particularly dangerous — you cannot tell if food is contaminated.
• Spoilage bacteria cause food to deteriorate — they produce visible signs such as slime, off-smells and discolouration. While eating spoiled food may make you feel unwell, it is not usually as dangerous as food poisoning from pathogenic bacteria.

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.

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Yeasts

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.

Key Facts About Yeasts

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

Fermentation

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).

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Moulds

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.

Key Facts About Moulds

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

Important Safety Note About Moulds

Some moulds produce mycotoxins — poisonous substances that can cause illness. You should not simply cut mould off food and eat the rest, because:

• Mould hyphae can penetrate deep into soft foods (bread, soft cheese, fruit)
• Mycotoxins may have spread beyond the visible mould growth
• The exception is hard cheese — you can cut at least 2.5 cm around and below the mould, because the dense structure prevents deep penetration

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Conditions for Microbial Growth

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

1. Food (Nutrients)

Microorganisms need nutrients to grow. They prefer foods that are high in protein and moisture, such as:

• Meat and poultry
• Fish and shellfish
• Dairy products (milk, cream, soft cheese)
• Cooked rice and pasta
• Eggs

These are known as high-risk foods and will be covered in detail in the next lesson.

2. Acidity (pH)

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

3. Temperature

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.

4. Time

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:

• Food should not be left at room temperature for more than 2 hours
• Leftovers should be cooled quickly and refrigerated within 1–2 hours
• Reheated food should reach 75°C as quickly as possible

5. Oxygen

• Aerobic bacteria need oxygen to grow (e.g., some spoilage bacteria)
• Anaerobic bacteria grow without oxygen (e.g., Clostridium botulinum, which causes botulism in incorrectly canned foods)
• Facultative bacteria can grow with or without oxygen (e.g., E. coli, Salmonella)

This is why vacuum packing does not eliminate all bacterial risk — anaerobic bacteria can still grow in the absence of oxygen.

6. Moisture

Bacteria need water to grow. Foods with high moisture content (known as high water activity) support bacterial growth more readily. This is why:

• Dried foods (pasta, rice, dried fruit) have a longer shelf life
• Salting and sugaring preserve food by reducing available water
• Dehydration is one of the oldest methods of food preservation

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Bacterial Growth Curve

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.

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Comparing the Three Microorganisms

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

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Common Exam Mistakes

• Confusing pathogenic and spoilage bacteria. Pathogenic bacteria cause food poisoning but do not change the appearance, taste or smell of food. Spoilage bacteria cause visible changes but are generally less dangerous.
• Stating that freezing kills bacteria. Freezing makes bacteria dormant — they stop multiplying but are not killed. When food is thawed, bacteria resume growth.
• Forgetting that some bacteria are anaerobic. Vacuum packing and canning do not eliminate all bacterial risks.
• Not knowing the specific danger zone temperatures. Always quote 5°C to 63°C.

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Summary

• Microorganisms include bacteria, yeasts and moulds.
• Bacteria are the main cause of food poisoning and reproduce by binary fission every 10–20 minutes in ideal conditions.
• Yeasts are single-celled fungi that reproduce by budding and cause fermentation; they are used beneficially in bread making and brewing.
• Moulds are multicellular fungi that reproduce via spores and are visible as fuzzy growths; some produce harmful mycotoxins.
• All microorganisms need food, acidity (neutral pH), temperature (5–63°C danger zone), time, oxygen (some do not), and moisture to grow — remembered as FATTOM.
• Pathogenic bacteria are especially dangerous because they do not change the appearance, taste or smell of food.
• The bacterial growth curve has four phases: lag, log, stationary and death.
• Controlling the conditions for growth (especially temperature and time) is the basis of all food safety practices.

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."