What you'll learn
This topic examines how microorganisms affect food safety, the conditions that promote or prevent their growth, and the practical measures needed to prevent food poisoning. Understanding bacterial contamination, toxin production and safe food handling practices is essential for answering questions worth 8-12 marks in Paper 1 and extended response questions in Paper 2.
Key terms and definitions
Microorganisms — microscopic living organisms including bacteria, yeasts, moulds and viruses that can cause food spoilage or food poisoning.
Pathogenic bacteria — harmful bacteria that cause foodborne illness when consumed in sufficient numbers or when their toxins are ingested.
Cross-contamination — the transfer of harmful bacteria from one food item (typically raw) to another (typically ready-to-eat) via hands, surfaces, equipment or utensils.
Food poisoning — illness resulting from consuming food contaminated with pathogenic bacteria, their toxins, viruses or other harmful substances, characterised by symptoms such as nausea, vomiting, diarrhoea and abdominal pain.
Danger zone — the temperature range between 5°C and 63°C where bacteria multiply most rapidly, doubling in number every 10-20 minutes under optimal conditions.
Binary fission — the method by which bacteria reproduce asexually, splitting into two identical cells, enabling rapid population growth when conditions are favourable.
Spores — resistant structures formed by certain bacteria (such as Clostridium botulinum) that can survive extreme conditions including high temperatures and lack of moisture.
High-risk foods — foods that provide ideal conditions for bacterial growth, typically high in protein and moisture content, including cooked meat, dairy products, cooked rice, and seafood.
Core concepts
Types of microorganisms in food
Bacteria are single-celled organisms that reproduce rapidly under suitable conditions. While some bacteria are beneficial (used in yoghurt and cheese production), pathogenic bacteria cause food poisoning. The main types include:
Aerobic bacteria require oxygen to multiply. Examples include Salmonella species found on raw poultry and eggs.
Anaerobic bacteria multiply without oxygen, often in vacuum-packed foods or canned products. Clostridium botulinum produces a deadly toxin in improperly canned low-acid foods.
Facultative anaerobes can multiply with or without oxygen. Staphylococcus aureus is commonly found on human skin and in the nose, and produces heat-resistant toxins when allowed to multiply in food.
Yeasts are single-celled fungi used beneficially in bread and alcoholic beverage production, but can also cause food spoilage through fermentation, creating off-flavours and gas production.
Moulds are multi-cellular fungi that grow as thread-like structures. Some moulds are beneficial (Penicillium in blue cheese), whilst others produce dangerous mycotoxins. Visible mould growth indicates deeper contamination within the food.
Conditions for bacterial growth
Bacteria require specific conditions to multiply. These are remembered using the mnemonic FATTOM:
Food — bacteria need nutrients, particularly protein. High-risk foods include cooked meat, poultry, fish, eggs, dairy products, cooked rice, pasta and foods containing these ingredients.
Acidity (pH) — most pathogenic bacteria prefer neutral conditions (pH 6.5-7.5). Foods with pH below 4.5 (pickles, citrus fruits) or above 9 inhibit bacterial growth. This principle is used in food preservation through pickling and acidification.
Temperature — the danger zone (5-63°C) allows rapid multiplication. Bacteria multiply fastest at body temperature (37°C). Below 5°C, multiplication slows significantly; above 63°C, most bacteria are destroyed. Spore-forming bacteria survive boiling and require temperatures above 121°C (achieved in pressure cooking or commercial canning).
Time — given suitable conditions, bacteria double every 10-20 minutes. A single bacterium can produce over 16 million bacteria in 8 hours. The 2-hour rule states that high-risk foods should not remain in the danger zone for more than 2 hours cumulatively.
Oxygen — requirements vary by bacterial type. Modified atmosphere packaging reduces oxygen to slow aerobic bacterial growth.
Moisture — bacteria require moisture to absorb nutrients. Water activity (aw) measures available moisture; most pathogenic bacteria need aw above 0.85. Drying, salting and adding sugar reduce water activity, preserving foods by making moisture unavailable to bacteria.
Major foodborne bacteria and their characteristics
Salmonella causes salmonellosis with symptoms appearing 12-72 hours after consumption. Sources include raw poultry, eggs, meat and unpasteurised milk. Thorough cooking to 75°C core temperature kills Salmonella. Cross-contamination from raw chicken to salad vegetables is a common cause of outbreaks.
Campylobacter is the most common cause of bacterial food poisoning in the UK. Found in raw poultry, unpasteurised milk and untreated water, it causes severe diarrhoea and abdominal cramps 2-5 days after consumption. Campylobacter is easily destroyed by proper cooking but spreads readily through cross-contamination.
Escherichia coli (E. coli) — most strains are harmless, but pathogenic strains like E. coli O157:H7 produce dangerous toxins causing severe illness, particularly in young children and elderly people. Sources include undercooked minced beef, unpasteurised milk and contaminated water. Cooking to 75°C kills E. coli.
Staphylococcus aureus produces heat-stable enterotoxins that cause rapid-onset illness (1-6 hours). The bacteria are destroyed by cooking, but toxins remain active even after reheating. Sources include foods handled extensively (sandwiches, cream cakes) where the handler has infected cuts or is carrying the bacteria in their nose or throat.
Clostridium perfringens forms heat-resistant spores commonly found in soil and on raw meat. The bacteria multiply rapidly in cooked meat dishes cooled slowly or kept warm rather than hot. Symptoms appear 8-16 hours after consumption. This is a particular risk with large portions of stews, curries and casseroles.
Listeria monocytogenes multiplies at refrigeration temperatures, making it particularly dangerous. High-risk groups include pregnant women, as it can cause miscarriage. Sources include soft ripened cheeses (Brie, Camembert), pâté, and pre-prepared salads. Thorough cooking kills Listeria.
Bacillus cereus is associated with cooked rice kept warm or left at room temperature. The bacteria produce heat-stable toxins causing vomiting 1-5 hours after consumption. Cooked rice should be cooled quickly, refrigerated, and reheated only once to piping hot.
Types of contamination
Biological contamination involves microorganisms (bacteria, viruses, parasites) or their toxins. This is the most common cause of food poisoning. Sources include raw food, food handlers, pests and contaminated water.
Chemical contamination occurs through cleaning chemicals, pesticides, food additives used incorrectly, or toxins from bacteria. Storage of cleaning products away from food preparation areas and following manufacturer instructions prevents chemical contamination.
Physical contamination involves foreign objects such as glass, metal, plastic, hair, jewellery, dirt or pest droppings. Food handlers must tie back hair, remove jewellery (except plain wedding bands), cover cuts with blue waterproof plasters, and report damaged equipment immediately.
Cross-contamination pathways include:
- Raw meat juices dripping onto cooked foods in refrigerators
- Using the same chopping board for raw chicken then salad vegetables
- Handling raw foods then ready-to-eat foods without washing hands
- Using the same cloth to wipe raw meat preparation surfaces then clean plates
- Storing raw and cooked foods together without adequate separation
Food safety controls and prevention
Personal hygiene for food handlers:
- Wash hands thoroughly with soap and hot water for 20 seconds before handling food, after using the toilet, after handling raw foods, after touching face/hair, and after handling waste
- Cover cuts and sores with blue waterproof plasters
- Tie back long hair and cover with a hairnet or hat
- Wear clean protective clothing designated for food preparation only
- Do not wear jewellery except plain wedding bands
- Keep fingernails short and clean; avoid nail polish
- Report illness, particularly vomiting and diarrhoea; do not handle food for 48 hours after symptoms cease
Temperature control:
- Store chilled foods below 5°C; check refrigerator temperatures daily
- Cook food to 75°C core temperature (use a temperature probe)
- Cool cooked foods rapidly (within 90 minutes to below 5°C)
- Reheat food only once to 75°C throughout
- Keep hot food above 63°C during service
- Defrost frozen foods thoroughly in the refrigerator, never at room temperature
Prevention of cross-contamination:
- Use separate colour-coded chopping boards: red for raw meat, blue for raw fish, yellow for cooked meat, green for salad/fruit, white for dairy/bakery, brown for vegetables
- Store raw meat on the bottom shelf of the refrigerator in sealed containers
- Store cooked and ready-to-eat foods on upper shelves, covered
- Use separate equipment and utensils for raw and cooked foods
- Clean and sanitise work surfaces between tasks
- Wash hands between handling raw and cooked foods
Food storage principles:
- Follow FIFO (First In, First Out) stock rotation
- Check use-by dates; do not use foods past these dates
- Store foods in appropriate containers with lids
- Keep dry goods in cool, dry, well-ventilated storage areas
- Maintain cold chain during delivery and storage
- Store raw and cooked foods separately
Food preservation methods
Understanding preservation links directly to preventing bacterial growth by removing one or more conditions needed for multiplication:
Heating methods destroy bacteria and enzymes. Pasteurisation heats milk to 72°C for 15 seconds, killing pathogenic bacteria whilst maintaining flavour. Sterilisation uses ultra-high temperature (UHT) treatment at 132°C for 1-2 seconds, destroying all microorganisms and spores. Canning uses high temperatures (116-121°C) under pressure to destroy spores in sealed containers.
Chilling and freezing slow or stop bacterial multiplication but do not kill bacteria. When frozen foods thaw, bacteria resume multiplying. Blast freezing creates small ice crystals, maintaining food quality better than slow freezing.
Drying and dehydration remove moisture, preventing bacterial growth. Examples include dried herbs, pasta, powdered milk and freeze-dried coffee.
Chemical preservation uses additives: sulphur dioxide (E220) in dried fruit prevents browning and microbial growth; sodium nitrite (E250) in cured meats prevents Clostridium botulinum and maintains colour; benzoic acid in soft drinks inhibits yeast and mould growth.
Pickling uses vinegar (acetic acid) to create acidic conditions (pH below 4.5) preventing bacterial growth in products like pickled onions and chutneys.
Salting and sugaring reduce water activity, making moisture unavailable to bacteria. Used in jam (high sugar), bacon (salt) and salted fish.
Modified atmosphere packaging (MAP) replaces air with gases (carbon dioxide, nitrogen) that slow bacterial growth and extend shelf life in products like pre-packed salads and fresh meat.
Worked examples
Example 1: A restaurant received complaints of food poisoning from customers who ate chicken curry the previous evening. The curry was cooked thoroughly at lunchtime, left in a warm kitchen to cool for 4 hours, then refrigerated overnight and reheated before evening service.
(a) Identify which bacterium most likely caused the food poisoning. [1 mark]
Answer: Clostridium perfringens [1]
(b) Explain why this bacterium was able to cause food poisoning in this situation. [4 marks]
Answer:
- The bacterium forms heat-resistant spores that survived the initial cooking [1]
- Slow cooling for 4 hours allowed the food to remain in the danger zone (5-63°C) [1]
- During this time the spores germinated and bacteria multiplied rapidly in the high-protein food [1]
- Reheating may have killed the bacteria but the curry had already reached dangerous contamination levels [1]
(c) Suggest two changes the restaurant should make to prevent this happening again. [2 marks]
Answer:
- Cool the curry rapidly (within 90 minutes) by dividing into shallow containers and using a blast chiller [1]
- Refrigerate immediately once cooled to below 5°C to prevent bacterial multiplication [1]
Example 2:
(a) Explain the term 'cross-contamination' giving a specific example. [3 marks]
Answer:
- Cross-contamination is the transfer of harmful bacteria from one food to another [1]
- This typically occurs from raw foods to ready-to-eat foods [1]
- Example: Using the same chopping board for raw chicken then for preparing a salad without washing in between, transferring Salmonella or Campylobacter bacteria to the salad [1]
(b) Describe three different ways cross-contamination can occur in a kitchen. [3 marks]
Answer:
- Through food handlers touching raw meat then handling cooked foods without washing hands [1]
- Raw meat juices dripping from upper refrigerator shelves onto cooked foods stored below [1]
- Using the same cloth to wipe surfaces after preparing raw meat then wiping clean plates [1]
Example 3: A nursery cook is planning meals for young children.
(a) State why young children are considered a high-risk group for food poisoning. [1 mark]
Answer: Their immune systems are not fully developed / they have lower resistance to infection [1]
(b) The menu includes cooked rice. Explain why Bacillus cereus food poisoning is a particular risk with cooked rice and state two precautions the cook should take. [4 marks]
Answer:
- Bacillus cereus spores survive cooking and germinate when rice is kept warm or at room temperature [1]
- The bacteria multiply rapidly and produce heat-stable toxins [1]
- Precaution 1: Cool cooked rice quickly and refrigerate within 90 minutes [1]
- Precaution 2: Reheat rice only once to piping hot (75°C) throughout or do not reheat at all [1]
Common mistakes and how to avoid them
Mistake: Stating that refrigeration kills bacteria. Correction: Refrigeration (below 5°C) slows bacterial multiplication but does not kill bacteria. When food returns to room temperature, bacteria resume multiplying. Freezing also stops multiplication but does not kill bacteria. Only adequate heat treatment (cooking to 75°C) kills most pathogenic bacteria.
Mistake: Confusing use-by dates with best-before dates in exam answers. Correction: Use-by dates appear on high-risk, perishable foods (fresh meat, dairy products) and relate to food safety; food should not be eaten after this date. Best-before dates appear on longer-life foods (biscuits, canned goods) and relate to quality; food is safe but may have reduced quality after this date.
Mistake: Describing bacterial multiplication as "breeding" or "growing bigger." Correction: Bacteria reproduce through binary fission, a process where one bacterial cell divides into two identical cells. Under optimal conditions this occurs every 10-20 minutes. Use the term "multiplication" rather than "growth" when referring to increasing bacterial numbers.
Mistake: Listing food poisoning symptoms without linking to the specific bacterium asked about. Correction: Different bacteria cause different patterns: Staphylococcus aureus causes rapid onset (1-6 hours) vomiting; Salmonella causes diarrhoea and fever after 12-72 hours; Clostridium perfringens causes abdominal pain and diarrhoea after 8-16 hours. Match symptoms and timing to the bacterium specified in the question.
Mistake: Writing that washing hands "gets rid of germs" without explaining proper technique. Correction: Explain that hands must be washed with soap and hot water for at least 20 seconds, covering all surfaces including between fingers and under nails, then dried with disposable paper towels. State specific times when handwashing is required: before handling food, after touching raw foods, after using the toilet, after handling waste.
Mistake: Giving vague answers about temperature control such as "keep food hot or cold." Correction: Use specific temperatures: refrigerate below 5°C, cook to 75°C core temperature, keep hot food above 63°C, and identify the danger zone as 5-63°C. Measurements and precision earn marks in CIE IGCSE Food and Nutrition examinations.
Exam technique for "Food Safety: Microbiology and Contamination"
Command words and mark allocation: Questions using "state" or "identify" require brief factual answers (1 mark each). "Describe" questions need more detail about what/how (2-3 marks). "Explain" questions require reasons why something happens, using connectives like "because," "therefore," "this means that" (3-4 marks). Allocate approximately 1 minute per mark.
Structuring extended answers: For 4-6 mark questions, write in clear, separate points. Start each point on a new line or use bullet points. Make sure each point contains a distinct piece of information. For "explain" questions, follow the point-reason-consequence structure: state what happens, explain why, describe the result.
Using scientific terminology: Marks are awarded for precise language. Use terms like "pathogenic bacteria," "binary fission," "core temperature," "water activity," and specific bacterial names in italics (Salmonella, Campylobacter). Avoid colloquial terms like "germs" or "bugs."
Linking theory to practical contexts: Questions often present scenarios (restaurant, school kitchen, home cooking). Apply your knowledge to the specific situation given. If the question mentions raw chicken, discuss Salmonella and Campylobacter; if it mentions rice, mention Bacillus cereus; if it involves slow-cooled stew, reference Clostridium perfringens.
Quick revision summary
Pathogenic bacteria cause food poisoning when conditions allow multiplication: nutrients, suitable temperature (danger zone 5-63°C), time, moisture, correct pH and oxygen. High-risk foods include cooked meat, rice, dairy and seafood. Major bacteria include Salmonella, Campylobacter, E. coli, Staphylococcus aureus, Clostridium perfringens, Listeria and Bacillus cereus. Prevent contamination through personal hygiene, temperature control, preventing cross-contamination using colour-coded equipment, and proper food storage. Preservation methods work by removing conditions bacteria need. Cook food to 75°C, chill below 5°C, and follow the 2-hour rule for foods in the danger zone.