5.4 Preventing Bacterial Growth

A free GCSE Biology lesson for AQA (8461) from OBA School: conditions for growth, keeping food safe, antiseptics, disinfectants and antibiotics, and calculating areas of clear zones. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe the conditions bacteria need to grow and how temperature affects their growth
• Explain how refrigeration, freezing, cooking, salt, sugar and vinegar keep food safe
• Describe the difference between disinfectants, antiseptics and antibiotics
• Calculate numbers of bacteria at different temperatures and the area of a clear zone using πr²

Key words

• Disinfectant: A chemical that kills bacteria on non-living surfaces such as floors and toilets.
• Antiseptic: A chemical that kills or slows the growth of bacteria on skin or living tissue.
• Antibiotic: A medicine that kills bacteria, or stops them growing, inside the body. It has no effect on viruses.
• Clear zone: A clear area around a disc on an agar plate where bacteria have been killed or could not grow. Also called a zone of inhibition.
• Preserve: Treat food so that microorganisms cannot grow in it quickly, so it stays safe for longer.

What bacteria need to grow

Bacteria grow fastest when they have nutrients, water and a warm temperature. Many also need oxygen, and most prefer a neutral pH. Remove one of these and growth slows or stops.

• At low temperatures bacteria grow very slowly. Freezing stops growth but does not kill most bacteria.
• Bacteria that live in humans grow fastest near body temperature, 37 °C.
• At high temperatures the bacteria are killed because their proteins, including enzymes, are denatured.

Keeping food safe

• Fridge (0 to 5 °C); How it prevents bacterial growth Cold slows growth, so bacteria divide much less often.
• Freezer (about −18 °C); How it prevents bacterial growth Stops growth (water is frozen), but bacteria survive and grow again when food thaws.
• Cooking thoroughly; How it prevents bacterial growth High temperature kills bacteria. The centre of the food must get hot enough.
• Salt or sugar; How it prevents bacterial growth Concentrated solutions draw water out of bacterial cells by osmosis, so they cannot grow.
• Pickling in vinegar; How it prevents bacterial growth The low (acidic) pH stops most bacteria growing.
• Drying, canning, vacuum packing; How it prevents bacterial growth Remove water, or seal food from microorganisms and oxygen.

Worked example: room temperature or fridge?

A chicken sandwich contains 20 bacteria. At room temperature they divide every 45 minutes. In a fridge they divide every 4 hours 30 minutes. How many bacteria are there after 9 hours in each place?
Room: 9 h = 540 min. 540 ÷ 45 = 12 divisions. 20 × 2¹² = 20 × 4096 = 81 920.
Fridge: 9 ÷ 4.5 = 2 divisions. 20 × 2² = 80.

Answer: 81 920 bacteria at room temperature, but only 80 in the fridge

Disinfectants, antiseptics and antibiotics

• Disinfectant; Used on Non-living surfaces. Too harsh for skin.; Example use Cleaning a kitchen worktop or toilet
• Antiseptic; Used on Skin and other living tissue; Example use Cleaning a cut, hand gel in a hospital
• Antibiotic; Used on Inside the body (tablets or injection); Example use Treating a bacterial infection such as gonorrhoea

To compare how well chemicals work, paper discs soaked in each one are placed on an agar plate covered in bacteria. After incubation, a clear zone forms around a disc where bacteria were killed. The bigger the clear zone, the more effective the chemical (at that concentration).

Key fact: Antibiotics do not kill viruses. Viruses live inside your own cells, so drugs that harmed them would also damage your cells.

Calculating the area of a clear zone or colony

Clear zones and colonies are roughly circular. Measure the diameter, halve it to get the radius, then use:

area = π r² (r = radius)

Worked example: comparing two antiseptics

Antiseptic X gives a clear zone 18 mm across. Antiseptic Y gives a clear zone 12 mm across.
X: r = 9 mm. Area = π × 9² = 254.5 mm². Y: r = 6 mm. Area = π × 6² = 113.1 mm².
X ÷ Y = 254.5 ÷ 113.1 = 2.25, so X's clear zone is 2.25 times bigger in area.

Answer: X is more effective: 254.5 mm² compared with 113.1 mm²

Exam tip: If the diameter goes up 1.5 times, the area goes up 1.5² = 2.25 times. Always square the radius, not the diameter.

Common mistakes

• Saying freezing kills bacteria. It stops them growing, but they survive.
• Using the diameter instead of the radius in π r².
• Saying antibiotics can treat colds or flu. Antibiotics do not work on viruses.
• Mixing up antiseptics (living tissue) and disinfectants (non-living surfaces).
• Saying high temperatures "kill the enzymes". Enzymes are not alive; they are denatured, which kills the bacteria.

Revision questions

Try these, then check your answers below each one.

Question 1. What does putting food in a freezer do to most bacteria? (Choose from: Kills them all, Stops them growing but does not kill them, Makes them grow slowly, Has no effect.)

Answer: Stops them growing but does not kill them

Working: Freezing stops bacteria growing, but most survive and start growing again when the food thaws. Only cooking kills them.

Question 2. True or false? Antiseptics are used on living tissue such as skin.

Answer: True

Working: True. Antiseptics are used on skin and living tissue. Disinfectants are used on non-living surfaces and are too harsh for skin.

Question 3. A clear zone around an antiseptic disc is 14 mm across. Calculate its area to 1 decimal place.

Answer: 153.9 mm²

Working: r = 7 mm. Area = π × 7² = 153.9 mm².

Related lessons

• Previous lesson: 5.3 Growing Bacteria in the Lab
• Next lesson: 5.5 Preventing Infections
• In this chapter: 5.1 Health and Disease
• In this chapter: 5.2 Pathogens and Disease
• In this chapter: 5.6 Viral Diseases

Continue in the full course

This lesson is section 5.4 (AGCBH0504) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.