Why are plates incubated at no more than 25 °C in a school lab?

To reduce the chance of growing pathogens that thrive at body temperature

Why is the inoculating loop heated until red-hot?

To kill any microorganisms on it

Disc A has a clear zone of 20 mm and disc B a clear zone of 8 mm. What does this show?

A is more effective at stopping bacterial growth

Core practical: antiseptics, antibiotics and aseptic technique

Core practical 5.18 tests how well antiseptics, antibiotics or plant extracts stop bacteria growing. Aseptic technique keeps the culture pure and keeps you safe.

GCSE · 1BI0 · 5.17B, 5.18B, 5.19B (Biology only)
About 16 minutes

Practise this lesson free (6 questions)

Aseptic technique

1. Sterilise the inoculating loop by heating it in a Bunsen flame until red-hot, then let it cool.
2. Use sterile agar plates and lift the lid only a little, near a lit Bunsen burner (the rising air carries microbes away).
3. Secure the lid with two pieces of tape, not sealed all round, so oxygen can get in and harmful anaerobic bacteria do not grow.
4. Incubate at 25 °C at most in school, so pathogens that grow at body temperature (37 °C) are not encouraged.
5. Store the plate upside down so condensation does not drip onto the agar.

The method

1. Spread bacteria evenly over the agar.
2. Soak paper discs in different antiseptics (and one in sterile water as a control).
3. Place the discs on the agar, tape the lid and incubate.
4. A clear zone (zone of inhibition) appears where bacteria could not grow. Measure its diameter.
The bigger the clear zone, the more effective the substance.

Area of a clear zone

area = π r², where r is the radius (half the diameter).
Example: diameter 12 mm, so r = 6 mm. Area = π × 6² = 113.1 mm² (to 1 decimal place).

Worked example

A clear zone has a diameter of 16 mm. Calculate its area to 1 decimal place.

r = 16 ÷ 2 = 8 mm
Area = π × 8² = π × 64
= 201.1 mm²

Answer: 201.1 mm²

Key idea

Flame the loop, keep lids mostly closed, tape (not seal) the lid and incubate at 25 °C. Discs soaked in antiseptic make clear zones; a bigger zone means a more effective substance. Area = πr².

Quick check

Try two quick questions

See if it has sunk in. Answers are marked straight away.

Extra practice

2 more questions

Try each one, then tap to check.

Why should cultures not be grown at 37 °C in a school lab?
1. It is too cold
2. Agar melts at 37 °C
3. Pathogens that infect humans grow well at body temperature
4. Bacteria cannot grow at 37 °C
Show the answer
Answer: Pathogens that infect humans grow well at body temperature
Lower temperatures reduce the risk.
Why are Petri dishes and agar sterilised before use?
1. To help bacteria grow faster
2. To kill any microorganisms already present
3. To make the agar melt
4. To add nutrients
Show the answer
Answer: To kill any microorganisms already present
Only the chosen bacteria should grow.

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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