New

Year 11

AQA

Higher

The effect of antimicrobial substances on bacterial growth: analysis and conclusions

I can compare the effectiveness of antimicrobial substances on bacterial growth on an agar plate by measuring clear ‘zones of inhibition’.

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New

Year 11

AQA

Higher

The effect of antimicrobial substances on bacterial growth: analysis and conclusions

I can compare the effectiveness of antimicrobial substances on bacterial growth on an agar plate by measuring clear ‘zones of inhibition’.

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Share activities with pupils

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Lesson details

Key learning points

The diameter of a clear zone of inhibition around an antimicrobial disc can be measured.
The areas of clear zones can be calculated using 𝜋r² and compared to investigate antimicrobial effectiveness.
The larger the clear zone of inhibition, the more effective the antimicrobial has been in killing the bacteria.
The importance of testing the effectiveness of antimicrobials in clinical settings.

Keywords

Antimicrobial - A substance that slows down or stops the growth of microorganisms, such as bacteria.
Agar plate - A Petri dish containing a growth medium solidified with agar jelly, used for culturing microorganisms.
Zone of inhibition - The clear area around an antimicrobial disc on an agar plate where bacterial growth has been stopped.
Aseptic technique - Practical methods used to prevent contamination by unwanted microorganisms.

Common misconception

Pupils often use the diameter instead of the radius when calculating the area of a clear zone of inhibition around a disc.

Remind pupils that we use 𝜋r² to calculate the area, so they need to find the radius first.

It is simpler to use the value of 3.14 for 𝜋, rather than using the 𝜋 button on a calculator, as this yields answers to fewer decimal places and makes answers easier for pupils to round and work with. Examination questions often instruct pupils to use 3.14 for 𝜋.

Teacher tip

Equipment

agar plate upon which bacteria have been cultured with antimicrobial discs, ruler, calculator

Content guidance

Risk assessment required - equipment

Supervision

Adult supervision required

Licence

This content is © Oak National Academy Limited (2025), licensed on

Open Government Licence version 3.0

except where otherwise stated. See Oak's terms & conditions (Collection 2).

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Worksheet

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Starter quiz

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6 Questions

Q1.

What is this?

Correct Answer: Agar plate, agar plate, agar

Q2.

What letter represents the radius of this circle?

Correct answer: A

Q3.

Using a value of 3.14 for 𝜋, calculate the area of this circle.

28.3 cm²

9.4 cm²

Correct answer: 7.1 cm²

4.7 cm²

Q4.

Match the aseptic technique to its reason.

Correct Answer:flaming the inoculating loop,sterilises the wire to avoid contaminating the bacteria bottle

sterilises the wire to avoid contaminating the bacteria bottle

Correct Answer:lift the plate lid to a 45° angle,prevents unwanted microorganisms from settling on the agar

prevents unwanted microorganisms from settling on the agar

Correct Answer:flaming the neck of the bottle,prevents bacteria contaminating the air

prevents bacteria contaminating the air

Correct Answer:spraying disinfectant,kills microorganisms on work surfaces

kills microorganisms on work surfaces

Q5.

The Oak pupils are discussing aseptic technique. Who is correct?

Lucas: You shouldn't use a Bunsen burner because you could burn yourself.

Correct answer: Sofia: You should use a Bunsen burner so microorganisms in the air won't settle.

Laura: You should use a Bunsen burner to kill bacteria in your agar plate.

Jun: You shouldn't use a Bunsen burner as agar is flammable.

Q6.

Which variables should be controlled in the investigation into the effect of antimicrobial substances on bacterial growth.

the size of the clear zone

the type of antiseptic

Correct answer: incubation time

Correct answer: incubation temperature

Correct answer: the size of the paper discs

Exit quiz

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6 Questions

Q1.

What are the clear areas around the paper discs?

areas with lots of bacterial growth

areas of antiseptic

Correct answer: areas where bacteria can not grow or have been killed

areas where the agar can not grow

areas where the antiseptic has covered via active transport

Q2.

Which antibiotic is the most effective?

Correct answer: b

Q3.

Put these steps in order to show how you would test which antibiotics should (and should not) be used to treat a patient's disease.

1 - Take a sample of bacteria from the patient.

2 - Grow the sample bacteria on an agar plate in culture.

3 - Add antibiotic discs to the bacteria and culture the plate again.

4 - Compare the sizes of any clear zones of inhibition around the discs.

Q4.

The area of the zone of inhibition is mm².

Correct Answer: 200.96, 201, 201.06, 201.1

Q5.

Which antibiotic are bacteria resistant to?

Correct answer: a

Q6.

Which antimicrobial is the least effective?

Correct Answer: C, c, antimicrobial c