Culturing Microorganisms
Recap Culturing Microorganisms for GCSE Biology with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. A separate (triple) biology topic covering aseptic technique and calculating the area of an inhibition zone with πr² — a favourite Higher-tier calculation.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Bacteria can be grown in the laboratory on an agar gel or in nutrient broth. This is a required practical, and the aseptic technique it uses is examined as much as the results.
Uncontaminated cultures are essential, because stray microorganisms would make the results meaningless. Petri dishes and culture media must be sterilised before use, and inoculating loops are sterilised by passing them through a flame.
In schools, cultures are incubated at a maximum of 25 °C. This is a safety requirement — higher temperatures are closer to body temperature and would encourage the growth of pathogens that could harm humans.
Revision notes
Aseptic technique
Sterilise Petri dishes and culture media before use to kill unwanted microorganisms. Sterilise the inoculating loop by passing it through a flame and letting it cool.
Lift the lid only slightly when inoculating, and secure it with adhesive tape afterwards — but do not seal it completely, as that would allow harmful anaerobic bacteria to grow.
Incubation temperature
School cultures are incubated at a maximum of 25 °C.
The reason is safety: temperatures near 37 °C encourage the growth of pathogens that are harmful to humans. Store the dish upside down so condensation does not drip onto the agar.
Measuring the zone of inhibition
When testing antibiotics or disinfectants, measure the clear zone around each disc where bacteria have not grown.
Area = πr², where r is the radius of the clear zone. A larger zone means the substance is more effective. Always include a control disc so the comparison is fair.
Key points
- Cultures must be uncontaminated to give valid results.
- Sterilise Petri dishes, media and inoculating loops.
- Incubate school cultures at a maximum of 25 °C for safety.
- Tape the lid but do not seal it completely.
- Store dishes upside down to stop condensation dripping.
- Zone of inhibition area = πr².
Worked examples
Example 1
Explain why Petri dishes and culture media must be sterilised before use. [2 marks]
Model answer
Sterilising kills any unwanted microorganisms already present (1 mark), so that the culture is not contaminated and only the microorganism being investigated grows (1 mark).
Example 2
Explain why cultures in schools are incubated at a maximum of 25 °C. [2 marks]
Model answer
Temperatures closer to body temperature would encourage the growth of pathogens (1 mark) that could be harmful to humans (1 mark).
Example 3
A clear zone around an antibiotic disc has a radius of 6 mm. Calculate its area. [2 marks]
Model answer
Area = πr² = π × 6² (1 mark) = 113 mm² to 3 significant figures (1 mark).
Common mistakes
Saying dishes are sealed completely with tape.
They are taped but not fully sealed, because a complete seal allows harmful anaerobic bacteria to grow.
Giving the wrong reason for the 25 °C limit.
It is a safety measure to avoid growing human pathogens, not to slow growth down for convenience.
Using the diameter as r in the area formula.
The formula needs the radius, so halve the diameter first.
Forgetting a control.
A disc with no antibiotic is needed so any effect can be attributed to the antibiotic itself.
Exam tips
- Learn the aseptic steps as a sequence — you may be asked to describe the method.
- Give safety as the reason for the 25 °C limit.
- Halve the diameter before using πr².
- Mention the control disc when describing an antibiotic investigation.
Key terms
- Aseptic technique
- Methods used to prevent contamination by unwanted microorganisms.
- Agar
- The gel medium on which bacterial cultures are grown.
- Inoculating loop
- A wire loop used to transfer microorganisms, sterilised in a flame.
- Zone of inhibition
- The clear area around a disc where bacteria have not grown.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.