Describe how aseptic techniques are used when culturing microorganisms in a school laboratory. Explain why each precaution is necessary.

WJEC A-Level Biology (Wales) — 3.3 Microbiology · Describe · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A student wants to grow a culture of bacteria on an agar plate in the school laboratory. Before beginning, the student is reminded to follow aseptic techniques throughout the procedure.

Model answer (5 marks)

1. Sterilise the agar and all equipment (e.g. loops, pipettes) by autoclaving or boiling before use – this removes any existing microorganisms.
2. Work close to a Bunsen‑burner flame (or inside a laminar‑flow cabinet) so that the upward current of hot air sweeps airborne microbes away from the work surface.
3. Pass the inoculating loop or the neck of the culture bottle through the flame before touching the agar – this kills any microbes on the surface.
4. After inoculation, seal the agar plate with tape or partially close the lid – this prevents environmental contaminants from entering the plate.
5. Incubate the plates at a temperature no higher than 25 °C – this limits the growth of potential human pathogens that could be present in the sample.

Examiner tips

  • Use the exact wording from the mark scheme (e.g. "autoclave", "Bunsen burner", "seal with tape").
  • Explain the purpose of each step – examiners look for the link between technique and contamination control.

Common mistakes

  • Assuming any temperature is fine; students often ignore the 25 °C restriction.
  • Omitting the step of sealing the plate, which is a key control against airborne contamination.

Mark scheme (5 marks)

  1. Sterilise / autoclave the growth medium (agar) and equipment before use
  2. Work near a Bunsen burner flame (or in a laminar flow cabinet) to create an upward current of air that prevents airborne microorganisms from settling
  3. Pass the inoculating loop / neck of the culture bottle through the Bunsen flame to sterilise it before use
  4. Seal the agar plate with tape / partially seal the lid after inoculation to prevent contamination from the environment
  5. Incubate at a temperature no higher than 25 °C to reduce the risk of culturing pathogens that could be harmful to humans

Key terms in this question

aseptic techniques · microorganisms

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