A student is culturing bacteria on an agar plate in a school laboratory. Describe the aseptic techniques the student should use when culturing the bacteria, and explain why each technique is important.

Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 5.1 Microbial growth and culture · Describe · 5 marks · View as Markdown

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

A student is growing a culture of bacteria on an agar plate. They must follow aseptic techniques to ensure the culture is not contaminated and that no harmful microorganisms are released into the environment.

Model answer (5 marks)

1. Sterilise the inoculating loop by passing it through a Bunsen burner flame until it glows red.
2. Use a freshly sterilised agar plate; the medium is autoclaved to kill any existing microbes.
3. Keep the Petri dish lid closed, lifting it only a few centimetres or taping it down while inoculating.
4. Work close to the flame or in a laminar‑flow hood so that the rising hot air creates a sterile zone.
5. Handle all equipment with clean gloves and avoid touching the agar surface with unsterilised objects.

Each step removes or prevents the introduction of unwanted microorganisms, ensuring that the observed growth is due solely to the bacteria being studied and protecting the environment from accidental release of harmful microbes.

Examiner tips

  • Use the exact wording from the mark scheme (e.g., "sterilised", "autoclave", "Bunsen burner flame").
  • Show the logical link between each technique and its purpose.
  • Keep the answer concise – 5 points, 1 sentence each.

Common mistakes

  • Forgetting to mention the lid should be lifted minimally or taped down.
  • Using vague terms like "clean" instead of "sterilised".

Mark scheme (5 marks)

  1. The inoculating loop / equipment must be sterilised, e.g. by passing it through a flame or using an autoclave
  2. Sterilising equipment prevents contamination of the culture with unwanted microorganisms / prevents introduction of foreign bacteria
  3. The agar plate / growth medium should be sterilised before use, e.g. using an autoclave, to kill any pre-existing microorganisms
  4. The Petri dish lid should be lifted as little as possible / taped down / only partially opened when inoculating
  5. Working near a Bunsen burner flame / in sterile conditions reduces airborne contamination because the rising hot air prevents microorganisms from settling onto the plate

Key terms in this question

aseptic techniques · agar plate

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