A student wants to grow bacteria on an agar plate to investigate the effect of temperature on bacterial growth. Explain why the student must use aseptic techniques throughout the procedure and describe two specific aseptic techniques the student should use when preparing the culture.

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

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

The student uses an autoclave to sterilise equipment before beginning the investigation.

Model answer (5 marks)

Aseptic techniques are required so that only the bacteria of interest grow on the agar plate; any contaminant would give unreliable, invalid results.

1. Flame sterilise the inoculating loop before and after use – this kills any microbes on the loop.
2. Flame the neck of the culture bottle/flask when opening – this prevents airborne organisms from entering the medium.

These steps keep the culture free from unwanted microorganisms, ensuring the observed growth is due to the intended species.

Examiner tips

  • Explain the purpose of asepsis before giving the techniques; use the words "prevent contamination" and "unreliable results".
  • Show the two techniques in separate bullet points or numbered sentences.
  • Mention the quick replacement or sealing of the agar plate lid to minimise exposure.

Common mistakes

  • Writing only one technique or omitting the purpose of asepsis.
  • Using vague terms like "keep clean" instead of specific actions such as flaming.
  • Leaving the agar plate open for too long or not mentioning the lid.

Mark scheme (5 marks)

  1. Aseptic techniques are needed to prevent contamination of the culture by unwanted microorganisms
  2. Contamination would mean results are unreliable / invalid because the bacteria present would not be the intended species
  3. Specific aseptic technique 1: flaming / passing the inoculating loop through a Bunsen burner flame to sterilise it before and after use
  4. Specific aseptic technique 2: briefly passing / flaming the neck of the culture bottle/flask when it is opened to prevent airborne contamination entering
  5. The lid of the agar plate should be replaced quickly / taped down / not left open to minimise the time the culture is exposed to the air

Key terms in this question

aseptic techniques · culture · agar plate

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