A scientist wishes to investigate whether a new antiseptic is effective against a bacterial pathogen. The scientist plans to culture the bacteria on agar plates, apply the antiseptic, and then incubate the plates. Explain why the scientist must use aseptic techniques throughout this procedure, and describe two specific aseptic techniques the scientist should use when preparing the agar plates.

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).

Antiseptics are chemicals that kill bacterial and fungal pathogens. When culturing bacteria on agar plates in a laboratory, contamination from unwanted microorganisms can make results unreliable.

Model answer (5 marks)

Aseptic techniques are required to keep the culture free of unwanted microorganisms; any contamination would make the results unreliable because it would be impossible to know whether the antiseptic was acting on the target pathogen or on a contaminant.

Two specific aseptic techniques:
1. Sterilise the agar plates (e.g. by autoclaving) before use to kill any existing microorganisms.
2. Sterilise the inoculating loop or instrument by passing it through a flame until it glows red hot before each transfer of bacteria.

Additionally, keep the plate lid closed and open the plate only briefly when inoculating to minimise airborne contamination.

Examiner tips

  • Use the exact phrase "prevent contamination" and explain the consequence of contamination. Mention the two techniques explicitly and give a brief method for each. Keep the answer concise – 5 marks can be earned with 3–4 short sentences.

Common mistakes

  • Writing a general statement about cleanliness without linking it to contamination. Forgetting to mention that the plates must be sterilised before use. Using the wrong method for sterilising the loop (e.g. saying "wash with soap" instead of flaming).

Mark scheme (5 marks)

  1. Aseptic techniques are needed to prevent contamination of the culture by unwanted microorganisms
  2. Contamination would make results unreliable / it would be unclear whether the antiseptic was affecting the target pathogen or a contaminant
  3. The agar plates / growth medium should be sterilised (e.g. using an autoclave) before use to kill any existing microorganisms
  4. The inoculating loop / instrument used to transfer bacteria should be sterilised, e.g. by passing it through a flame until it glows red hot
  5. The agar plate should be opened as briefly as possible / the lid should be kept on except when inoculating, to prevent airborne contaminants entering

Key terms in this question

aseptic techniques · culture · agar plate

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