A student is investigating the effect of different antiseptics on the growth of bacteria. The student places filter paper discs soaked in different antiseptics onto an agar plate that has been inoculated with bacteria, then incubates the plate at 25 °C for 48 hours. After incubation, the student observes clear zones around some of the discs where no bacterial colonies have grown. Explain why aseptic techniques must be used throughout this investigation and describe TWO specific aseptic techniques the student should use when preparing the inoculated agar plate.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Antiseptics are chemicals that kill bacterial and fungal pathogens. When testing their effectiveness, it is important that any results are due only to the antiseptic being tested and not to contamination from other sources.
Model answer (5 marks)
Aseptic techniques are essential to prevent contamination of the agar plate by unwanted microorganisms from the environment, which would give misleading results and make it impossible to attribute zones of inhibition to the antiseptic alone.
1. The inoculating loop or spreader must be sterilised by flaming in a Bunsen‑burner flame and allowed to cool before touching the agar, to kill any microbes on the instrument.
2. The agar plate lid should be kept closed as much as possible; it should only be briefly lifted at an angle when inoculating, so that airborne organisms do not fall onto the surface.
1. The inoculating loop or spreader must be sterilised by flaming in a Bunsen‑burner flame and allowed to cool before touching the agar, to kill any microbes on the instrument.
2. The agar plate lid should be kept closed as much as possible; it should only be briefly lifted at an angle when inoculating, so that airborne organisms do not fall onto the surface.
Examiner tips
- Use the exact phrase "zones of inhibition" to show understanding of the assay.
- Show the two specific techniques in separate numbered points.
- Explain the purpose of each technique in one sentence.
Mark scheme (5 marks)
- Aseptic techniques are needed to prevent contamination of the agar plate by unwanted microorganisms from the environment
- Contamination would produce misleading results / make it impossible to determine whether zones of inhibition are due to the antiseptic or unwanted microorganisms
- Specific aseptic technique 1: The inoculating loop / spreader should be sterilised by flaming / passed through a Bunsen burner flame before use (and allowed to cool) to kill any microorganisms on it
- Specific aseptic technique 2: The lid of the agar plate should be kept closed as much as possible / only briefly lifted at an angle when inoculating to prevent airborne microorganisms falling onto the agar
- The neck of any container holding the bacterial culture / broth should be flamed before and after use OR work should be carried out near a Bunsen burner flame to create an upward convection current that reduces airborne contamination
Key terms in this question
aseptic techniques · agar plate · antiseptics
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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