A student is culturing bacteria on agar plates in a school laboratory. Describe the aseptic techniques the student should use when preparing and incubating the agar plates to minimise contamination.

Pearson Edexcel A-Level Biology A (9BI0) — 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 wants to grow a pure culture of a known bacterium. They inoculate an agar plate with the bacterium and leave it to incubate. They must follow strict aseptic techniques throughout the procedure.

Model answer (5 marks)

1. Sterilise the inoculating loop by passing it through a Bunsen burner flame before and after use.
2. Autoclave the agar plates and all other equipment before use.
3. Work close to the Bunsen burner flame so that the upward air current keeps airborne microbes away from the plates.
4. After inoculation, seal the lid of each agar plate with adhesive tape or Parafilm to prevent contamination.
5. Incubate the plates upside‑down at a temperature no higher than 25 °C to avoid condensation dripping onto colonies and to limit growth of human pathogens.

Examiner tips

  • Use the exact wording from the mark scheme – e.g. "sterilise the inoculating loop". Include the order of operations (before and after use). Mention the purpose of each step (e.g. upward air current, sealing, upside‑down incubation).

Common mistakes

  • Forgetting to sterilise the loop after use. Not specifying the temperature limit or the upside‑down orientation. Using the wrong word for the sealing method (e.g. "glue" instead of tape or Parafilm).

Mark scheme (5 marks)

  1. Sterilise the inoculating loop (e.g. by passing it through a flame) before and after use
  2. Sterilise the agar plate / equipment using an autoclave before use
  3. Work near a Bunsen burner flame to create an upward air current that reduces the settling of airborne microorganisms
  4. Seal the lid of the agar plate with adhesive tape / Parafilm after inoculation to prevent contamination
  5. Incubate the plates upside down at no more than 25 °C (in a school laboratory) to prevent condensation dripping onto colonies and to limit growth of human pathogens

Key terms in this question

aseptic techniques · agar plate · contamination

Related

More Microbial growth and culture questions

▶ Try answering this question with AI marking (free) →