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Use only the teacher-approved culture and prepared laboratory method. Aseptic technique reduces unwanted contamination and helps keep the comparison about the intended organism. The centre controls handling, incubation and disposal; do not open incubated plates or culture unknown environmental samples as an unsupervised extension.
Video by Malmesbury Education. Use the demonstration alongside this lesson’s specification reference and written explanations.
Viewing focus: Explain aseptic technique and appropriate controls
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Why include a solvent-only disc?
To test whether the carrier affects growth. The carrier is a possible alternative explanation.
After watching 2: What is the radius of a 16 mm diameter zone?
8 mm. Halve the diameter before using πr².
What you'll be able to do
Use only the teacher-approved culture and prepared laboratory method. Aseptic technique reduces unwanted contamination and helps keep the comparison about the intended organism. The centre controls handling, incubation and disposal; do not open incubated plates or culture unknown environmental samples as an unsupervised extension.
Apply the same planned inoculation method and compare defined antimicrobial treatments with a suitable blank or solvent control. Keep disc size, loading, agar conditions, organism and observation interval comparable. Label the plate base so that a removed lid does not detach the identity from the culture.
Pearson CP5.18B specifies pre-poured agar inoculated with bacteria and treatment discs; its aseptic-technique requirements include use of a Bunsen burner. Keep this supervised microbiology procedure separate from flammable-solvent food testing.
A clear region around a treatment disc indicates inhibited visible growth in that assay. Record how the boundary is defined and measure diameter rather than guessing relative size. An irregular zone may need two perpendicular measurements or the approved analysis rule. Report units and retain raw readings.
If the required quantity is clear-zone area excluding the disc, subtract the disc area from the total circular area. Radius is half the diameter. Do not switch between total zone and disc-excluded area partway through a comparison, and avoid overlapping zones that make boundaries hard to attribute.
Repeat the comparison with appropriate independent plates or preparations and consider variation. Several discs on one plate share conditions, so their number is not automatically the number of independent experiments. An untreated or solvent-only disc helps show whether the carrier itself affects growth.
A larger zone is evidence about the tested preparation, loading and organism. Diffusion through agar also matters, so the assay does not automatically rank all products, establish a human treatment dose or prove that a bacterium has evolved resistance. State a controlled conclusion and a specific limitation.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: The same zone contains a disc of diameter 6 mm. Find clear area excluding the disc.
About 286 mm², using the stated circular model.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.