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Use the centre’s approved non-pathogenic culture and microbiological procedures. Aseptic handling reduces unwanted contamination and helps keep the measured growth attributable to the intended organism. Sterile materials, minimal exposure and suitable disinfection protect the validity of the test as well as safe practice.
Video by Malmesbury Education. Foundation recap: the written lesson develops the A-level detail.
Viewing focus: Explain aseptic agar and broth techniques
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: An aseptic technique helps reduce:
Contamination by unintended organisms. It supports both validity and appropriate handling.
After watching 2: A solvent control tests whether:
The solvent or handling contributes to inhibition. Separate treatment effects from the vehicle.
What you'll be able to do
Use the centre’s approved non-pathogenic culture and microbiological procedures. Aseptic handling reduces unwanted contamination and helps keep the measured growth attributable to the intended organism. Sterile materials, minimal exposure and suitable disinfection protect the validity of the test as well as safe practice.
Apply antimicrobial treatments in a standardised way, for example using appropriately prepared discs in the approved agar method. Keep organism, inoculum, medium, treatment quantity and incubation conditions comparable. Include an untreated or solvent control to separate the antimicrobial effect from handling or solvent effects.
After the supervised incubation and observation procedure, measure the inhibition zone consistently. If approximately circular, record diameters in perpendicular directions and state whether the disc is included. If calculating area, use radius rather than diameter and subtract disc area only if reporting clear area outside the disc.
Use independent plates or suitable replicates as designed, and avoid choosing only the clearest zone. Plate handling, sealing, incubation and disposal follow the centre’s procedure; do not open an incubated culture simply to inspect a result more closely. The experimental record should include the actual controls and measurements.
A larger zone can indicate greater inhibition under the assay conditions, but zone size depends on diffusion through agar as well as microbial susceptibility. Different substances can spread at different rates. Disc concentration, solubility and growth conditions can therefore confound a direct ranking of intrinsic effectiveness.
This activity links to natural selection, but one short inhibition assay does not by itself demonstrate evolutionary change in allele frequency or predict clinical performance. A resistance conclusion should be supported by an appropriate design and interpretation. Explain whether the data concern immediate growth suppression, inherited susceptibility or population change.
A suitable supervised dilution series can reduce a non-pathogenic culture to a countable sample. Record the total dilution factor and plated volume if estimating viable colony-forming units per volume. A colony-forming unit need not be a single cell: clumps can form one colony, and cells unable to grow under those conditions are not counted.
A broth’s turbidity can provide an optical growth proxy after an appropriate blank and calibration, but it does not automatically distinguish live from dead cells. Agar growth and broth absorbance answer different measurement questions. Use only teacher-approved organisms, handling, incubation, sealing and disposal; examine prepared results as directed rather than reopening an incubated culture.
Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.
Common mistakes to avoid
Key takeaways
Question 1: Two compounds have different zone diameters but also very different diffusion rates through agar. Why is a simple potency ranking uncertain?
Identify the measurement confounder rather than assuming larger always means better in every context.
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.