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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. The demonstration develops the shared agar and aseptic methods. This lesson applies those methods to plant extracts, with the extraction and solvent controls made explicit.
Viewing focus: Prepare a controlled plant-extract comparison
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.
Define plant species and material, extraction and any dilution using the centre’s method. Equal disc size does not guarantee equal active-compound amount: extraction efficiency, solvent and concentration can vary. Keep preparation and loading comparable and include a solvent-only control processed in the same way.
An inhibition zone supports a growth-suppression effect of that preparation on the tested organism under the assay conditions. It does not identify the active molecule or establish that an extract is a safe or effective medicine. Repeat independent preparations where the question concerns variation in plant material, rather than only applying one extract to several positions.
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.