Compare school-approved antimicrobials on a supplied safe culture growing on agar. The clear inhibition zone shows reduced visible growth under these conditions, not proof that all organisms were killed.Labelled apparatus schematic; not to scale. Follow the measurements and connections, not the drawn dimensions.
Use sterile pre-poured plates, supplied culture, sterile swabs/tools, forceps and equal paper discs. Change treatment identity or concentration separately; measure zone diameter or area.
Control culture density, species, agar depth, disc size, treatment volume and incubation conditions. Use a matching water/solvent-only negative control.
Use supervised aseptic technique including the Bunsen burner specified by Pearson. Work only with teacher-approved strains and methods, not unknown environmental cultures at home.
School incubation is typically around 25°C rather than 37°C, reducing the likelihood of growing human pathogens. Never open incubated plates; staff arrange safe disposal.
Method and measurement
Disinfect the bench, wash hands and arrange labelled sterile equipment. Keep the lid open briefly and work near the supervised flame; rising air helps reduce falling contamination.
Spread equal amounts of the bacterial culture (inoculum) evenly across the agar to form a continuous layer called a lawn. Use sterile forceps to add equal-sized discs with equal treatment doses. Space them far enough apart that clear zones do not overlap.
If extracts have a solvent carrier, use a matching carrier control. Equal liquid volumes do not automatically mean equal active-ingredient doses.
Tape the closed lid at a few places rather than sealing the whole rim, allowing gas exchange. Incubate inverted for a fixed duration to reduce condensation dripping on agar.
Measure zones through the closed lid. State consistently whether diameter includes the disc. For non-circular zones use two perpendicular diameters and acknowledge the limitation.
Circular area = πr², with radius = diameter/2. Subtract disc area only if asked for the clear agar area excluding the disc.
Evaluation and conclusions
Repeat using separate plates with comparable bacterial cultures. This tests variation between cultures. Measuring one zone several times checks measurement consistency but does not replace repeating the culture experiment.
Zone size depends on dose, diffusion and stability as well as biological effect. A larger zone does not directly establish the best clinical drug or treatment.
A known effective treatment is a positive control: it checks that the test can show reduced bacterial growth (inhibition). A solvent-only control shows whether the solvent itself has an effect. Poor growth across the lawn may mean the bacteria were not spread successfully.
Match agar depth and incubation conditions, measure with a clear scale or calibrated closed-plate image, and investigate overlaps, label mistakes or contamination.
This test tells you about the treatment’s effect on this organism under these conditions. Choosing a treatment for patients also needs evidence about antibiotic resistance, toxicity and clinical trials.
Flaming tools is only done under the school procedure; avoid bringing flammable treatment solvent near a flame. Keep lids closed and minimise handling after incubation.
Exam skills: planning, precision and evaluation
State what you change (the independent variable), what you measure (the dependent variable) and what you keep the same (control variables). Explain how you keep each control variable constant, rather than just saying “make it fair”.
Accuracy means how close a result is to the true value. Precision means how close repeated measurements are to each other. Resolution is the smallest change an instrument can show. More digits on a display do not automatically mean a more accurate result.
Repeat measurements for each condition, calculate a mean and describe how spread out the results are. This helps assess and reduce the effect of random errors. Repeating cannot fix an error that pushes results consistently in one direction (a systematic error), such as different agar depth for one treatment.
Repeatability means getting similar results when the same person repeats the same method with the same equipment. Reproducibility means getting similar results when someone else, or different suitable equipment, repeats the experiment. Results can be consistent but still inaccurate.
Check that instruments read zero correctly and are calibrated where needed. Read scales at eye level: looking from an angle can give a wrong reading (parallax error). Choose suitable ranges, measurement intervals and scale divisions (resolution).
Write down the original readings straight away in a table, with units in the headings. Use decimal places that match the instrument’s resolution. Keep the original data and round only when needed. Do not discard a result just because it differs from your prediction.
An anomalous result does not fit the pattern of the other results. Repeat that measurement and check the method. Only leave it out of a mean if you have a clear reason; state which result you excluded and why.
For continuous variables, plot the independent variable on the horizontal axis and the dependent variable vertically. Use sensible scales, units and a best-fit line or curve; do not automatically join every point or force the graph through zero.
Find the gradient of a straight best-fit line using a large triangle: vertical change ÷ horizontal change. For a curve, draw a tangent to estimate the gradient at one point. Explain what the gradient shows in this experiment, include its units and use measured values to support your conclusion.
Uncertainty describes the possible range around a measurement. For one reading on a scale, half the smallest division is a useful classroom estimate unless the question says otherwise. If you subtract two readings, both have uncertainty. Percentage uncertainty = absolute uncertainty ÷ measured value × 100. Follow the method specified in the question.
Use results as evidence and then explain what they mean. A pattern linking variables (a correlation) does not prove that one causes the other. If the ranges of repeat results overlap, a claimed difference may be less convincing. Keep conclusions within the range tested and suggest an improvement that tackles a specific error.
Mr Kidman's Classroom