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AQA GCSE Biology (9-1): Getting the most out of Required Practicals - Microbiology

Posted by James Simms on November 25, 2019

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Specification location: AQA GCSE Biology Paper 1, Section 4.1 - Cell biology

Summary of method: Using aseptic technique, bacteria are plated onto agar in a petri dish. Discs soaked in various antiseptics/antibiotics are placed on the agar and the dish is incubated for 24-48 hours. Effective antibiotics/antiseptics will show a large clear zone around the discs where bacteria have died; ineffective ones will show a small or no clear zone around the discs. The size of the clear zones can then be measured and the size of the clear zone calculated.

For more detail on the set-up of this experiment, watch our high-quality required practical tutorial on YouTube: Part One and Part Two.

Microbiology required practical variables:

Independent variable: Chemical that the discs are soaked in

Dependent variable: Size of the clear zone

Control variables: Chemical concentration, bacteria type, incubation temperature, size of discs, material of discs

Common issues	Potential solutions
Variability of results	This experiment can show a high degree of variation between repeated results, depending on the conditions a plate is incubated in, how effectively aseptic technique is carried out, how effectively the bacteria were plated and many other factors. Because of this, it would be advisable to have an exemplar petri dish prepared so that results from this can be used instead. Alternatively, an exemplar set of results is included below for analysis.
Student knowledge of aseptic technique	Unfortunately, this experiment requires that aseptic technique be carried out accurately and rapidly. When coupled with the demand on technician time to produce the agar plates, the number of spare agar plates available to compensate for errors will probably be minimal. It is, therefore, imperative that students clearly understand exactly what to do and in what order before they start doing anything. To facilitate this, students should be given a clear set of steps as a method, they should have the technique modelled for them by their teacher and they should view a high-quality tutorial video (TEL YouTube link) beforehand. It may be desirable for pupils to preload the aseptic technique methodology through a flipped learning technique (e.g. viewing and note-taking tutorials and undertaking quizzes on https://www.theeverlearner.com/). This, combined with the other approaches, should ensure that students are as clear as they can be about how they are to plate the bacteria effectively.
Shape of clear zones	It is likely that the clear zones produced by this experiment will not be perfectly circular. To combat this, the diameter of the zone should be measured twice (at right angles to each other) and the mean found. This can then be used to calculate the area of the zone, if required.

Sample microbiology required practical results:

Type of antiseptic	Diameter of clear zone (mm)
Type of antiseptic	1	2	Mean
Kitchen cleaner	11.0	13.0	12.0
Antiseptic liquid	17.0	14.0	15.5
Hand sanitiser	19.0	20.0	19.5
Bleach	24.0	26.0	25.0
Toilet cleaner	7.0	8.0	7.5
Pine disinfectant	10.0	10.0	10.0

Source: AQA GCSE Biology (9-1) required practical handbook

AQA GCSE Science subject-specific terminology:

As students will undoubtedly find variation between their repeated sets of results, this is an ideal opportunity to discuss accuracy and precision.
Again, as the results from this practical are so variable, reproducibility and repeatability in investigations could be discussed.