AQA GCSE Testing Antiseptics, Antibiotics and Disinfectants (Biology)
Testing Antiseptics, Antibiotics and Disinfectants
Antiseptics, antibiotics and disinfectants can be tested to see how effective they are at stopping the growth of bacteria.
Uncontaminated cultures of microorganisms are required for investigating the action of disinfectants, antiseptics and antibiotics. This is to ensure that only the microorganisms under test are present.
Method
Small paper discs are soaked in the antiseptic, antibiotic or disinfectant.
Each disc is labelled for reference purposes so we know what it was soaked in.
It might be we are looking at how different concentrations of antibiotics affect microorganism growth, or which antibiotic is most effective at controlling growth of microrganisms.
In the experiment below, 4 different types of antibiotic were tested (Antibotic A,B,C and D).
The soaked discs are then placed onto agar which contains the bacteria under test.
The petri dish is incubated for 24 hours. After incubation, the size of the clear zone or zone of inhibition is measured around each paper disc. The larger the clear zone the more effective the substance on the disc is at controlling the growth of microorganisms.
Results of zone of inhibition/clear zone experiment
Antibiotic A had a small clear zone, so it had a mild effect on the growth of microorganisms
Antibiotic B had no clear zone, so it is has no effect on growth of microorganisms
Antibiotic C has a moderate sized clear zone so it has a greater effect on growth of microorganisms
Antibiotic D has the largest clear zone, so this antibiotic has the strongest effect on stopping growth of the microorganisms.
Calculating the area of the clear zone
In the following calculations we will assume the value of π = 3.14
The larger the area of the clear zone the more effective the antibiotic, antiseptic or disinfectant is at killing the microorganisms.
Rearranging the formula
The formula can be rearranged to obtain the radius of the clear zone.
Practice Question
1.When investigating how effective antibiotics are at controlling the population of bacteria, it is important to use uncontaminated cultures. Explain why.
2. Four different disinfectants were placed onto different paper discs. The paper discs were then placed onto the surface of agar as shown and incubated.
Use the diagram above to answer the following questions
2a. Which disinfectant was the most effective at killing the bacteria?
2b. Which disinfectant was least effective at killing the bacteria?
3. Samantha measured the diameter of a clear zone on her agar dish. She found the value to be 2.5cm. Calculate the area of this clear zone.