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This lesson brings together everything you have learned about infection and response, focusing on the key exam skills you need. It also covers the Required Practical: investigating the effect of antiseptics or antibiotics on bacterial growth, which is directly examined in AQA GCSE Biology.
This is one of the required practicals for AQA GCSE Biology. You need to know the method, how to calculate the area of the zone of inhibition, and how to evaluate the results.
To investigate the effect of different antiseptics or antibiotics on the growth of bacteria.
| Precaution | Reason |
|---|---|
| Sterilise equipment (inoculation loop, forceps) | Kill any unwanted bacteria that could contaminate the experiment |
| Pass inoculation loop through Bunsen flame | Sterilises the loop by killing bacteria with heat |
| Incubate at maximum 25 degrees Celsius (in schools) | Prevents the growth of harmful human pathogens that thrive at 37 degrees Celsius |
| Seal plate with tape (not completely) | Prevents contamination from airborne microorganisms while allowing aerobic respiration |
| Incubate plates upside down | Prevents condensation from dripping onto the bacterial colonies, which could spread bacteria |
| Use a control disc (sterile water) | Provides a baseline for comparison — shows that the clear zone is due to the antibiotic, not the disc itself |
Exam Tip: The temperature used in school labs is 25 degrees Celsius, not 37 degrees Celsius. At 37 degrees Celsius (human body temperature), dangerous pathogens could grow. In industrial and university labs, 37 degrees Celsius can be used because of more advanced safety equipment. This is a commonly examined distinction.
The zone of inhibition is the clear area around an antibiotic disc where bacteria have been killed or prevented from growing. A larger zone indicates a more effective antibiotic.
Area = pi x r x r
Where pi = 3.14 (approximately) and r = radius in mm.
If the diameter of the clear zone is 14 mm:
| Antibiotic | Diameter of Zone (mm) | Radius (mm) | Area of Zone (mm squared) |
|---|---|---|---|
| Penicillin | 14 | 7 | 154 |
| Streptomycin | 22 | 11 | 380 |
| Tetracycline | 18 | 9 | 254 |
| Control | 0 | 0 | 0 |
In this example, streptomycin is the most effective antibiotic because it has the largest zone of inhibition.
Exam Tip: The formula for the area of a circle (Area = pi x r x r) is given on the equation sheet, but you must be confident using it. Practise converting diameter to radius first, then substituting into the formula. Show all your working in the exam — even if your final answer is wrong, you can still pick up method marks.
| Disease | Pathogen Type | Transmission | Key Symptoms | Prevention / Treatment |
|---|---|---|---|---|
| Measles | Virus | Airborne droplets | Fever, red rash | MMR vaccine |
| HIV | Virus | Body fluids (sexual contact, needles) | Flu-like, then immune system failure | Condoms, antiretroviral drugs |
| TMV | Virus | Contact, tools, insects | Mosaic discolouration on leaves | Resistant varieties, hygiene |
| Salmonella | Bacterium | Contaminated food | Fever, vomiting, diarrhoea | Food hygiene, poultry vaccination |
| Gonorrhoea | Bacterium | Sexual contact | Discharge, pain on urination | Condoms, antibiotics (resistance growing) |
| Rose black spot | Fungus | Wind, water splash (spores) | Black spots on leaves, leaf drop | Fungicides, remove infected leaves |
| Malaria | Protist | Mosquito vector | Recurring fever, chills | Bed nets, insecticides, antimalarial drugs |
Use this checklist to ensure you have covered all the essential content:
Here are the types of extended response questions that frequently appear:
Model structure:
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