Plant-derived antimicrobials and drug testingEdexcel International A Level Biology: Revision notes
Section 1
Conditions for bacterial growth
Bacteria multiply quickly when their needs are met. They need:
- nutrients, such as a carbon source (for example glucose) and a nitrogen source, supplied in nutrient agar or broth
- a suitable temperature: enzymes work faster when warm, but they denature if too hot. Human pathogens grow best at 37 °C, so school cultures are incubated at 25 °C or below 30 °C to reduce the risk
- a suitable pH and water
- oxygen for aerobes, with a few kinds growing without it
In school, a culture is grown on sterile agar, in a Petri dish with a lid.
Section 2
Aseptic technique
Aseptic technique prevents contamination of the culture, and prevents the release of pathogens.
- Disinfect the bench and wash hands.
- Use sterile equipment and agar (autoclaved).
- Flame the inoculating loop until red hot, and let it cool before use. Flame the neck of the bottle.
- Open lids for as short a time as possible, working close to a Bunsen flame.
- Tape the lid with two short strips (not all the way round) so that oxygen can enter and harmful anaerobic bacteria do not grow. Invert the dish so that condensation does not drip on the agar.
- Sterilise or autoclave used cultures before disposal.
The loop is flamed to sterilise it. It is not flamed to warm the bacteria.
Section 3
Core Practical 9: antimicrobial properties of plants
- Using aseptic technique, spread a lawn of bacteria on sterile nutrient agar.
- Soak sterile paper discs in equal volumes of plant extracts (for example garlic or mint) and place them on the lawn with sterile forceps.
- Add a negative control (a disc with sterile water or solvent) and a positive control (a disc with a known antibiotic).
- Tape, invert and incubate at 25 °C for 48 hours.
- Measure the diameter of each clear zone (zone of inhibition) and calculate the area using πr².
A larger clear zone shows a stronger antibacterial effect. Repeat and take a mean.
The clear zone is measured as a diameter and the area is worked out. Compare areas, not diameters, because the area shows the effect.
Section 4
Plants as sources of medicines
Many plants make compounds that protect them against pests and infection, and some are used in medicine.
- Foxglove (Digitalis) gives digitalis for heart conditions.
- Willow bark contains salicin, which led to aspirin.
- Garlic contains allicin, with antimicrobial properties.
- Cinchona bark gives quinine for malaria.
- Yew gives taxol for cancer.
Plant extracts must be tested, because the dose is hard to control and some are toxic. Many drugs are made synthetically once the active compound is known.
Section 5
From Withering to modern drug testing
In 1785 William Withering published his results of using foxglove leaves, called digitalis soup, to treat dropsy. He adjusted the dose by each patient's response. His method had weaknesses: no control group, variable dose, no blinding and no earlier safety testing.
Modern testing:
- Pre-clinical: tests on cells and animals for toxicity.
- Phase 1: small group of healthy volunteers: safety and dose.
- Phase 2: small group of patients: effectiveness and side effects.
- Phase 3: large group of patients: compared with the existing treatment or a placebo.
Double-blind: neither patients nor doctors know who is receiving the drug. A placebo is an inactive treatment that looks identical. Together they remove bias and the placebo effect.
Do not say that a placebo has no effect. Patients may improve because they expect to, which is why a placebo group is needed.
Must Know
- Bacteria need nutrients, warmth, suitable pH, water and (usually) oxygen. School cultures are kept at or below 25–30 °C.
- Aseptic technique: sterilise, flame, minimal lid opening, tape and invert.
- CP9: lawn, discs, controls, clear zones. Area = πr².
- Plant-derived drugs: digitalis, aspirin, quinine.
- Withering used trial and error. Modern drugs pass pre-clinical, then phases 1, 2 and 3.
- Double-blind, randomised and placebo-controlled trials remove bias.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Plant-derived antimicrobials and drug testing
- A student investigates whether extracts from garlic and mint leaves kill bacteria. She spreads a non-pathogenic bacterium over the surface of sterile nutrient agar in a Petri dish and places on it small paper discs that have been soaked in garlic extract, mint extract or sterile distilled water. The dish is sealed and incubated at 25 °C for 48 hours.Explain how the results would show that garlic extract has antibacterial properties.2 marks
- A school technician prepares agar plates for a class and transfers a broth culture of bacteria onto them. The technician uses aseptic technique throughout to avoid contamination of the cultures and to avoid growing pathogens.Explain two precautions in aseptic technique that prevent the cultures from being contaminated by microorganisms from the air or the bench.2 marks
- In the 1780s the physician William Withering treated patients with dropsy using a mixture of foxglove (Digitalis) leaves, which he called digitalis soup. He adjusted the amount he gave each patient according to how they responded, and he recorded the outcomes in detail. Today a drug that is derived from a plant must go through a much more rigorous testing process before it is licensed.Suggest three reasons why Withering's testing of digitalis soup was less reliable than the testing of a modern drug.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).