What you need to know
- 4.10Conditions required for bacterial growth.
- 4.11Plant-derived substances can have antimicrobialDescribes a substance that kills microorganisms or slows their growth. and other therapeutic properties.
- 4.12Core Practical 9: antimicrobial properties of plants and aseptic techniqueMethods that stop unwanted microorganisms contaminating a culture and stop cultures escaping, such as flaming loops and using sterile equipment..
A zone of inhibition is evidence of reduced growth under those conditions, not a licensed medicine. Do not taste, apply or recommend plant extracts as treatment. School incubations are commonly kept near 25 °C rather than 37 °C to reduce the chance of culturing human pathogens.
By the end, you should be able to…
- State conditions bacteria need in order to grow.
- Explain why plant products can be antimicrobial without being a completed drug.
- Identify aseptic controls and what a zone of inhibitionThe clear area around an antibiotic disc on a bacterial lawn, where bacteria have not grown. can and cannot show.
Build your understanding
What bacteria need
Bacterial growth requires a suitable temperature, pHA measure of how acidic or alkaline a solution is, based on its H⁺ ion concentration. Below 7 is acidic, above 7 is alkaline., water and nutrients, including a carbon source and a nitrogen source. Many also need mineral ions. Oxygen demand varies: some are obligate aerobes, some facultative, some anaerobic. “All bacteria need oxygen” is false.
In the laboratory, nutrient agar supplies food in a solid gel so colonies grow on a surface. The agar polymer is the setting agent; the nutrients are what the cells use. Time matters: a plate examined too early underestimates growth.
Plants as a chemical source
Specification 4.11: substances from plants can inhibit microbes and have other therapeutic properties. Examples used in teaching include garlic, thyme or mint extracts, and historically willow bark (salicylates) or foxglove (cardiac glycosides) — the last of those is not an antimicrobialDescribes a substance that kills microorganisms or slows their growth.. Name the idea: a plant secondary metabolite can affect another organism.
An extract is a mixture. Activity in a dish does not identify the active molecule, the safe dose, or an effect in a person. Toxicity to human cells, stability and whether resistance can evolve are separate questions.
A fair, safe test
aseptic techniqueMethods that stop unwanted microorganisms contaminating a culture and stop cultures escaping, such as flaming loops and using sterile equipment. reduces unwanted microbes: work near a lit Bunsen if that is the school method, flame tools, open lids minimally, do not talk over plates, and dispose of cultures as directed. Tape plates, incubate inverted, and do not seal all the way around so anaerobic pathogens are less favoured.
Soak paper discs in extract, include a solvent-only control and, if permitted, a known antimicrobial comparison. The dependent variable is zone diameter after a stated time at a stated temperature. A larger zone suggests more inhibition in that agar, not necessarily a better medicine: molecules diffuse at different rates, and extracts differ in concentration.
Follow the process
Grow the test bacterium in broth → spread it evenly on agar → soak equal discs for the same time in each extract, plus one disc in the solventA liquid in which other substances (solutes) dissolve. Water is a good solvent for ions and polar molecules. only → space the discs, tape the lid at intervals, invert the plate → incubate near 25 °C for 24–48 hours, not at human body temperature in a school lab → measure each clear-zone diameter in millimetres and use the mean of repeats. Area, if asked, is πr² with r equal to half the diameter.
A larger zone means more inhibition of that culture in that agar under those conditions. It does not mean an infection was cured, a safe dose was found, or the extract is a licensed medicine. Quote the diameters and the control before you judge.
NOT: a clear zone does not prove the extract is safe to take, and 37 °C is not the school incubation.
- A zone means reduced growth under these conditions
- Diffusion distance affects the diameter
- The control disc checks that water and the disc itself are not the cause
- Do not taste or apply the extract
Two traps this diagram blocks
- Do not omit the control disc.
- Do not describe the extract as an antibiotic drug because a zone appeared.
From the diagram alone
Question
Explain why a control disc soaked in water is included when testing a plant extract on bacteria. [2]
Show a model answer
The water-soaked disc is a negative control: it shows whether the paper disc and water alone inhibit growth. If a clear zone appears only around the extract, the extract is a better explanation than the disc or the water. If the control disc also has a zone, the result cannot be blamed on the extract alone.
| Claim | What the class practical can show | What it cannot show |
|---|---|---|
| Bacteria need named conditions | No growth if water, nutrients or temperature are unsuitable | The exact optimum for every species |
| A plant extract is antimicrobial | A zone of inhibitionThe clear area around an antibiotic disc on a bacterial lawn, where bacteria have not grown. versus a solvent control | Safety, dose or clinical effectiveness |
| Aseptic technique | Fewer contaminant colonies when rules are followed | A sterile room or a guaranteed pathogen-free plate |
Make your reasoning visible
Build the explanation
X happens because Y, leading to Z. X: the extract disc has a larger clear zone than the solventA liquid in which other substances (solutes) dissolve. Water is a good solvent for ions and polar molecules. control disc. Y: something in the extract inhibited growth of that bacterium in the agar. Z: you can compare diameters, not claim a cure. Incomplete: “The big circle means the plant cures infection.” Improved: “The 14 mm zone, against a 0 mm solvent control, shows inhibition on this plate; it does not show that an infection was cured.” The improvement uses the control and stays inside the evidence.
Quote diameter, not “a big circle”
Measure zone diameter, state units, and compare means of repeats.
Key explanations
- The plate is seeded so bacteria would form a lawn if nothing inhibited them.
- The extract diffuses out of the disc into the agar.
- A clear zone shows bacteria did not grow there under these conditions.
- A larger zone can mean more inhibition, but it also depends on how far the substance diffuses.
Avoid this misconception
The claim: A plant that makes a clear zone is a proven antibiotic tablet.
The correction: The plate shows inhibition of that culture under those conditions. Drug development needs identification, safety, dose and clinical testing.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
List four conditions required for bacterial growth and explain why “warmth” alone is incomplete.
Show suggested answer
Nutrients, water, a suitable temperature and a suitable pH (plus oxygen matching the species). Warmth without nutrients or water cannot support growth, and too much heat denatures proteins.
Practice question
Retrieval and explanation · suggested answer
Why incubate school plates at about 25 °C rather than 37 °C?
Show suggested answer
25 °C is less likely to select for microbes that grow well at human body temperature, reducing risk. Follow the school’s approved temperature.
Practice question
Retrieval and explanation · suggested answer
Garlic extract gives a 14 mm zone; the ethanol control gives 0 mm. What can you conclude?
Show suggested answer
Under these conditions the garlic preparation inhibited that bacterium more than the solvent alone. You cannot conclude it is a safe medicine or that it would work in the body.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
Extract A produces a 20 mm zone and extract B an 8 mm zone, but A was ten times more concentrated and used a more diffusive solvent. Can you rank their antimicrobial potency? What extra control would you add?
Show a hint
Separate concentration and diffusion from intrinsic activity.
Explore the reasoning
No. Zone size mixes concentration, diffusion and activity. Standardise concentration, solvent and disc load, and include a solvent control. Measuring the amount of extract loaded, or using a dilution series, would make potency comparable.
Recall this lesson
Lesson recall questions
Question 1 of 5: How does agar help when culturing bacteria on a plate?
Conditions for bacterial growth
Practice question
Retrieval and explanation · suggested answer
How does agar help when culturing bacteria on a plate?
Lesson: 4A 6: Plant-based Medicines
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Plant medicines still need evidence
4A · Plant-based MedicinesWHO notes that traditional, complementary and integrative medicine is widely used, yet receives a small share of health-research funding. A research agenda is needed so safety, quality and…Specification 4.11 · 4.12Read & use the scienceClose article
Why this matters
WHO notes that traditional, complementary and integrative medicine is widely used, yet receives a small share of health-research funding. A research agenda is needed so safety, quality and effectiveness can be tested rather than assumed. That is a policy and evidence point, not a claim that every herbal product works. An inhibition zone on an agar plate may suggest that a plant extract restricts bacterial growth under those conditions. It does not identify the active molecule, prove that it is safe in humans, or show that it will work as a medicine. Aseptic technique prevents contamination and protects the investigator. Researchers then isolate compounds and compare doses and controls. The WHO research agenda supplies the public-health context; the exam focus remains bacterial growth conditions, antimicrobial testing and evidence.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 4.11 know that substances derived from plants can have antimicrobial and other therapeutic properties
- 4.12 CORE PRACTICAL 9: investigate the antimicrobial properties of plants, including aseptic techniques for the safe handling of bacteria
Use the science
Why might a widely used traditional preparation still need a laboratory and clinical protocol?
Explore a possible response
Use does not prove a consistent dose, a known active substance, freedom from contaminants, or a net benefit over harm. Tests can support, refine or reject the preparation.
Explore the source
- WHO: traditional, complementary and integrative medicine research agenda 2025–2034World Health Organization
From a plant extract to an antimalarial
4A · Plant-based MedicinesTu Youyou’s work on artemisinin began with a plant source and continued through extraction, purification and testing. The Nobel account is an example of a plant-derived therapeutic substance…Specification 4.11 · 4.13Read & use the scienceClose article
Why this matters
Tu Youyou’s work on artemisinin began with a plant source and continued through extraction, purification and testing. The Nobel account is an example of a plant-derived therapeutic substance whose value depended on evidence, not simply traditional use. Artemisinin acts against the malaria parasite, so this is not an antimicrobial-zone result against bacteria. That distinction helps students interpret Core Practical 9: an agar assay can investigate a possible antibacterial effect, but cannot stand in for testing a treatment of another disease. For the specification, explain how a useful plant compound becomes a candidate and why controlled safety and effectiveness studies remain necessary.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 4.11 know that substances derived from plants can have antimicrobial and other therapeutic properties
- 4.13 understand the development of drug testing from historic to contemporary protocols, including William Withering’s digitalis soup, double blind trials, placebo and three-phased testing
Use the science
Why would a clear zone around a plant extract on bacterial agar not prove it treats malaria?
Explore a possible response
Malaria is caused by a parasite, not the bacteria in that assay. The active compound, dose, safety and activity against the parasite need separate tests.
Explore the source
- 2015 Medicine Nobel Prize: artemisininNobel Prize Outreach