A level-readyAll subjects · Biology · WBI12 (Unit 2)
Bacterial growth and core practical 9: antimicrobial plants
Many plants make chemicals that kill or slow bacteria. Core practical 9 tests plant extracts on bacteria using aseptic technique.
What bacteria need
Bacteria grow when they have nutrients (a carbon source such as glucose and a nitrogen source for proteins), water, a suitable temperature and pH, and oxygen if they are aerobic (some grow without it).
Some plants produce antimicrobial substances to defend themselves, such as compounds in garlic, mint and tea tree. Plants have also given us other therapeutic drugs (for example digitalis from foxglove and aspirin, developed from a compound in willow bark).
Method
1. Crush plant material with ethanol (a solvent), then leave it to soak and filter the extract.
2. Soak sterile paper discs in each extract; soak a control disc in ethanol only. Let the ethanol evaporate.
3. Spread a lawn of bacteria evenly over an agar plate with a sterile spreader.
4. Place the discs on the agar with sterile forceps, evenly spaced.
5. Tape the lid with two strips (not sealed all round), label the base, and incubate upside down at 25 °C for 24 to 48 hours.
6. Measure the diameter of each clear zone of inhibition and calculate area = πr2. A bigger zone means stronger antimicrobial action.
Aseptic technique and safety
- Wipe the bench with disinfectant; work near a lit Bunsen burner so the updraft carries microbes away.
- Flame the necks of bottles; lift the Petri dish lid only slightly and briefly.
- Use sterile equipment; never open the plates after incubation; dispose of everything by autoclaving.
- Incubating at 25 °C, not 37 °C, reduces the chance of growing human pathogens. Taping only partly stops anaerobic conditions that favour harmful anaerobes. Incubating upside down stops condensation dripping onto the agar.
Worked example
A clear zone has a diameter of 14 mm. Calculate its area.
- Radius = 14 / 2 = 7 mm
- Area = π x 72
Answer: 154 mm2 (to 3 significant figures)
Check yourself
- A zone of inhibition has a diameter of 12 mm. Calculate its area in mm2 to the nearest whole number.
Show answer
113 mm2. r = 6 mm; π x 36 = 113 mm2.
- A zone of inhibition has a radius of 5 mm. Calculate its area to 1 decimal place.
Show answer
78.5 mm2. π x 25 = 78.5 mm2.
- Why is one disc soaked in ethanol only?
- To kill all the bacteria
- To feed the bacteria
- To make the agar set
- To show that any effect is due to the plant extract, not the solvent
Show answer
To show that any effect is due to the plant extract, not the solvent. The control rules out ethanol as the cause of a clear zone.
- Why are the plates incubated at 25 °C rather than 37 °C?
- To reduce the chance of growing human pathogens
- Bacteria cannot grow at 37 °C
- Agar melts at 37 °C
- Plant extracts only work at 25 °C
Show answer
To reduce the chance of growing human pathogens. Many pathogens grow best at body temperature.
- Why is the lid taped with only two strips, not sealed all round?
- So the plate can be opened easily later
- To let the bacteria escape
- So oxygen can enter, preventing anaerobic conditions that favour harmful bacteria
- To save tape
Show answer
So oxygen can enter, preventing anaerobic conditions that favour harmful bacteria. Fully sealing could encourage anaerobic pathogens.
- Which is part of aseptic technique?
- Flaming the neck of the culture bottle
- Opening the lid fully for a long time
- Breathing over the plate
- Opening plates after incubation to smell them
Show answer
Flaming the neck of the culture bottle. Flaming kills microbes at the opening.
- Which condition is NOT needed for bacterial growth in general?
- Nutrients
- Water
- Light
- Suitable temperature
Show answer
Light. Most bacteria do not need light.
Sources
Last updated 7 November 2026. Spotted a mistake? Tell us.