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4.2.9Topic 4 · Biodiversity and Natural Resources

Antimicrobial Properties of Plants — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

5 min read 8 parts Lesson 20 of 22 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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4.2.9 Antimicrobial Properties of Plants — Edexcel A Level Biology A (SNAB) 9BI0 notes: Explain why plants produce antimicrobial compounds and describe how to investigate their effect on bacterial growth using aseptic technique.Plants cannot move away from pathogens or herbivores…

  • negative control
  • positive control
  • zone of inhibition
  • bactericidal
  • bacteriostatic
  • secondary metabolites

Part 1 of 6Plants as a source of antimicrobials

Plants cannot move away from pathogens or herbivores, so many produce chemical defences.

1
Some of these compounds, such as those in garlic (allicin), mint, thyme and tea tree, inhibit the growth of bacteria or fungi.
2
Humans have used these extracts as traditional remedies, and they provide leads for new antibiotics.
3
Plants may also be useful as a source of preservatives and disinfectants.
4
Because bacteria are evolving resistance to existing antibiotics, searching for novel compounds, in the rainforest and other habitats, is a reason to conserve biodiversity.

Part 2 of 6Aseptic technique

When culturing bacteria, aseptic technique prevents contamination of the culture by unwanted microbes and protects the worker from pathogens:

1
Disinfect the working surface and wash hands.
2
Sterilise inoculating loops by flaming until red hot and allow them to cool near the flame without touching anything.
3
Flame the neck of the culture bottle on opening and closing.
4
Hold the lid of a Petri dish only slightly open and for as short a time as possible.
5
Seal the lid with two or four short strips of tape (not all the way round, to let oxygen in and prevent anaerobic pathogens developing); invert the plate to stop condensation falling on the culture.
6
Incubate at about 25 degrees Celsius in school, below 30 degrees, to avoid growing human pathogens, then dispose of plates by autoclaving.

Part 3 of 6Investigating antimicrobial effect

A common method uses an agar plate seeded with a bacterial lawn:

Pipette a standard volume of broth culture onto sterile nutrient agar and spread evenly with a sterile spreader.
Negative control
Place sterile paper discs soaked in the plant extracts (for example garlic, mint and clove) on the agar using sterile forceps.
  • Include a disc soaked in sterile water or solvent as a negative control and a disc of a known antibiotic as a positive control.
Incubate for 24 to 48 hours.
Zone of inhibition
Measure the diameter of the clear zone of inhibition around each disc, where bacteria have not grown.

The larger the clear zone, the more effective the extract. Because zones are circular, the area, pi x r2, may be calculated to compare treatments, or the diameter measured in two directions and averaged.

Part 4 of 6Variables and limitations

Controlled variables are the concentration and volume of extract, size of discs, species and density of bacteria, agar depth, temperature and time.

Variables and limitations
Replicates on several plates improve reliability.
Results from a plate show only that the extract inhibits growth in vitro: it may not be safe or effective in a human body,
  • and the active ingredient needs to be identified, purified and tested through clinical trials.
Zones may also vary because compounds diffuse through agar at different rates.

Linkdrug testing (4.2.8) and biodiversity conservation (Topic 4.1) explain why unexplored plants are valuable.

Part 5 of 6Antibiotics, resistance and conservation

An antibiotic that targets bacteria, for example by preventing cell wall synthesis, is described as bactericidal if it kills cells or bacteriostatic if it only stops them multiplying.

Antibiotics, resistance and conservation
A clear zone on a plate does not tell you which, though a follow-up test can.
Bacteria that survive exposure to low doses can develop resistance by natural selection: a mutation that gives protection is passed on,
  • and the resistant strain increases,
  • which is why antibiotics should not be overused.
The search for new compounds from plants, fungi and soil microorganisms is vital because few new classes have been discovered in recent decades.

Plants produce secondary metabolites such as alkaloids, phenols and terpenes.

1
These are not needed for basic metabolism, but defend against pests and pathogens.
2
Their chemical diversity makes them promising candidates.
3
Ethically, collection of material from wild plants must be sustainable and respect the rights of the people who used them first.
4
In an exam, remember that a good investigation needs a clear independent variable, a measurable dependent variable such as clear-zone area, sensible controls and safe practice.
5
Mention that cultures of unknown bacteria are never opened after incubation, and that agar plates are disposed of by autoclaving.

Additional practical pointsuse a standard-sized cork borer to cut wells in the agar if you want to test liquid extracts directly, as an alternative to discs.

Antibiotics, resistance and conservation
Label the base of the dish, not the lid, so lids cannot be swapped by accident.
Extracts made with alcohol need a solvent-only control because the alcohol itself may inhibit bacteria.
Check that discs are placed far enough apart that zones cannot overlap, otherwise they cannot be measured accurately.

Part 6 of 6Exam-style questions

  1. Describe how you would test the effect of garlic extract on bacterial growth. [5]5 marks

  2. Zones of inhibition around discs were 12 mm for clove and 8 mm for mint. Calculate the area of each zone (including disc) and the ratio of clove to mint. [3]3 marks

  3. Explain why agar plates are incubated at no more than 25 degrees Celsius in schools. [2]2 marks

Key terms

TermMeaning
Aseptic techniqueMethods that prevent contamination of cultures and workers.
Zone of inhibitionClear area on agar where bacterial growth is prevented.
AntimicrobialSubstance that kills or inhibits microorganisms.
Negative controlTreatment that should have no effect, for comparison.
Positive controlTreatment known to work, to show the method is valid.
AgarJelly made from seaweed used as a culture medium.
AutoclaveMachine using high-pressure steam to sterilise equipment.
In vitroIn an artificial environment such as a culture plate.

Avoid these mistakes

1
Sealing the plate completely with tape, which could encourage anaerobic pathogens.
2
Not including negative or positive controls.
3
Measuring the disc rather than the clear zone, or forgetting to state whether the disc is included.
4
Concluding that an extract would work as a medicine from plate results alone.
5
Mixing up sterilising (killing all microbes) with disinfecting (reducing numbers).

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