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4.3.3Unit 4: Energy, Environment, Microbiology and Immunity

Core Practical 9: Antimicrobial Properties of Plants — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 5 parts Lesson 19 of 28 Unit quiz

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Part 1 of 5Preparing the Investigation

4.3.3 Core Practical 9: Antimicrobial Properties of Plants

The bacteria and extracts must be prepared before the discs are applied.

1
Spread bacteria from a broth culture evenly over an agar plate using a sterile pipette and plastic spreader.
2
Open the lid as little as possible and replace it immediately to avoid contamination from the air.
3
Dry and finely grind the plant tissue, soak it in ethanol to extract the antimicrobial substances, then filter it.
4
Dip equal-sized sterile paper discs in each extract using sterile forceps, for the same length of time, and dip a control disc in ethanol only.
5
Space the discs evenly on the plate, tape the lid, invert the plate and incubate at about 25 °C for 24 to 48 hours.

Part 2 of 5Analysis

Compare the cleared areas that appear around the discs.

Clear zone
  • The area around a disc where bacteria cannot grow because the extract has killed them or stopped their growth.
Interpretation
  • A larger clear zone shows a more effective antimicrobial extract.
Measuring
  • Measure the diameter of each zone, or calculate its area using πr², with the radius as half the diameter.
Repeats
  • Repeat at least three times and calculate a mean for each extract.
Control disc
  • The ethanol-only disc shows that any effect is due to the plant extract and not to the paper or the solvent.

Part 3 of 5Aseptic Technique

Aseptic technique keeps unwanted microbes out of the culture.

Aseptic Technique
Environment
  • Keep windows and doors closed and disinfect work surfaces and utensils before and after use.
Sterile equipment
  • Use sterile equipment and discard plastic items safely afterwards.
Flame
  • Work near a Bunsen flame so rising hot air draws microbes away, and flame the neck of the broth container when opening it.
Why
  • Contamination would affect the growth of the bacteria under study and could introduce harmful pathogens.

Part 4 of 5Worked Example

Question

  • Extract A produced a clear zone of mean diameter 12 mm and extract B produced one of mean diameter 18 mm.
  • Calculate the area of each zone and say which extract is more effective.

  1. Radius A = 12 ÷ 2 = 6 mm, so area A = π × 6² = 113.1 mm².
  2. Radius B = 18 ÷ 2 = 9 mm, so area B = π × 9² = 254.5 mm².
  3. Area B is larger than area A, so more bacterial growth was prevented by extract B.

Answer Area A = 113.1 mm² and area B = 254.5 mm², so extract B is more effective.

Part 5 of 5Exam Checklist & Tips

4.3.3 – Key Points for the Exam

  • Aseptic technique Methods that prevent contamination of cultures by unwanted microorganisms.
  • Clear zone An area on an agar plate with no bacterial growth around an antimicrobial.
  • Control A treatment that shows what happens without the factor being tested.
  • Incubate To keep cultures at a set temperature to allow growth.

Common Mistakes

  • Using the diameter instead of the radius when calculating the area of the clear zone with πr².
  • Leaving out the ethanol control disc, so effects of the solvent cannot be separated from the plant extract.
  • Incubating the plate right way up with the lid loose, which risks condensation and contamination.
  • Opening the plate lid for longer than needed, which lets airborne microbes contaminate the culture.

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