garlic and mint

• IV


  Substance whose antimicrobial properties are being tested (garlic or mint)
• DV


  Zone of inhibition made by substance (area measured in cm²)
• control variables

  • Same concentration of plant material used
  • Type and amount of bacteria used – E.coli will be used as the bacteria and it will be evenly spread across an agar medium
  • Volume of plant material used for each disc – 0.1 cm³ can be used per sterile paper disc each time
  • Contamination of culture – aseptic techniques and sterile equipment used to avoid contamination of bacteria culture
  • Temperature of cultures – all Petri dishes should be incubated overnight at the same temperature
  • pH of medium – an agar jelly will be used
• control


  Four sterile paper discs soaked in distilled water can be placed on a Petri dish seeded with E.coli bacteria. This will act as a negative control to see if the bacteria die regardless of plant material being used.
• method

  1. Crush 3 g of garlic- use a measuring cylinder to add 10 cm³ of denatured alcohol to the mixture. Shake the mixture occasionally for 10 minutes.
  2. Repeat using 3g of the mint plant material.
  3. Pipette 0.1 cm³ of the garlic extract onto 4 of the sterile paper discs. Allow each disc to dry. Repeat this process for the mint extract solution.
  4. Label the other Petri dishes for garlic, mint and control solutions
  5. Use the sterile forceps to place all 4 discs. Close each dish. small gap is left so that oxygen can enter and there is no build-up of anaerobic bacteria.
• results and calculations

  • Open each Petri dish and use a ruler to work out the zone of inhibition for each paper disc.
  • Use a ruler to calculate the radius for the circular clear zones around each paper disc – this is the zone of inhibition.
  • Use the equation A=πr² where r, cm, is the radius to work out the area of the zone of inhibition in cm².