Effect of garlic and mint on bacterial growth

Question 1

Independant variable

Answer 1

presence of garlic or mint

Question 2

Dependant variable

Answer 2

zone of inhibition around disc

Question 3

Other variables to control (4)

Answer 3

• concentration of plant material
• lawn of bacteria on petri dish
• contamination of petri dish by other microbes
• same volume of plant material on each disc

Question 4

Equipment (10)

Answer 4

• plate seeded with bacteria
• plant material e.g. garlic & mint
• pestle & mortar
• 10cm3 industrial denatured alcohol
• sterile pipette
• paper discs
• sterile petri dish
• sterile forceps
• hazard tape
• marker pen.

Question 5

Outline method

Answer 5

make plant extract by crushing 3g of plant material with 10cm3 industrial denatured alcohol. Shake occasionally for 10 mins. Pipette 0.1cm3 of extract onto sterile paper disc. Allow to dry on sterile petri dish. Meanwhile label agar plates with date and split into 4 sections. 1 for each type of plant extract. Place 1 disc of each extract in each quadrant of the agar plate, close and tape with hazard tape. Leave to incubate over night and observe zone of inhibition. Carry out controls with just distilled water on discs.

Question 6

Possible evaluation issues (5)

Answer 6

• Growth of unwanted microbes on agar plates due to bad aseptic techniques
• Not shaking extract enough to ensure enough active ingredient
• Inconsistency when adding plant extract to paper discs.
• Contaminating controls
• Using wrong species of bacteria for lawn