Plant-derived antimicrobials and drug testingEdexcel International A Level Biology: Subtopic test

1. 1
   A student investigates whether extracts from garlic and mint leaves kill bacteria. She spreads a non-pathogenic bacterium over the surface of sterile nutrient agar in a Petri dish and places on it small paper discs that have been soaked in garlic extract, mint extract or sterile distilled water. The dish is sealed and incubated at 25 °C for 48 hours.

   (a)

   Why is the dish incubated at 25 °C rather than at 37 °C?

   [1 mark]

   • AIt reduces the chance of growing bacteria that are harmful to humans
   • BBacteria cannot grow at 37 °C
   • CEnzymes in the bacteria are denatured at 25 °C
   • DThe agar would melt at 37 °C

   (b)

   Which is the most suitable negative control for this investigation?

   [1 mark]

   • AA disc soaked in a known antibiotic
   • BA disc soaked in sterile distilled water
   • CA disc soaked in garlic extract at twice the concentration
   • DA dish with no bacteria and no discs

   (c)

   Explain how the results would show that garlic extract has antibacterial properties.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A school technician prepares agar plates for a class and transfers a broth culture of bacteria onto them. The technician uses aseptic technique throughout to avoid contamination of the cultures and to avoid growing pathogens.

   (a)

   Why is an inoculating loop passed through a flame before and after it is used?

   [1 mark]

   • ATo cool the loop so it does not damage the bacteria
   • BTo dry the loop so the broth sticks to it
   • CTo sterilise the loop by killing microorganisms on it
   • DTo warm the broth so the bacteria grow faster

   (b)

   After inoculation, the lid is secured with two short pieces of tape rather than sealed completely. What is the reason for this?

   [1 mark]

   • AIt lets the bacteria escape into the air
   • BIt keeps the agar dry
   • CIt lets carbon dioxide leave the dish
   • DIt lets in oxygen so that harmful anaerobic bacteria do not grow

   (c)

   Explain two precautions in aseptic technique that prevent the cultures from being contaminated by microorganisms from the air or the bench.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In the 1780s the physician William Withering treated patients with dropsy using a mixture of foxglove (Digitalis) leaves, which he called digitalis soup. He adjusted the amount he gave each patient according to how they responded, and he recorded the outcomes in detail. Today a drug that is derived from a plant must go through a much more rigorous testing process before it is licensed.

   (a)

   Suggest three reasons why Withering's testing of digitalis soup was less reliable than the testing of a modern drug.

   [3 marks]

   (b)

   Describe how a modern double-blind, placebo-controlled trial would be carried out, and explain why each of these features is used.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A pharmaceutical company has found that an extract from the bark of a tropical tree prevents the growth of bacteria on agar plates. The company wants to find out how effective the extract is and whether it could be developed into a licensed antibiotic for people.

   (a)

   Describe the stages of testing that the extract would have to pass before it could be licensed as an antibiotic, explaining the purpose of each stage.

   [6 marks]

   (b)

   Describe how the company's scientists could compare the antibacterial effect of extracts from the bark of two different tree species, using aseptic technique.

   [6 marks]

   Total for question 4: 12 marks

End of questions