RP 6 - The use of aseptic techniques to investigate the effect of antimicrobial substances on microbial growth

Question 1

Describe two aseptic techniques you would use when transferring a sample of broth culture (bacteria) on to an agar plate.
Explain why each was important.
[4 marks]

Answer 1

• Keep lid on Petri dish OR Open lid of Petri dish as little as possible (at a 450 angle only);
• To prevent unwanted bacteria contaminating the dish OR prevent the bacteria getting out;
• Wear gloves / Wear mask / Wash hands;
• To prevent contamination of agar with bacteria on hands/mouth OR
• Prevent spread of bacteria from agar to outside the lab;
• Use sterile pipette / Flame the loop / Flame the neck of the container of the culture;
• To maintain a pure culture of bacteria;

Question 2

Explain why there is a clear zone around each disc?

Answer 2

Bacteria killed

Question 3

The student investigated information on this disinfectant prior to carrying out his investigation. On the basis of this research, the student used a maximum concentration of disinfectant of 40%. Use the diagram to explain why

Answer 3

Clear zone would be too large
OR
Clear zones would overlap/merge

Question 4

Suggest TWO variables the student should control in using the filter paper discs in this investigation

Answer 4

(Same) size;
Accept: any measure of size e.g. thickness, area, diameter.
Ignore: ‘same shape’ as shape shown on the diagram.
2. (Same) material/absorbency;
3. In solution for same time period;
Ignore: reference to volume of disinfectant.

Question 5

Describe TWO asceptic techniques the student would have used when transferring a sample of broth culture on to an agar plate. Suggest why each was important. (4 MARKS)

Answer 5

1. Keep lid on Petri dish OR Open lid of Petri dish as little as possible.
2. To prevent unwanted bacteria contaminating the dish.
   OR
   L. monocytogenes may be dangerous / may get out.

OR

3. Wear gloves OR Wear mask OR Wash hands;
4. To prevent contamination from bacteria on hands / mouth
   OR
   Prevent spread of bacteria outside the lab;

OR

5. Use sterile pipette OR Flame the loop OR
   Flame the neck of the container of the culture;
6. To maintain a pure culture of bacteria

Question 6

Explain the purpose of boiling the agar

Answer 6

So no contamination / other bacteria;
1. Accept sterilisation / kills all (bacteria)

Question 7

Explain the purpose of transferring the same volume of liquid culture onto each agar plate

Answer 7

So same number of bacteria transferred to allow comparison;
2. Allow amount / concentration for number