RP6: Use of aseptic techniques to investigate the effect of antimicrobial substances on microbial growth

What is the method for this experiment? (9)

What are aseptic techniques? (1)

Methods used to prevent contamination by microbes.

How can cultures of microorganisms be grown? (1)

They can be grown on a sterile Petri dish containing agar jelly.

What are the three types of antimicrobials? (3)

- Disinfectants - Applied to a non-living surface to kill microorganisms.

- Antiseptics - Applied to living tissues (e.g., skin) to kill microorganisms.

- Antibiotics - Kill or prevent the division of bacteria in the body.

State aseptic techniques that could be used. (5)

- Wash hands with soap / disinfect surfaces.

- Use a sterile pipette/syringe to transfer bacteria.

- Remove bottle lid and flame the neck of the bottle.

- Lift the lid of the agar plate at an angle.

- Place pipette/spreader into disinfectant immediately after use.

Describe two aseptic techniques that could be used & explain their importance. (4)

- Use sterile equipment / flame the neck of the container: Maintains a pure culture of bacteria.

- Wear gloves / wash hands: Prevents the spread of bacteria outside the lab.

Give one reason why aseptic technique should be used. (2)

- Minimises contamination of the agar plate from airborne bacteria

- Which could give imprecise results or be hazardous if pathogenic

Explain why the agar was boiled before pouring into the Petri dish. (1)

To sterilise the agar and kill all bacteria

Why should the Petri dish be stored upside down in the incubator? (1)

To prevent condensation from dripping onto the agar surface

Explain why the plates were not incubated at temperatures higher than 25°C. (1)

Higher temperatures would be closer to the optimum for bacteria that cause human disease

Why should the Petri dish not be sealed all the way around? (1)

To prevent the growth of anaerobic bacteria

Why was the original bacterial culture diluted before spreading on the agar plate? (2)

- To prevent too many cells from overlapping.

- To allow for an accurate count of bacterial colonies

Why was the same volume of culture added to each agar plate? (1)

To ensure the same number of bacteria are transferred, allowing fair comparison

Explain why the size of the clear zone is a measure of antimicrobial properties. (2)

- The clear zone indicates where bacteria cannot grow.

- A larger clear zone means the antimicrobial is more effective

State two variables the student should control regarding the paper discs in the investigation. (2)

- Use paper discs of the same size and material.

- Ensure the discs are soaked in the solution for the same length of time.

Predict the results if bacteria are completely resistant to an antibiotic. (1)

No clear zone would form around that particular paper disc