Practical

Question 1

Describe how to an uncontaminated culture using aseptic technique

Answer 1

1. Sterilise the petri dish, nutrient broth and agar jelly before using them to destroy any unwanted microorganisms and to prevent contamination
2. Then sterilise an incocculating loop by passing it through a bunsen burner flame to prevent contamination
3. Then use the inocculating loop to transfer the bacteria to the agar jelly in the petrie dish. After this place a lid on the petri dish and seal te lid with adhesive tape- this will prevent the lid from falling off and prevent any unwanted bacteria from entering
4. Then place the lid upside down, this will prevent moisture from dripping onto the bacteria and dsrupting the colonies
5. Then incubate the culture in a school lab at 25 degrees celcius. This temperature will reduce the risk of harmful bacteria from growing

Question 2

Why must petri dishes and culture media be sterilised before use in this practical?

Answer 2

To kill any unwanted bacteria and prevent contamination

Question 3

How is the inocculating loop sterilised?

Answer 3

By passing it through a bunsen burner flame

Question 4

Why must the inocculating loop be sterilised?

Answer 4

To kill any harmful bacteria and prevent contamination

Question 5

Why is the lid of the petri dish sealed with adhesive tape when culturing microorganisms?

Answer 5

To prevent the lid from falling off
and to prevent any unwanted microorganisms from entering

Question 6

Why is the petri dish stored upside down after culturing microorganisms?

Answer 6

To prevent mositure from dripping onto the bacteria and disrupting the colonies

Question 7

Why is it that in school labatories, cultures need to be stored at 25 degrees celcius?

Answer 7

This reduces the risk of harmful bacteria growing

Question 8

Describe how to investigate the effect of antibiotics on bacterial growth

Answer 8

1. Clean the practical bench using a disinfectant, this will destroy unwanted bacteria and prevent contamination
2. Sterilise an inocculating loop by passing it through a bunsen burner flame- this will kill unwanted bacteria and prevent contaimination
3. Then open a petri dish with agar jelly inside near to a bunsen burner flame- the flame eill kill any unwanted microorganisms in the air and prevent contamination
4. Use the incocculating loop to evenly transfer the bacteria on to the sterile agar jelly plate
5. Place sterile paper discs that have been soaked in antibiotics onto the agar jelly- make sure to place a control aswell, a sterile paper disc which was soaked in sterile water- this will ensure that it was the antibiotics killing the microorganisms and not other factors like the paper
6. Place a lid on the petri dish and seal it with adhesive tape, this will prevent the lid from falling off and prevent any unwanted bacteria from entering, then leave the petri dish upside down, this prevent moisture from dripping onto the bacteria
7. Incubate the petri dish in a lab for 48 hours at 25 degrees celcius- this temperature will prevent any harmful bacteria from growing
8. The most efficient antibiotic is the one where there is the largest inhibition zone around the paper disc

Question 9

What is an inhibition zone?

Answer 9

A clear area where all of the bacteria has been destroyed

Question 10

Why is the adhesive tape on the lid to seal a petri dish place on incompletely?

Answer 10

To allow oxygen to still enter the dish so that the bacteria respire aeroibically- if they respire anaerobically it can produce harmful pathogens

Question 11

Name three control varaibles in this practical

Answer 11

The volume of the antibiotic
The concentration of the antibiotic
The incubation time and temperature

Question 12

Explain how you calculate the area of an inhibition zone around a paper disc

[3 marks]

Answer 12

1.Measure the diameter of the inhibition zone using a ruler
2.Half the diameter to find the radius
3. Square the radius and multiply it by pi