RPs Paper 1

Question 1

Describe the preparation of an uncontaminated culture using aseptic technique. (RP1)

Answer 1

1. Use pre-sterilised plastic Petri dishes or sterilise glass Petri dishes and agar
   gel before using with an autoclave.
2. Pour the sterile agar gel into the Petri dish and allow time to set.
3. Sterilise the inoculating loop by passing it through a Bunsen burner flame.
4. Dip the inoculating loop into the solution of microorganisms and make streaks
   with the loop on the surface of the agar.
5. Put the lid on the Petri dish and secure it with tape. Label accordingly then
   turn and store upside down.
6. Incubate the culture at 25oC in school laboratories.

Question 2

Why must Petri dishes, culture media and the inoculating loop be sterilised before use? (RP1)

Answer 2

To kill any existing bacteria

Question 3

Why must the Petri dish lid be secured with tape, stored upside down and not fully sealed? (3) (RP1)

Answer 3

Stop bacteria in the air from contaminating the experiment

To prevent the growth of anaerobic bacteria in a lack of oxygen, the lid isn’t fully sealed

Upside down to prevent condensation from forming and dripping onto the colonies

Question 4

Why are cultures incubated at 25°C in schools

Answer 4

Harmful pathogens are less likely to grow at this temperature

Question 5

Formula to calculate cross-sectional area of bacterial colony

Answer 5

Pi × radius²

Question 6

How is the number of bacteria in a population after a certain time calculated from the mean division time

Answer 6

1. Calculate the number of times the bacteria will divide in the given time period
   from the mean division time.
2. Use the following equation to calculate the number of bacteria:
   Number of bacteria in population at end of time period = number of bacteria at
   the beginning of the time period x 2number of divisions in the time period.