OpenRevise

Exam questions · Biology · Cell Structure

Culturing Microorganisms

  • 8 exam questions
  • 23 marks
  • 8 quick checks
  1. 1 Explain [2 marks]

    Petri dishes and agar are sterilised before use. Explain why.

    Show answerHide answer

    Model answer

    To kill any microorganisms that are already on them. If they were not sterilised, these microorganisms could grow and contaminate the culture.

    Mark scheme

    • Kill microorganisms already present — 1 mark
    • Prevents contamination of the culture — 1 mark
  2. 2 Describe [4 marks]

    Describe how to prepare an uncontaminated culture of bacteria on an agar plate.

    Show answerHide answer

    Model answer

    Sterilise the Petri dish and the agar before use. Sterilise the inoculating loop by passing it through a flame. Use the loop to spread the bacteria over the agar, lifting the lid only as little as needed. Seal the lid with adhesive tape and incubate the dish upside down at no more than 25 °C.

    Mark scheme

    • Petri dish and agar sterilised first — 1 mark
    • Inoculating loop passed through a flame — 1 mark
    • Lid taped on to keep microorganisms out — 1 mark
    • Incubated at 25 °C (or below), upside down — 1 mark
  3. 3 Explain [2 marks]

    In school, cultures of bacteria are incubated at no more than 25 °C. Explain why.

    Show answerHide answer

    Model answer

    At higher temperatures, such as 37 °C, harmful pathogens are more likely to grow. Keeping the temperature lower makes the culture safer.

    Mark scheme

    • At higher temperatures pathogens are more likely to grow — 1 mark
    • This could harm humans / is a safety risk — 1 mark
  4. 4 Calculate [2 marks]

    A bacterium divides every 20 minutes. Starting with one bacterium, calculate how many bacteria there will be after 2 hours.

    Show answerHide answer

    Model answer

    2 hours = 120 minutes, which is 6 divisions. 2 to the power 6 = 64 bacteria.

    Mark scheme

    • 120 ÷ 20 = 6 divisions — 1 mark
    • \(2^{6} = 64\) — 1 mark
  5. 5 Calculate [3 marks]

    A culture contains 200 bacteria. The mean division time is 30 minutes. Calculate how many bacteria there will be after 6 hours. Give your answer in standard form to 3 significant figures.

    Show answerHide answer

    Model answer

    6 hours = 360 minutes, which is 12 divisions. \(200 \times 2^{12} = 200 \times 4096 = 819\,200\), which is \(8.19 \times 10^{5}\).

    Mark scheme

    • 360 ÷ 30 = 12 divisions — 1 mark
    • \(200 \times 2^{12} = 819\,200\) — 1 mark
    • \(8.19 \times 10^{5}\) — 1 mark
  6. 6 Calculate [2 marks]

    Figure 1 shows an agar plate with three antibiotic discs, A, B and C, and a control disc, D. The clear zone around disc A has a diameter of 28 mm. Calculate the cross-sectional area of this zone. Use the formula area = πr².

    A Petri dish with four discs and a key giving the clear zone diameters as 28 mm, 18 mm, 10 mm and 0 mm.
    Show answerHide answer

    Model answer

    radius = 28 ÷ 2 = 14 mm. area = π × 14² = π × 196 = 616 mm² (to 3 significant figures).

    Mark scheme

    • Radius = 14 mm — 1 mark
    • \(\pi \times 14^{2} = 616\text{ mm}^{2}\) — 1 mark
  7. 7 Explain [2 marks]

    Use Figure 1 to state which antibiotic is the most effective at killing the bacteria, and explain how you can tell.

    A Petri dish with four discs and a key giving the clear zone diameters.
    Show answerHide answer

    Model answer

    Antibiotic A is the most effective. It has the largest clear zone, which means it killed or stopped the growth of the most bacteria.

    Mark scheme

    • Antibiotic A — 1 mark
    • Largest clear zone / killed the most bacteria — 1 mark
  8. 8 Describe [6 marks]

    Describe how you would carry out an investigation to compare the effectiveness of three different antibiotics on the growth of bacteria.

    Show answerHide answer

    Model answer

    Use aseptic technique to spread a lawn of bacteria over a sterile agar plate. Soak identical paper discs in each of the three antibiotics, and one in sterile water as a control. Place the discs, evenly spaced, on the agar, then tape the lid on and incubate the plate upside down at 25 °C for 48 hours. Measure the diameter of the clear zone around each disc with a ruler. The antibiotic with the largest clear zone is the most effective. To make it a fair test, use the same type and size of discs, the same concentration of antibiotic and the same incubation time and temperature.

    Mark scheme

    • Aseptic technique used to prepare the plate and spread the bacteria — 1 mark
    • Discs soaked in each antibiotic, placed on the agar — 1 mark
    • Control disc soaked in sterile water — 1 mark
    • Incubated upside down at 25 °C or below, with the lid taped on — 1 mark
    • Measures the diameter (or area) of the clear zones — 1 mark
    • Largest zone means most effective, with a control variable to make it a fair test — 1 mark

Quick check

  1. 1

    How do bacteria multiply?

    1. ABy mitosis in a nucleus
    2. BBy fertilisation
    3. CBy binary fission
    4. DBy making seeds
    Show answerHide answer

    C: By binary fission

    Bacteria multiply by binary fission, in which one cell splits into two identical cells.

  2. 2

    Why is an inoculating loop passed through a flame?

    1. ATo make it cool
    2. BTo kill microorganisms on it
    3. CTo help bacteria grow
    4. DTo make it magnetic
    Show answerHide answer

    B: To kill microorganisms on it

    The flame sterilises the loop by killing any microorganisms on it, so only the wanted bacteria are transferred.

  3. 3

    Why is the lid of a Petri dish taped on?

    1. ATo stop microorganisms from the air getting in
    2. BTo make the agar set
    3. CTo keep the dish warm
    4. DTo stop the bacteria growing
    Show answerHide answer

    A: To stop microorganisms from the air getting in

    The tape stops microorganisms from the air getting into the culture.

  4. 4

    What is the highest temperature at which cultures are normally incubated in school?

    1. A10 °C
    2. B20 °C
    3. C37 °C
    4. D25 °C
    Show answerHide answer

    D: 25 °C

    School cultures are kept at 25 °C or below, because higher temperatures make it more likely that harmful pathogens grow.

  5. 5

    A bacterium divides every 20 minutes. How many bacteria are there after 1 hour, starting with one?

    1. A3
    2. B6
    3. C8
    4. D16
    Show answerHide answer

    C: 8

    In 60 minutes there are 3 divisions, so \(1 \to 2 \to 4 \to 8\).

  6. 6

    What does a large clear zone around an antibiotic disc show?

    1. AThe antibiotic does not work
    2. BThe antibiotic is very effective
    3. CThe bacteria are resistant
    4. DThe dish was contaminated
    Show answerHide answer

    B: The antibiotic is very effective

    A clear zone is where bacteria have been killed or stopped from growing, so a larger zone means the antibiotic is more effective.

  7. 7

    The clear zone around a disc has a radius of 10 mm. What is its area, to the nearest whole number?

    1. A31 mm²
    2. B63 mm²
    3. C100 mm²
    4. D314 mm²
    Show answerHide answer

    D: 314 mm²

    Area = \(\pi r^{2} = \pi \times 10^{2} = 314\text{ mm}^{2}\) (to the nearest whole number).

  8. 8

    A culture of 100 bacteria has a mean division time of 30 minutes. How many bacteria are there after 2 hours?

    1. A400
    2. B1600
    3. C800
    4. D12 000
    Show answerHide answer

    B: 1600

    2 hours is 120 minutes, which is 4 divisions, so \(100 \times 2^{4} = 1600\).