Exam questions · Biology · Culturing Microorganisms and Developing Medicines
Aseptic Technique and Culturing Microorganisms
- 6 exam questions
- 18 marks
- 9 quick checks
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1 State [1 mark]
State what is meant by an aseptic technique.
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Model answer
A method that prevents unwanted microorganisms from contaminating the culture.
Mark scheme
- Preventing contamination by unwanted microorganisms — 1 mark
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2 Describe [2 marks]
Describe how an autoclave is used to prepare agar and petri dishes for culturing microorganisms.
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Model answer
The agar and glass dishes are heated by steam under pressure, at about 121 °C, for about 15 to 20 minutes, which kills all microorganisms and their spores, so they are sterile.
Mark scheme
- Steam under pressure at about 121 °C — 1 mark
- Kills all microorganisms, so they are sterile — 1 mark
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3 Explain [3 marks]
Explain why an inoculating loop is held in a flame and then cooled before it is used.
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Model answer
The flame kills any microorganisms on the loop, so that only the microorganisms being studied are transferred. It is then cooled because a hot loop would kill the bacteria that it picks up.
Mark scheme
- Flame kills microorganisms on the loop — 1 mark
- So only the wanted microorganisms are transferred — 1 mark
- Cooled so that it does not kill the bacteria picked up — 1 mark
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4 Explain [3 marks]
A student taped the lid on a petri dish with four short pieces of tape and put it in an incubator at 25 °C. Explain why the lid was not sealed completely, and why the temperature was 25 °C.
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Model answer
The lid is not sealed completely so that oxygen can enter, and harmful anaerobic bacteria cannot grow. The temperature is 25 °C because human pathogens grow best at body temperature, so fewer of them will grow at this lower temperature.
Mark scheme
- Oxygen can get in — 1 mark
- Prevents growth of anaerobic pathogens — 1 mark
- 25 °C is too low for most human pathogens — 1 mark
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5 Calculate [3 marks]
A bacterium divides every 30 minutes. Calculate the number of bacteria after 5 hours, starting with one cell, assuming that none die.
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Model answer
5 hours is 300 minutes, so there are 300 ÷ 30 = 10 divisions. The number is 2 to the power 10, which is 1024 bacteria.
Mark scheme
- 10 divisions — 1 mark
- 2 to the power 10 — 1 mark
- 1024 — 1 mark
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6 Evaluate [6 marks]
A technician found that a petri dish she had prepared contained several different colonies, when only one type of bacterium should have been present. Suggest three ways in which the dish could have been contaminated, and say how each could be avoided.
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Model answer
The loop may not have been sterilised, which can be avoided by heating it until red hot before use. The dish may have been left open for too long, so that microorganisms in the air landed on it, which can be avoided by lifting the lid only slightly and for as short a time as possible. The agar or the dish may not have been sterile, which can be avoided by using an autoclave. The neck of the culture bottle may not have been flamed, or the bench and hands may not have been clean, which can be avoided by flaming the neck, and using disinfectant and washing hands.
Mark scheme
- Unsterilised loop — 1 mark
- Flame until red hot — 1 mark
- Lid open too long — 1 mark
- Lift the lid only slightly and briefly — 1 mark
- Non-sterile agar or dish — 1 mark
- Use an autoclave, or flame the bottle neck — 1 mark
Quick check
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1
What does an autoclave use to sterilise equipment?
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C: Steam under pressure
Steam under pressure, at about 121 °C, kills all microorganisms.
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2
Why is an inoculating loop heated in a flame?
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A: To kill microorganisms on it
To kill any microorganisms on it, so only the wanted ones are transferred.
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3
Why is a dish stored upside down?
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D: So that condensation does not drip onto the agar
Condensation then falls on the lid, not on the agar.
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4
Why is the lid of a petri dish not completely sealed?
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B: To let oxygen in
Oxygen must get in, so that anaerobic pathogens do not grow.
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5
At which temperature are school cultures usually incubated?
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A: 25 °C
25 °C or below, which is too cool for most human pathogens.
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6
What is agar?
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C: A jelly used as a growth medium
A jelly, made from seaweed, that forms a solid medium.
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7
What does the word sterile mean?
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D: Free from all living microorganisms
Sterile means free from all living microorganisms.
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8
One bacterium divides every 20 minutes. How many are there after 1 hour?
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B: 8
60 ÷ 20 = 3 divisions, and \(2^3 = 8\).
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9
Why is aseptic technique especially important when culturing bacteria in a school laboratory?
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A: It protects the culture from contamination and the students from pathogens
It stops contamination of the culture, and prevents pathogens being released or breathed in.