RP6 Aseptic technique and microbial growth Questions

Specification route: RP6
Question bank: 14 questions
Course stage: AS / Year 12

Sample questions

QUESTION 1 · RP6 · LEVEL 2

During aseptic technique, why is it recommended to work close to a lit Bunsen burner?

The flame creates a convection current that draws air upwards, preventing airborne microbes from falling onto the agar plate.
The heat instantly sterilises all the air within a one-metre radius of the workspace.
The light from the flame kills bacteria via intense UV radiation.
It keeps the agar jelly warm enough to remain in a liquid state during inoculation.

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Hot air rises.

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Answer: The flame creates a convection current that draws air upwards, preventing airborne microbes from falling onto the agar plate.

Required practical 6. Working near a Bunsen flame creates an updraft (convection current). This prevents airborne fungal spores and bacteria from settling onto the open agar plate.

QUESTION 2 · RP6 · LEVEL 3

A student measures the clear zone of inhibition around an antibiotic disc. The diameter is $16\ mm$. What is the area of the zone of inhibition? (Use $\pi = 3.14$)

$200.96\ mm^2$
$803.84\ mm^2$
$50.24\ mm^2$
$25.12\ mm^2$

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Area of a circle is $\pi r^2$. Remember to halve the diameter to get the radius first.

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Answer: $200.96\ mm^2$

Required practical 6. The radius ($r$) is half the diameter ($16 / 2 = 8\ mm$). Area = $\pi r^2$. $3.14 \times 8^2 = 3.14 \times 64 = 200.96\ mm^2$.

QUESTION 3 · RP6 · LEVEL 2

During an experiment testing antibiotics, why are the inoculated agar plates incubated at $25^\circ C$ rather than $37^\circ C$ in a school laboratory?

To prevent the growth of human pathogenic bacteria, which thrive at human body temperature ($37^\circ C$).
Because $25^\circ C$ is the optimum temperature for the antibiotic to diffuse through the agar.
To slow down the bacterial growth so the clear zones can be measured accurately over several weeks.
Because the agar jelly will melt into a liquid state if heated to $37^\circ C$.

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Safety first. You do not want to accidentally breed dangerous microbes.

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Answer: To prevent the growth of human pathogenic bacteria, which thrive at human body temperature ($37^\circ C$).

Required practical 6. Culturing bacteria below $30^\circ C$ (usually $25^\circ C$ in schools) prevents the unintentional selection and growth of human pathogens, ensuring student safety.

QUESTION 4 · RP6 · LEVEL 3

When transferring bacteria from a broth to an agar plate, the neck of the glass culture bottle is briefly passed through a Bunsen burner flame. What does this achieve?

It heats the air inside the bottle, causing it to expand and push outwards, preventing airborne microorganisms from falling in.
It burns off the lip of the glass to make it smooth and safe to handle.
It boils the broth at the surface to sterilise the bacteria before transfer.
It creates a vacuum inside the bottle that draws the sterile pipette liquid downwards.

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Think about the physics of hot air and convection currents.

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Answer: It heats the air inside the bottle, causing it to expand and push outwards, preventing airborne microorganisms from falling in.

Required practical 6 / Aseptic technique. Flaming the neck creates an outward convection current of hot air, which physically pushes airborne contaminants away from the opening.

QUESTION 5 · RP6 · LEVEL 2

In an experiment investigating the effect of antibiotics on bacterial growth, why are the agar plates incubated upside down?

To prevent condensation that forms on the lid from dripping down onto the agar surface and disrupting the bacterial colonies or diluting the antibiotic.
To force the bacteria to grow downwards against gravity, proving they exhibit gravitropism.
To ensure the antibiotic discs remain perfectly centered through gravitational pull.
To block all light from reaching the bacteria, ensuring strictly anaerobic conditions.

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Warm, moist agar produces water vapor. If the lid is on top, water droplets form and fall.

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Answer: To prevent condensation that forms on the lid from dripping down onto the agar surface and disrupting the bacterial colonies or diluting the antibiotic.

Required practical 6 / Aseptic technique. Condensation forms on the inside of the lid during incubation. Turning the plate upside down ensures that water droplets do not fall onto the agar, which would smear colonies and alter the diffusion gradient of the antibiotic.

QUESTION 6 · RP6 · LEVEL 2

When spreading bacteria onto an agar plate, a student dips a glass spreader into ethanol and then passes it through a Bunsen burner flame. Why is it important to let the ethanol burn off completely and allow the spreader to cool before touching the agar?

If the spreader is too hot, it will instantly kill the bacteria being spread and melt the agar jelly.
If the ethanol does not burn off, the spreader will chemically react with the plastic Petri dish.
The burning ethanol provides the necessary activation energy to start bacterial mitosis.
A hot spreader will cause the antibiotic discs to instantly evaporate upon contact.

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Heat sterilises the tool, but heat also kills the organism you are trying to cultivate.

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Answer: If the spreader is too hot, it will instantly kill the bacteria being spread and melt the agar jelly.

Required practical 6. Flaming the ethanol sterilises the spreader. However, if the glass is still searing hot when it touches the bacterial culture, it will kill the cells you are trying to plate, resulting in no growth.

QUESTION 7 · RP6 · LEVEL 3

After placing antibiotic discs on the seeded agar, the Petri dish lid is secured with adhesive tape. Why should the lid NOT be completely sealed shut with tape all the way around the edge?

Sealing it completely creates anaerobic conditions inside the dish, which encourages the growth of dangerous pathogenic anaerobic bacteria.
Sealing it completely prevents the antibiotic from evaporating out of the dish.
Sealing it completely stops the bacteria from undergoing aerobic respiration, causing them to die instantly.
The tape chemically reacts with the agar, producing toxic fumes that ruin the experiment.

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Some of the nastiest human diseases (like tetanus and botulism) are caused by bacteria that thrive when oxygen is absent.

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Answer: Sealing it completely creates anaerobic conditions inside the dish, which encourages the growth of dangerous pathogenic anaerobic bacteria.

Required practical 6 / CPAC 3. Petri dishes are usually taped with two or four small pieces of tape. Sealing the entire circumference cuts off oxygen, potentially selecting for and growing dangerous anaerobic human pathogens.

QUESTION 8 · RP6 · LEVEL 3

A student measures the zone of inhibition around an antibiotic disc. The zone is not a perfect circle. How should they measure it to calculate a reliable area?

Measure the diameter across two perpendicular axes, calculate the mean diameter, halve it to find the mean radius, and then use Area = $\pi r^2$.
Measure only the longest possible diameter to show the maximum effectiveness of the drug.
Measure the circumference using a piece of string and divide by $\pi$.
It is impossible to calculate the area; the result must be discarded as an anomaly.

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You need an 'average' radius to mathematically approximate the irregular shape to a circle.

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Answer: Measure the diameter across two perpendicular axes, calculate the mean diameter, halve it to find the mean radius, and then use Area = $\pi r^2$.

Required practical 6 / MS 4.1. Because diffusion zones are rarely perfect circles, taking two measurements at right angles (perpendicular) and calculating a mean diameter provides a more accurate approximation of the area.

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