Why work near a Bunsen flame?

Convection currents carry airborne microorganisms away from the dish.

Why are Petri dishes not sealed completely with tape?

So oxygen can enter and anaerobic pathogens are not encouraged to grow.

Why incubate school cultures at 25 °C, not 37 °C?

To reduce the chance of growing human pathogens.

A polysaccharide gel used to make solid media so colonies can form.

What is a selective medium?

A medium that allows only certain microorganisms to grow, e.g. high salt or an antibiotic.

What does dilution plating measure?

The number of viable cells, from colony counts.

What happens in the lag phase?

Little division as cells adapt to the medium, making enzymes.

Why does the stationary phase occur?

Nutrients run low and toxic wastes build up, so deaths equal divisions.

Give the equation for the number of generations n.

n = (log₁₀ N − log₁₀ N₀) ÷ 0.301.

How is the growth rate constant k found?

k = n ÷ t, in generations per unit time.

What is streak plating for?

To isolate single species as separate colonies from a mixed culture.

Microbial techniques and culturingEdexcel A-Level Biology B: Flashcards

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Why is the inoculating loop flamed?

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Why is the inoculating loop flamed?: To kill microorganisms on it so it does not contaminate the culture or release the culture.
Why work near a Bunsen flame?: Convection currents carry airborne microorganisms away from the dish.
Why are Petri dishes not sealed completely with tape?: So oxygen can enter and anaerobic pathogens are not encouraged to grow.
Why incubate school cultures at 25 °C, not 37 °C?: To reduce the chance of growing human pathogens.
What is a broth?: A liquid nutrient medium.
What is agar?: A polysaccharide gel used to make solid media so colonies can form.
What is a selective medium?: A medium that allows only certain microorganisms to grow, e.g. high salt or an antibiotic.
What does dilution plating measure?: The number of viable cells, from colony counts.
Why can turbidity overestimate viable cells?: It also counts dead cells.
What happens in the lag phase?: Little division as cells adapt to the medium, making enzymes.
Why does the stationary phase occur?: Nutrients run low and toxic wastes build up, so deaths equal divisions.
Give the equation for the number of generations n.: n = (log₁₀ N − log₁₀ N₀) ÷ 0.301.
How is the growth rate constant k found?: k = n ÷ t, in generations per unit time.
What is streak plating for?: To isolate single species as separate colonies from a mixed culture.

Exam questions on Microbial techniques and culturing

A technician transfers a sample of a harmless strain of Escherichia coli from a nutrient broth culture to a nutrient agar plate using an inoculating loop, then incubates the plate at 25 °C.
The technician worked close to a lit Bunsen burner and lifted the lid of the Petri dish only slightly. Explain why these precautions reduce contamination.2 marks
A microbiologist wants to isolate a salt-tolerant bacterium from a seawater sample that contains many species of bacteria.
Describe how streak plating could be used to obtain isolated colonies of the bacterium from a mixed culture.2 marks
A broth culture of bacteria was serially diluted by transferring 1 cm³ into 9 cm³ of sterile diluent five times, giving a dilution of 10⁻⁵ in the final tube. A 0.1 cm³ sample of this final dilution was spread on nutrient agar and incubated, and 46 colonies were counted.
Calculate the number of viable bacteria per cm³ in the original culture. Give your answer in standard form.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).