GCSE · Biology · AQA · Spec 8461

Culturing microorganisms (biology only)

Growing bacteria safely means controlling contamination, temperature and the way growth is measured.

What you need to know

Bacteria reproduce by binary fission and can grow in nutrient broth or as colonies on agar.
Aseptic technique reduces contamination by using sterilised equipment, a sterilised and cooled loop, a taped lid and upside-down storage.
School cultures are incubated at a maximum of 25 °C to reduce the likelihood of growing harmful pathogens.
You need to calculate circular colony or inhibition-zone area and bacterial population size after repeated divisions.

The big picture

Bacteria reproduce by binary fission and can be cultured in nutrient broth or on agar plates. Aseptic technique is used to prepare uncontaminated cultures, with sterilised equipment, careful handling and school incubation at a maximum of 25 °C. Agar cultures can show colonies and clear zones of inhibition around antiseptic- or antibiotic-soaked discs. You also need to calculate circular areas and bacterial population growth from division time.

TONIGHT'S REVISION

Culturing microorganisms (biology only)

Grow bacteria safely, prevent contamination and measure how cultures change.

Preparing an uncontaminated culture

Follow how each aseptic precaution protects the culture or the people working with it.

Two ways to grow bacteria

Nutrient brothvsAgar plate

Both provide conditions for growth, but the visible evidence looks different.

Focus

Culture format

Nutrient broth

A nutrient broth solution

Agar plate

Agar gel in a Petri dish

The insight

The same type of organism can be cultured either in liquid broth or on a solid agar surface.

What growth can look like

Nutrient broth

The broth can become cloudy, or turbid

Agar plate

Bacteria can grow as colonies

Measurements in this topic

Nutrient broth

Population growth can be followed through bacterial-number calculations

Agar plate

Circular colonies or clear inhibition zones can have their cross-sectional area calculated

Measure a circular colony or clear zone

cross-sectional area = πr²

A circular colony or clear inhibition zone uses the area-of-a-circle relationship.

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Calculate bacterial population growth

number of bacteria = initial number × 2^(number of divisions)

Each division doubles the population, so repeated divisions are represented by powers of 2.

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Predict, then check

Picture what happens to water droplets inside a Petri dish.

A cultured Petri dish is stored the right way up. What problem could this cause?

Key points

1Given enough nutrients and a suitable temperature, some bacteria can divide about every 20 minutes.

2Nutrient broth can become cloudy as bacteria multiply; on agar, bacteria can form colonies.

3Store agar plates upside down so condensation does not drip onto the agar and spread colonies.

4A zone of inhibition is a clear area around an antiseptic- or antibiotic-soaked disc where bacteria have not grown.

5Higher Tier students must be able to express bacterial-population calculation results in standard form.

Worked example

Problem

A circular zone of inhibition has a radius of 4 mm. Calculate its cross-sectional area.

🧠

Memory hook

Think: sterile kit → flame → cool → tape → flip → 25 °C.

⚠ Watch out

Incubating a school culture near 37 °C instead of keeping it at a maximum of 25 °C.

✓

Check yourself

Why is an agar Petri dish stored upside down after inoculation?

Flashcards

(16)

How do bacteria reproduce in this topic?

By simple cell division called binary fission.

How quickly can some bacteria divide under suitable conditions?

As often as about every 20 minutes when there are enough nutrients and a suitable temperature.

What are two ways bacteria can be cultured?

In a nutrient broth solution or as colonies on an agar gel plate.

Why is an uncontaminated culture needed when testing disinfectants or antibiotics?

So the investigation uses a culture without unwanted contamination.

What must happen to Petri dishes and culture media before use?

They must be sterilised.

How is a metal inoculating loop sterilised and prepared for use?

It is heated in a flame until red hot and then allowed to cool before transferring microorganisms.

Why must the inoculating loop cool before microorganisms are transferred?

So the hot loop does not kill the microorganisms.

Why is a cultured Petri dish stored upside down?

So condensation does not drip onto the agar and spread the colonies.

What is the maximum incubation temperature for cultures in school laboratories?

25 °C.

Why are school cultures kept at a maximum of 25 °C?

To reduce the likelihood of growing pathogens that might be harmful to humans.

What is a zone of inhibition?

A clear area around an antiseptic- or antibiotic-soaked disc where bacteria have not grown.

What can happen to nutrient broth as bacteria multiply?

It can become cloudy, or turbid.

How can culture media and reusable glassware be sterilised?

By autoclaving; sterile disposable plastic Petri dishes can also be used.

What equation gives the cross-sectional area of a circular colony or clear zone?

cross-sectional area = πr², where r is the radius in mm and the area is in mm².

How do you calculate the number of bacteria after repeated divisions?

number of bacteria = initial number × 2^(number of divisions), where number of divisions = total time ÷ mean division time.

What extra requirement applies to bacterial-population calculations at Higher Tier?

Express the result in standard form.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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