GCSE Biology revision

Culturing microorganisms (biology only)

Cell structure

AQA 4.1.1.1

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AQA student objectives

Learning pathway

ALL 🎯

Describe how an uncontaminated culture of microorganisms is prepared using aseptic technique.

MOST 🎯🎯

Apply aseptic principles and calculate the cross-sectional area of colonies or clear zones using πr².

SOME 🎯🎯🎯

Analyse microbial growth data and, for Higher Tier, calculate bacterial populations from mean division time using standard form.

Revision summary

Read on screen, then print for Cornell-style active revision.

Bacteria reproduce by binary fission and can double in number every 20 minutes given sufficient nutrients and a suitable temperature.
Culturing bacteria allows scientists to grow and study large numbers of bacteria under controlled conditions.

A nutrient broth is a liquid solution containing all the nutrients bacteria need to grow and divide, appearing cloudy when bacteria are present.
An agar gel plate is made by setting nutrient broth into a jelly using a chemical called agar, which is poured into a Petri dish and allowed to set.
On an agar gel plate, bacteria grow into visible clusters known as colonies.

2. Methods of Culturing BacteriaAQA

A nutrient broth is a liquid solution containing all the nutrients bacteria need to grow and divide, appearing cloudy when bacteria are present.
An agar gel plate is made by setting nutrient broth into a jelly using a chemical called agar, which is poured into a Petri dish and allowed to set.
On an agar gel plate, bacteria grow into visible clusters known as colonies.

All Petri dishes, nutrient broth, and agar must be sterilised before use to kill any unwanted microorganisms and prevent contamination.
An inoculating loop is used to transfer bacteria into a culture and must be sterilised by passing it through a Bunsen burner flame before use.
The bench should be cleaned with a disinfectant solution to kill microorganisms that could contaminate the culture.
Opening the agar gel plate near a Bunsen burner flame helps kill bacteria in the surrounding air, reducing the risk of contamination.

3. Aseptic Technique: Preventing ContaminationAQA

All Petri dishes, nutrient broth, and agar must be sterilised before use to kill any unwanted microorganisms and prevent contamination.
An inoculating loop is used to transfer bacteria into a culture and must be sterilised by passing it through a Bunsen burner flame before use.
The bench should be cleaned with a disinfectant solution to kill microorganisms that could contaminate the culture.
Opening the agar gel plate near a Bunsen burner flame helps kill bacteria in the surrounding air, reducing the risk of contamination.

After transferring bacteria onto the agar plate, the lid is secured with adhesive tape to prevent it from falling off and to stop unwanted microorganisms from entering.
The agar plate is placed upside down in the incubator to prevent moisture from dripping onto the bacteria and disrupting the colonies.

In school laboratories, bacteria are incubated at 25°C rather than 37°C (body temperature) to reduce the risk of harmful pathogens growing.

Sterile filter paper discs soaked in antibiotic are placed onto an agar plate that has been evenly spread with the chosen bacteria.
After incubation at 25°C for a few days, a clear area around each antibiotic disc — called the zone of inhibition — shows where bacterial growth has been prevented.
The effectiveness of an antibiotic can be measured by calculating the area of the zone of inhibition using the equation A = \pi r², where r is the radius of the zone.

6. Investigating the Effect of Antibiotics on Bacterial GrowthAQA

Sterile filter paper discs soaked in antibiotic are placed onto an agar plate that has been evenly spread with the chosen bacteria.
After incubation at 25°C for a few days, a clear area around each antibiotic disc — called the zone of inhibition — shows where bacterial growth has been prevented.
The effectiveness of an antibiotic can be measured by calculating the area of the zone of inhibition using the equation A = \pi r², where r is the radius of the zone.

If the radius of the zone of inhibition is 12 mm and π = 3.142, the area is calculated as A = 3.142 × 12² = 452.45 mm² (to two decimal places).
A larger zone of inhibition indicates that the antibiotic is more effective at preventing bacterial growth.